POUR OBTENIR Université De Bordeaux

Thomas Par

Damestoy

Bastien Castagneyrol

Christophe Plomion

M Thiery

Président Mme Branco

Manuela Professeure

Rapporteure M Kneeshaw

Daniel Professeur

Rapporteur M Castagneyrol

Bastien

M Plomion

THÈSE PRÉSENTÉE

Keywords: Thaumetopoea processionea, Quercus, Biodiversity, Genetic, Leaf traits, Plantinsects Associational effects, Herbivory, Indirect trait-mediated effects, Leaf traits, Plant-insect interaction, Quercus

Invité

response to maintain OPM growth rate on well-defended leaves. Leaf traits involved in oak-OPM interactions were partially determined by the identity of oak neighbours. Third, we found evidence that OPM performance was partially under the genetic control of their host trees in Q. robur.

Overall, these results built toward a better understanding of the processes involved in the effect of tree species mixture on the defoliation caused by pest insects and provide new avenues to guide oak stand management for improved resistance to OPM. Based on the results of this study, it would seem preferable to favour Q. robur in oak stands because it is less attractive and more resistant to OPM attacks, both in the field and in the laboratory.

However, this statement goes against current recommendations to use Q. petraea to adapt the oak to the increased risk of water deficit and therefore adds to complexity of managing forests for resistance to multiple-interacting stresses.

Interactions entre les chênes et la chenille processionnaire du chêne, Thaumetopoea processionea L. : de l'arbre à la forêt.

Résumé : Les forêts fournissent de nombreux services écosystémiques qui peuvent être compromis par les insectes. Le maintien de la santé des forêts est un enjeu majeur de la gestion forestière qui nécessite l'acquisition de connaissances sur les interactions arbresravageurs. Dans ce contexte, nous nous sommes focalisés sur la question de la résistance des chênes à un ravageur, la chenille processionnaire du chêne (CPC).

La CPC est un herbivore spécialiste des chênes décidus en Europe. Elle est responsable de fortes défoliations, en plus de graves urtications chez les usagers de la forêt. Au travers d'une série d'observations et d'expériences menées à plusieurs niveaux d'intégration (de l'arbre jusqu'à la forêt), nous nous sommes intéressés à la réponse de la CPC à différents facteurs écologiques intervenant dans la résistance des arbres : la composition du peuplement, le voisinage et les traits foliaires liés à la défense.

La résistance des arbres aux herbivores est influencée par la composition des forêts. Pour une espèce donnée, les arbres subissent en général moins d'attaques dans les mélanges que dans les peuplements purs (résistance par association). Nous avons testé l'effet des associations entre espèces d'arbres sur la résistance des chênes à la CPC, en comparant le nombre de papillon capturé et les défoliations des larves sur le chêne pédonculé (Quercus robur) et le chêne sessile (Q. petraea) dans des peuplements purs ou mélangés. Nous avons exploré les mécanismes sous-jacents, en recherchant les traits foliaires des chênes impliqués dans la variabilité des performances de la CPC. Enfin, nous nous sommes interrogés sur les causes génétiques et environnementales de la variabilité interindividuelle dans l'expression de ces traits.

Premièrement, nous avons mis en évidence une plus grande susceptibilité de Q. petraea, caractérisée par une plus grande attractivité de Q. petraea pour les papillons de CPC, un niveau de défoliation plus élevé et une plus grande survie et prise de poids des larves sur cette essence. Nous avons aussi mis en évidence une plus grande susceptibilité des chênes dans les peuplements purs par rapport aux peuplements mélangés, en particulier ceux associant un chêne à une autre essence non hôte. Ensuite, nous avons étudié les mécanismes conférant une plus grande résistance à Q. robur et aux peuplements mélangés, en mesurant les traits foliaires (les défenses chimiques et la phénologie). La synchronisation entre le développement des feuilles et celui des larves est apparue comme un facteur clé déterminant les performances de la CPC. La concentration des défenses chimiques dans les feuilles était positivement corrélée au taux de consommation des feuilles par les chenilles, suggérant une compensation alimentaire pour maintenir la croissance sur les feuilles les plus défendues. Nous avons montré que l'expression de ces traits était influencée par l'identité des voisins. Enfin, chez Q. robur, nous avons montré l'existence d'un contrôle génétique de l'arbre sur les performances des chenilles.

L'ensemble des résultats permet de mieux comprendre les processus impliqués dans l'effet du mélange d'essences sur les défoliations d'insectes et apporte des éléments permettant d'orienter la gestion des peuplements de chênes pour une meilleure résistance à la CPC. Au vu de ces résultats, il semblerait préférable de favoriser Q. robur pour sa moindre attractivité et sa plus grande résistance à la CPC. Cette proposition va toutefois à l'encontre des préconisations d'utilisation de Q. petraea pour adapter les chênaies au risque de déficit hydrique et ne fait que rajouter un élément de complexité à la gestion de la résistance des forêts face à des perturbations multiples en interaction. A l'aune de la mise en évidence de l'effet du génotype de l'arbre et de ses traits foliaires dans le développement de la CPC, la caractérisation des gènes et des molécules de défenses impliquées reste un champ de recherche quasi inexploré, qu'il conviendra de défricher.

Mots clés : Thaumetopoea processionea ; Quercus ; Biodiversité ; Génétique ; Traits foliaires ; Interactions plantes-insectes Interactions between oaks and the oak processionary moth, Thaumetopoea processionea L.: from trees to forest Abstract: Forest ecosystems provide a series of ecosystem services that can be threatened by pest insects. Maintaining forest health is therefore a major challenge of forest management that requires a sound understanding of tree-pest interactions. In this context, we investigated oak resistance (and its drivers) to a pest insect, the oak processionary moth (OPM), Thaumetopoea processionea.

The OPM is a specialist herbivore feeding on oaks in Europe. It is responsible for significant defoliation, in addition to serious urtication in forest users and practitioners. Through a series of observations and experiments carried out at several levels of integration (from tree to forest stand levels), we investigated the response of this insect to various ecological factors involved in oak resistance: forest stand species composition, tree neighbourhood effects and leaf traits potentially associated with defence.

Forest species composition has well documented effects on tree resistance to herbivores. For a given species, trees are generally less attacked in mixtures than in pure stands (i.e. associational resistance). But the opposite -associational susceptibility -also exists. We tested whether tree species diversity generated associational resistance to OPM, by comparing moth captures and larval defoliation on the pedunculate (Quercus robur) and sessile oaks (Q. petraea) in pure and mixed stands. We also investigated the underlying mechanisms, by characterizing the leaf traits involved in the variability of OPM performance. Finally, we focused on the genetic and environmental causes of inter-individual variability in the expression of these traits.

First, we found that Q. petraea was generally more susceptible to the OPM than Q. robur.

Stands dominated by Q. petraea attracted more OPM moths, Q. petraea suffered more defoliation, and OPM larvae had greater survival and weight gain on this species. We also found greater susceptibility of both oak species in pure stands as compared to mixed stands, particularly those associating an oak with another non-host species. Second, we investigated the mechanisms conferring greater resistance to Q. robur and to mixed stands by quantifying leaf chemical and developmental traits involved in oak-OPM interactions. The synchronization between leaf development and larval development also appeared to be a key factor determining the OPM performance. The concentration of leaf chemical defences was positively correlated with OPM consumption rate, suggesting a compensatory feeding Bastien ! Juste, MERCI !!! Pour absolument tout : les manips (même les jours fériés), les stats (les fameuses), la rigueur, les névroses (si peu et qui m'ont fait voir que je n'étais pas seul), l'humour, le soutien permanent, l'écoute … LES bières … (bref je ne développerai pas ici … je n'aurai pas assez de place).

Merci à Xavier d'avoir vanté mes mérites auprès de nos chers collègues de Bordeaux. Tu ne leur as pas tout dit … ils ne savaient pas à quoi ils s'exposaient ! Merci ;) Merci, Hervé pour toutes ces négociations avec l'ONF et le DSF (je pense que j'y serai toujours).

Merci Benjamin B. pour l'aide plus que précieuse dans la compréhension de la génétique des chênes. Et merci pour la réécriture de l'introduction de mon article au dernier moment ;).

Merci à Hubert de m'avoir accueilli en Loraine et pour ces moments passés ensembles à collecter des pontes dans les froideurs hivernales de la Meuse profonde (c'est le moins qu'on puisse dire), ou à compter des nids dans la chaleur estivale du plateau lorrain … Merci pour ton enthousiasme, ta bonne humeur et les formations accélérées.

Merci à Louis-Michel et aux collègues de l'INRA Avignon pour leur aide dans la récolte des pontes de processionnaire (Promis Maurane, je ne prendrai pas tout).

Merci Thierry, pour ton aide plus que précieuse dans l'utilisation de QGis (j'ai fait de belle carte grâce à toi !!) plus rien dans ton bureau). Merci à tout l'Observatoire pour la bonne humeur au labo : Charlotte, Jean-Paul, Xavier, Gilles, Benoit, Laure, Arthur, Séverin ... Merci à Marie-Lise pour ces merveilleux TP à la fac, tu resteras la meilleure prof de la fac pour moi (et surtout la meilleur dessinatrice ;)).

. Les insectes ravageurs : une problématique écologique et économique

Les insectes jouent un rôle écologique majeur dans les écosystèmes forestiers. D'une part, en consommant les arbres et autres plantes, les insectes herbivores contribuent à la régénération des écosystèmes forestiers et participent activement au renouvellement des forêts en éliminant les arbres âgés et/ou vulnérables, en favorisant le recyclage des éléments nutritifs et en créant de nouveaux habitats et de nouvelles ressources pour d'autres espèces animales ou végétales [START_REF] Schowalter | Insect Herbivore Effects on Forest Ecosystem Services[END_REF][START_REF] Bagchi | Pathogens and insect herbivores drive rainforest plant diversity and composition[END_REF][START_REF] Metcalfe | Herbivory makes major contributions to ecosystem carbon and nutrient cycling in tropical forests[END_REF]). D'autre part, en cas de forte infestation les insectes herbivores peuvent causer de sévères dégâts sur les arbres, compromettant le bon fonctionnement de l'écosystème et des services écosystémiques associés [START_REF] Boyd | The Consequence of Tree Pests and Diseases for Ecosystem Services[END_REF]. Ils peuvent détruire de vastes étendues de forêts d'une valeur économique certaine, et qu'ils soient indigènes ou exotiques, ils sont alors considérés comme des ravageurs. Les dégâts occasionnés par la grande diversité d'herbivores présente dans les peuplements forestiers, réduisent fortement la croissance des arbres entraînant leur affaiblissement voire leur mort [START_REF] Dietze | A general ecophysiological framework for modelling the impact of pests and pathogens on forest ecosystems[END_REF][START_REF] Zhang | Insect-induced tree mortality of boreal forests in eastern Canada under a changing climate[END_REF][START_REF] Anderegg | Tree mortality from drought, insects, and their interactions in a changing climate[END_REF].

Qu'ils soient exotiques ou natifs des régions infestées, les ravageurs sont préoccupants pour la santé des arbres et des écosystèmes concernés. Par exemple, le scolyte Ips typographus est l'un des ravageurs les plus destructeurs des forêts d'épicéa en Europe, dont l'augmentation des dégâts semble liée à des facteurs climatiques stressant (sécheresse, forte température, tempête) pour les arbres, augmentant les densités de population de l'insecte [START_REF] Marini | Climate drivers of bark beetle outbreak dynamics in Norway spruce forests[END_REF][START_REF] Netherer | Acute Drought Is an Important Driver of Bark Beetle Infestation in Austrian Norway Spruce Stands[END_REF]. En Amérique du nord, l'agrile du frêne (Agrilus planipennis) est un ravageur forestier qui a détruit plusieurs millions de frênes indigènes [START_REF] Kovacs | Cost of potential emerald ash borer damage in U.S. communities, 2009-2019[END_REF][START_REF] Mckenney | Estimates of the potential cost of Emerald ash borer (Agrilus planipennis Fairmaire) in Canadian municipalities[END_REF]. Ce ravageur introduit en 2002, représente une grande menace économique et écologique pour les pays concernés. Enregistré pour la première fois en 2007 en Allemagne, la pyrale du buis (Cydalima perspectalis), a colonisé en moins de 10 ans plus de 30 pays en Europe. Espèce invasive originaire d'Asie du Sud-Est, elle fait des ravages sur les buis d'ornement et de sous-bois [START_REF] Kenis | The box tree moth, Cydalima perspectalis, in Europe: horticultural pest or environmental disaster? Aliens[END_REF][START_REF] Bras | A complex invasion story underlies the fast spread of the invasive box tree moth (Cydalima perspectalis) across Europe[END_REF]. Avec l'augmentation des températures hivernales, la chenille processionnaire du pin (Thaumetopoea pytiocampa)

voit son aire de répartition s'étendre en altitude et en latitude. Autrefois cantonnée aux climats méditerranéens, elle est maintenant présente jusque-dans le bassin parisien [START_REF] Battisti | Expansion of geographic range in the pine processionary moth caused by increased winter temperatures[END_REF][START_REF] Robinet | Direct impacts of recent climate warming on insect populations[END_REF]. Introduite en Grande-Bretagne en 2006 [START_REF] Tomlinson | Managing tree pests and diseases in urban settings: The case of Oak Processionary Moth in London, 2006-2012[END_REF] et originaire d'Europe centrale, la processionnaire du chêne (Thaumetopoea processionea) est un ravageur des chênes dans les forêts naturelles et urbaines et pose de sérieux problèmes de santé humaine du fait de son caractère très urticant [START_REF] Maier | The oak processionary caterpillar as the cause of an epidemic airborne disease: survey and analysis[END_REF][START_REF] Mindlin | The arrival of oak processionary moth, a novel cause of itchy dermatitis, in the UK: Experience, lessons and recommendations[END_REF]. Ceux-ci sont autant d'insectes qualifiés de ravageurs et en combinaison avec d'autres facteurs biotiques (autres insectes ou pathogènes) et/ou abiotiques (par exemple, la sécheresse), ils constituent un facteur important contribuant au dépérissement des forêts et, dans certains cas, à la mortalité des arbres. Ces quelques exemples illustrent l'importance cruciale de comprendre la dynamique des insectes forestiers ravageurs et de caractériser les processus écologiques susceptibles de réguler ou d'atténuer leurs effets sur les arbres et les forêts afin de prendre les mesures de gestion appropriées.

Traits foliaires impliqués dans la résistance des arbres aux insectes défoliateurs

Depuis le peuplement forestier jusqu'à l'arbre, la résistance de l'arbre aux insectes ravageurs passe par l'intervention de plusieurs facteurs écologiques. La composition du peuplement, l'identité des voisins les plus proches mais également la qualité des tissus consommés de l'arbre, sont autant de facteurs interagissant entre l'insecte et l'arbre. Dans un environnement changeant, les insectes et les plantes dont ils se nourrissent sont le résultat d'un processus coévolutif en cours depuis 400 millions d'années [START_REF] Ehrlich | Butterflies and Plants : A Study in Coevolution[END_REF]Raven 1964, Labandeira 2013).

Les insectes herbivores ont développé un système sensoriel finement réglé pour reconnaître et distinguer rapidement les signaux physiques et chimiques des espèces de plantes sur lesquelles ils se nourrissent et/ou se reproduisent (plantes hôtes) [START_REF] Ehrlich | Butterflies and Plants : A Study in Coevolution[END_REF]Raven 1964, Martin et al. 2011). Par ailleurs ils subissent une pression de sélection exercée par les plantes, non seulement pour trouver des hôtes de qualité mais également pour contourner les mécanismes de défense développés par la plante hôte (Fraenkel 1959, Mithöfer and[START_REF] Mithöfer | Plant Defense Against Herbivores: Chemical Aspects[END_REF]. Ces mécanismes de défenses auxquels l'insecte doit faire face sont directement liés à la qualité nutritive des tissus consommés et déterminent les performances de l'insecte qui dépend de la consommation du matériel végétal pour acquérir les éléments nutritifs essentiels à sa survie et à son développement.

Qualité nutritive et métabolites spécialisés : des défenses contre les herbivores défoliateurs

Chez les plantes la qualité nutritive (teneur en azote, en carbone ou en eau par exemple) joue un rôle important sur les performances des herbivores. L'azote est un élément limitant important chez les insectes herbivores [START_REF] Mattson | Herbivory in realtion to plant nitrogen content ![END_REF]) et une faible teneur en azote (ou un fort ratio C:N) est généralement associée à une faible qualité nutritive et réduit les performances des insectes herbivores qui ont tendance à préférer les feuilles riches en azote à celle plus riches en composés carbonés complexes [START_REF] Feeny | Seasonal changes in oak leaf tannins and nutrients as a cause of spring feeding by winter moth caterpillars[END_REF][START_REF] Mattson | Herbivory in realtion to plant nitrogen content ![END_REF][START_REF] White | The abundance of invertebrate herbivores in relation to the availability of nitrogen in stressed food plants[END_REF]. De plus, la qualité nutritive des plantes est liée non seulement à la teneur en nutriments mais également à la concentration en composés de défense. En particulier, chez les arbres les composés phytochimiques liés à la défense de la plante sont souvent considérés comme des traits essentiels impliqués dans la résistance aux insectes [START_REF] Fraenkel | The Raison d'Etre Substances of Secondary Plant[END_REF][START_REF] Ehrlich | Butterflies and Plants : A Study in Coevolution[END_REF][START_REF] Mithöfer | Plant Defense Against Herbivores: Chemical Aspects[END_REF][START_REF] War | Mechanisms of plant defense against insect herbivores[END_REF][START_REF] Agrawal | On the study of plant defence and herbivory using comparative approaches: How important are secondary plant compounds[END_REF]. Les plantes produisent des centaines voire des milliers de molécules différentes qui, prises ensembles, constituent des profils phytochimiques souvent très variables dans les populations naturelles [START_REF] Geber | Inheritance and Natural Selection on Functional Traits[END_REF]Griffen 2003, Barbour et al. 2015). Parmi ces composés, les métabolites secondaires, dits spécialisés, ont un rôle dans les défenses de la plante et dans la réduction des performances des insectes herbivores à travers divers mode d'action [START_REF] Fraenkel | The Raison d'Etre Substances of Secondary Plant[END_REF][START_REF] Mithöfer | Plant Defense Against Herbivores: Chemical Aspects[END_REF][START_REF] Agrawal | On the study of plant defence and herbivory using comparative approaches: How important are secondary plant compounds[END_REF]. Parmi eux, les composés phénoliques sont communément considérés comme des défenses efficaces de la plante contre de nombreux herbivores défoliateurs [START_REF] Feeny | Plant apparency and chemical defense[END_REF][START_REF] Lill | The effects of leaf quality on herbivore performance and attack from natural enemies[END_REF][START_REF] Forkner | Feeny revisited: condensed tannins as antiherbivore defences in leaf chewing herbivore communitites of Quercus[END_REF][START_REF] War | Mechanisms of plant defense against insect herbivores[END_REF]. Ces composés sont souvent considérés comme toxiques pour l'herbivore [START_REF] Salminen | Chemical ecology of tannins and other phenolics: We need a change in approach[END_REF][START_REF] Mithöfer | Plant Defense Against Herbivores: Chemical Aspects[END_REF][START_REF] War | Mechanisms of plant defense against insect herbivores[END_REF], et ont pour la plupart, la capacité à réduire la digestibilité des tissus consommés par les herbivores, et ainsi potentiellement réduire les défoliations [START_REF] Feeny | Seasonal changes in oak leaf tannins and nutrients as a cause of spring feeding by winter moth caterpillars[END_REF][START_REF] Roslin | Specialization pays off : contrasting effects of two types of tannins on oak specialist and generalist moth species[END_REF][START_REF] War | Mechanisms of plant defense against insect herbivores[END_REF], Abdala-Roberts et al. 2016, Moreira et al. 2017a). Les tannins, ainsi que les flavonoïdes réduisent la digestibilité des plantes en se fixant aux enzymes digestives et altèrent les tissus du tube digestif des herbivores par la production de dérivés réactifs de l'oxygène [START_REF] Barbehenn | Hydrolyzable tannins as "quantitative defenses": Limited impact against Lymantria dispar caterpillars on hybrid poplar[END_REF][START_REF] Barbehenn | Tannins in plant-herbivore interactions[END_REF][START_REF] Falcone Ferreyra | Flavonoids: biosynthesis, biological functions, and biotechnological applications[END_REF][START_REF] War | Mechanisms of plant defense against insect herbivores[END_REF].

Les lignines sont également des composés phénoliques toxiques. Constituants de la paroi cellulaire des plantes elles contribuent aussi à augmenter la dureté des feuilles [START_REF] Bidlack | Molecular structure and component integration of secondary cell walls in plants[END_REF][START_REF] Bonawitz | The Genetics of Lignin Biosynthesis: Connecting Genotype to Phenotype[END_REF][START_REF] War | Mechanisms of plant defense against insect herbivores[END_REF], un trait physique de défenses contre les défoliateurs commun aux plantes [START_REF] Clissold | The gastrointestinal tract as a nutrient-balancing organ[END_REF][START_REF] Pearse | The role of leaf defensive traits in oaks on the preference and performance of a polyphagous herbivore, Orgyia vetusta[END_REF][START_REF] Caldwell | Which leaf mechanical traits correlate with insect herbivory among feeding guilds?[END_REF].

Spécialiste vs généraliste : deux stratégies alimentaires

Bien que ces composés soient considérés comme efficaces en tant que défenses de la plante, le profil de défense d'une plante mis en place au cours de l'évolution est la résultante de différentes contraintes imposées par la diversité des insectes herbivores (Becerra 2007, Howe and[START_REF] Howe | Plant Immunity to Insect Herbivores[END_REF]. Un arbre doit faire face à une multitude d'espèces d'herbivores variant quant à leur façon de consommer les tissus de la plante mais également quant à leurs degrés de spécialisation [START_REF] Howe | Plant Immunity to Insect Herbivores[END_REF]. Ainsi la réponse des herbivores aux défenses des plantes varie parmi les différents groupes fonctionnels d'herbivores [START_REF] Slinn | Across Multiple Species, Phytochemical Diversity and Herbivore Diet Breadth Have Cascading Effects on Herbivore Immunity and Parasitism in a Tropical Model System[END_REF]. En particulier, les métabolites secondaires ont souvent des effets variables suivant que l'herbivore considéré est généraliste ou spécialiste [START_REF] Cornell | Herbivore Responses to Plant Secondary Compounds : A Test of Phytochemical Coevolution Theory[END_REF][START_REF] Lankau | Specialist and generalist herbivores exert opposing selection on a chemical defense[END_REF][START_REF] Bidart-Bouzat | An ecological genomic approach challenging the paradigm of differential plant responses to specialist versus generalist insect herbivores[END_REF][START_REF] Ali | Specialist versus generalist insect herbivores and plant defense[END_REF]. Bien que la distinction entre généraliste et spécialiste ait suscité de nombreux débats dans la littérature (e.g., [START_REF] Jorge | An integrated framework to improve the concept of resource specialisation[END_REF], on qualifie généralement de « spécialiste » un insecte herbivore ayant la capacité de se nourrir sur une gamme restreinte d'espèce hôtes au sein d'un même genre botanique. Au contraire, un insecte herbivore peut être qualifié de « généraliste » lorsqu'il a la capacité de se nourrir sur une large gamme de plantes hôtes appartenant à des familles botaniques différentes (e.g. [START_REF] Liebhold | Suitability of North American Tree Species to the Gypsy Moth: A Summary of Field and Laboratory Tests[END_REF]. Contrairement aux généralistes, les herbivores spécialistes surmonteraient, ou même bénéficieraient des faibles concentrations en composés spécifiques de défenses tout en étant plus sensibles aux composés produits à haute concentration [START_REF] Karban | Herbivore offense[END_REF], Coley et al. 2006[START_REF] Després | The evolutionary ecology of insect resistance to plant chemicals[END_REF][START_REF] Carmona | Plant traits that predict resistance to herbivores[END_REF]. Ainsi, la variabilité des traits foliaires de l'espèce hôte est un facteur déterminant non seulement le choix de l'insecte pour telle ou telle espèce, mais aussi les performances de ce dernier.

Bien que l'effet individuel des traits foliaires sur les performances des insectes herbivores, soit bien documenté, la totalité de ces traits foliaires agissent ensemble [START_REF] Agrawal | Induced responses to herbivory in wild radish: Effects on several herbivores and plant fitness[END_REF]Fishbein 2006, Howe and[START_REF] Howe | Plant Immunity to Insect Herbivores[END_REF]. Le comportement alimentaire et les performances de l'insecte herbivore sont déterminés par la combinaison de ces multiples traits, le plus souvent covariants et agissants dans un environnement complexe où l'identité et le choix de l'espèce sont cruciaux dans le développement de l'insecte. La décision prise par l'insecte est donc non seulement définie par une multitude de traits physiques et/ou chimiques déterminant ses performances mais également par la chimie, la morphologie, la distribution et/ou l'abondance des plantes hôtes ou non hôte présentes dans le voisinage de la plante focale. Dans l'environnement complexe que forment les écosystèmes forestiers, les insectes herbivores sont donc contrôlés par des mécanismes incluant les traits de défense [START_REF] Fraenkel | The Raison d'Etre Substances of Secondary Plant[END_REF][START_REF] Coley | Herbivory and Defensive Characteristics of Tree Species in a Lowland Tropical Forest[END_REF][START_REF] Mithöfer | Plant Defense Against Herbivores: Chemical Aspects[END_REF][START_REF] Agrawal | On the study of plant defence and herbivory using comparative approaches: How important are secondary plant compounds[END_REF] et également par les effets d'association fournis par la diversité des plantes [START_REF] Jactel | Tree diversity reduces herbivory by forest insects[END_REF][START_REF] Schuldt | Tree diversity promotes insect herbivory in subtropical forests of south-east China[END_REF], Castagneyrol et al. 2014b, Barton et al. 2015a[START_REF] Schuldt | Early positive effects of tree species richness on herbivory in a large-scale forest biodiversity experiment influence tree growth[END_REF][START_REF] Haase | Contrasting effects of tree diversity on young tree growth and resistance to insect herbivores across three biodiversity experiments[END_REF][START_REF] Guyot | Tree diversity reduces pest damage in mature forests across Europe[END_REF][START_REF] Wein | Tree Species Richness Promotes Invertebrate Herbivory on Congeneric Native and Exotic Tree Saplings in a Young Diversity Experiment[END_REF], Castagneyrol et al. 2018a).

Résistance et susceptibilité par association dans les peuplements mélangés : patterns et mécanismes

Différents résultats expérimentaux obtenus sur les écosystèmes herbacés [START_REF] Hooper | Effects of biodiversity on ecosystem functioning : a consensus of current knowledge[END_REF], ou à partir de données de placettes forestières [START_REF] Paquette | The effect of biodiversity on tree productivity: From temperate to boreal forests[END_REF]Messier 2011, Zhang et al. 2012) et de modélisation [START_REF] Loreau | Biodiversity and ecosystem functioning: a mechanistic model[END_REF][START_REF] Morin | Tree species richness promotes productivity in temperate forests through strong complementarity between species[END_REF], ont mis en évidence un effet positif de la diversité spécifique des plantes sur le fonctionnement des écosystèmes. Une des hypothèses qui expliquerait cette relation stipule que les effets de la diversité spécifique sur le fonctionnement d'un écosystème seraient liés aux interactions biotiques, notamment les interactions plantes-herbivores [START_REF] Andow | Vegetational diversity and arthropod population response[END_REF]. Bien qu'il existe plusieurs exemples d'espèces plus sensibles aux insectes herbivores dans les peuplements mélangés que dans les peuplements purs (i.e. susceptibilité par association) [START_REF] Schuldt | Tree diversity promotes insect herbivory in subtropical forests of south-east China[END_REF], 2015[START_REF] Haase | Contrasting effects of tree diversity on young tree growth and resistance to insect herbivores across three biodiversity experiments[END_REF][START_REF] Wein | Tree Species Richness Promotes Invertebrate Herbivory on Congeneric Native and Exotic Tree Saplings in a Young Diversity Experiment[END_REF], Castagneyrol et al. 2018a), l'hypothèse de résistance par association, rapportée par la plupart des études [START_REF] Jactel | Tree diversity reduces herbivory by forest insects[END_REF][START_REF] Vehviläinen | Tree species diversity influences herbivore abundance and damage: meta-analysis of long-term forest experiments[END_REF], Castagneyrol et al. 2014c[START_REF] Guyot | Tree diversity reduces pest damage in mature forests across Europe[END_REF], Jactel et al. 2017), stipule que les forêts pures sont plus susceptibles aux dégâts causés par les insectes ravageurs que les forêts mélangées.

La résistance par association pourrait s'expliquer par (i) une meilleure efficacité des ennemis naturels, (ii) une diminution de la probabilité de détection des plantes hôtes par les herbivores lorsque celles-ci sont entourées de plantes non hôtes, (iii) une diminution de la densité de la plante hôte dans le peuplement.

Tout d'abord, selon l'hypothèse des ennemis naturels [START_REF] Root | Organization of a Plant-Arthropod Association in Simple and Diverse Habitats: The Fauna of Collards (Brassica Oleracea)[END_REF], les prédateurs et parasitoïdes des insectes herbivores seraient à la fois plus nombreux et plus efficaces en tirant bénéfice des ressources alimentaires complémentaires dans les communautés végétales diversifiées [START_REF] Root | Organization of a Plant-Arthropod Association in Simple and Diverse Habitats: The Fauna of Collards (Brassica Oleracea)[END_REF][START_REF] Russell | Enemies Hypothesis: A Review of the Effect of Vegetational Diversity on Predatory Insects and Parasitoids[END_REF][START_REF] Jactel | 12 A Test of the Biodiversity -Stability Theory : Meta-analysis of Tree Species Diversity Effects on Insect Pest Infestations , and Re-examination 176[END_REF].

Ensuite, la recherche de nourriture ou de sites de ponte par les insectes herbivores dépend principalement de la reconnaissance de la plante hôte grâce à des traits physiques ou chimiques responsables de l'attractivité et de l'appétence de la plante hôte [START_REF] Barbosa | Associational Resistance and Associational Susceptibility: Having Right or Wrong Neighbors[END_REF]. Cependant, le voisinage de la plante hôte a lui aussi son rôle à jouer dans l'attractivité de cette dernière. Les caractéristiques physiologiques, morphologiques ou même la distribution et l'abondance d'une plante non hôte interagit directement avec l'insecte herbivore (Zhang andSchlyter 2004, Andersson et al. 2009). Dans les forêts mélangeant des espèces hôtes et non hôtes, les signaux attractifs de l'hôte peuvent ainsi être dilués ou masqués par des signaux visuels ou olfactifs non hôtes, ce qui réduit la probabilité de détection des plantes hôtes par les insectes herbivores [START_REF] Zhang | Olfactory recognition and behavioural avoidance of angiosperm nonhost volatiles by conifer-inhabiting bark beetles[END_REF][START_REF] Bruce | Insect host location: a volatile situation[END_REF][START_REF] Barbosa | Associational Resistance and Associational Susceptibility: Having Right or Wrong Neighbors[END_REF], Bruce and Pickett 2011a[START_REF] Castagneyrol | Plant apparency, an overlooked driver of associational resistance to insect herbivory[END_REF], 2014c[START_REF] Verschut | Disentangling associational effects: both resource density and resource frequency affect search behaviour in complex environments[END_REF]. Par exemple la présence d'une haie de feuillus devant les peuplements de pin maritime agit comme barrière physique qui réduit la détection des arbres hôtes par les femelles de la processionnaire du pin à la recherche de site de ponte [START_REF] Dulaurent | Hide and seek in forests: Colonization by the pine processionary moth is impeded by the presence of nonhost trees[END_REF].

Aussi, [START_REF] Jactel | Non-host volatiles mediate associational resistance to the pine processionary moth[END_REF] suggèrent que les composés volatils émis par le bouleau dans les peuplements de pin maritime pourraient expliquer la diminution de l'attraction des mâles de la processionnaire du pin vers les pièges à phéromones.

Enfin, le niveau de dégâts par les insectes herbivores peut également dépendre de la disponibilité de la ressource (plante hôte) dans le peuplement. Cependant suivant le degré de spécialisation de l'insecte herbivore considéré, il semblerait que l'insecte ne réagisse pas de la même manière. L'hypothèse de concentration de la ressource [START_REF] Root | Organization of a Plant-Arthropod Association in Simple and Diverse Habitats: The Fauna of Collards (Brassica Oleracea)[END_REF]) stipule que les herbivores spécialisés sont plus susceptibles de trouver, de rester et de se reproduire dans des peuplements où leurs plantes hôtes sont plus abondantes ou plus fréquentes que les plantes non hôtes. Une augmentation de la diversité spécifique diminuerait les dégâts par les herbivores spécialistes en diminuant la disponibilité de la ressource. À l'inverse, selon l'hypothèse de « mixing diet » [START_REF] Bernays | Dietary Mixing in a Generalist Herbivore : Tests of Two Hypotheses[END_REF][START_REF] Mody | Fitness related diet-mixing by intraspecific host-plant-switching of specialist insect herbivores[END_REF]

Effet indirect des plantes voisines sur les traits de résistance aux herbivores

La plupart de ces études sur les effets d'association portent sur la variabilité interpopulationnelle dans les préférences des plantes hôtes, par l'intervention de traits d'attraction ou de répulsion (physiques ou chimiques). Associée à cette variabilité, la performance de l'insecte est dépendante de la plante hôte et des traits nutritionnels et/ou de défenses associés.

Nous en savons cependant moins sur les effets du voisinage proche quant aux performances des herbivores. En effet, au-delà de la variation intra ou inter-spécifique dans les traits foliaires de l'hôte, il a été montré, indépendamment des processus impliqués dans les préférences et la colonisation de la plante hôte par l'insecte, que l'identité et la diversité des voisins d'une plante peuvent indirectement affecter les performances des herbivores en modifiant les traits de la plante hôte, en particulier ceux déterminant la qualité nutritive de la plante et les défenses anti-herbivores. Ces effets indirects des plantes voisines par la modification des traits peuvent résulter de différents mécanismes, incluant la compétition pour la ressource ou l'émission de composés organiques volatiles (COV) par les plantes voisines [START_REF] Arimura | Plant-plant interactions mediated by volatiles emitted from plants infested by spider mites[END_REF][START_REF] Turlings | Exploiting scents of distress: the prospect of manipulating herbivore-induced plant odours to enhance the control of agricultural pests[END_REF], Agrawal et al. 2006[START_REF] Barbosa | Associational Resistance and Associational Susceptibility: Having Right or Wrong Neighbors[END_REF][START_REF] Ballaré | Light Regulation of Plant Defense[END_REF], Kos et al. 2015b). La compétition pour la lumière, l'eau ou les nutriments agit, par exemple, sur les défenses contre les pathogènes ou les insectes, en modifiant l'intensité de l'allocation de la ressource aux défenses (Agrawal et al. 2006[START_REF] Ballaré | Light Regulation of Plant Defense[END_REF] L'allocation des ressources aux défenses, tels que les composés phénoliques, augmente avec la disponibilité en lumière, ainsi les plantes poussant en pleine lumière sont moins susceptibles aux attaques par les herbivores. Il existe également des interactions plantesplantes par le biais des COV modulant l'expression des traits de défense (Arimura et al. 2000a, 2001[START_REF] Karban | Plant behaviour and communication[END_REF][START_REF] Heil | Herbivore-induced plant volatiles: targets, perception and unanswered questions[END_REF][START_REF] Karban | Volatile communication between plants that affects herbivory: A meta-analysis[END_REF], Moreira et al. 2016b). Par exemple, [START_REF] Arimura | Plant-plant interactions mediated by volatiles emitted from plants infested by spider mites[END_REF]

Contribution de la diversité génétique aux effets d'association

Principalement rapportés par les études sur la diversité spécifique, les effets d'association ont plus récemment été décrits en intra-spécifique en considérant la diversité génétique des plantes hôtes [START_REF] Bailey | From genes to ecosystems: A synthesis of the effects of plant genetic factors across levels of organization[END_REF][START_REF] Tack | The relative importance of host-plant genetic diversity in structuring the associated herbivore community[END_REF][START_REF] Castagneyrol | Genetic Diversity Increases Insect Herbivory on Oak Saplings[END_REF], Barton et al. 2015b[START_REF] Koricheva | The relative importance of plant intraspecific diversity in structuring arthropod communities: A meta-analysis[END_REF]. Au cours des dernières décennies, de nombreuses études ont montré que la variation phénotypique des plantes hôtes était génétiquement déterminées et que cette variation pouvait avoir un effet sur les interactions plantes-insectes, certaines conduisant à des processus de co-évolution [START_REF] Crutsinger | Plant Genotypic Diversity Predicts Community Structure and Governs an Ecosystem Process[END_REF][START_REF] Johnson | An emerging synthesis between community ecology and evolutionary biology[END_REF][START_REF] Hughes | Ecological consequences of genetic diversity[END_REF][START_REF] Bidart-Bouzat | An ecological genomic approach challenging the paradigm of differential plant responses to specialist versus generalist insect herbivores[END_REF][START_REF] Mcart | Plant genotypic diversity reduces the rate of consumer resource utilization[END_REF] et donc, qu'au sein d'une population de plante (naturelle ou expérimentale), la diversité génétique influençait la composition des communautés d'insectes [START_REF] Crutsinger | Plant Genotypic Diversity Predicts Community Structure and Governs an Ecosystem Process[END_REF][START_REF] Johnson | Bottom-up effects of plant genotype on aphids, ants, and predators[END_REF][START_REF] Tack | The relative importance of host-plant genetic diversity in structuring the associated herbivore community[END_REF][START_REF] Crawford | Genetic diversity within a dominant plant outweighs plant species diversity in structuring an arthropod community[END_REF]. Parce que la variabilité phénotypique entre les génotypes devrait être inférieure à la variabilité entre les espèces, nous pourrions prédire un plus faible effet de la diversité génétique par rapport à la diversité spécifique. Il a cependant été montré que la plupart des fonctions écosystémiques fournit par la diversité spécifique étaient aussi apportées par la diversité génétique, incluant la productivité des plantes [START_REF] Crutsinger | Plant Genotypic Diversity Predicts Community Structure and Governs an Ecosystem Process[END_REF][START_REF] Cook-Patton | A direct comparison of the consequences of plant genotypic and species diversity on communities and ecosystem function[END_REF], le recyclage des éléments nutritifs [START_REF] Madritch | Phenotypic diversity influences ecosystem functioning in an oak sandhills community[END_REF] ou la stabilité des communautés [START_REF] Booth | Effects of Genetic Impoverishment on Plant Community Diversity[END_REF][START_REF] Reusch | Ecosystem recovery after climatic extremes enhanced by genotypic diversity[END_REF][START_REF] Gamfeldt | Increasing intraspecific diversity increases predictability in population survival in the face of perturbation[END_REF]. Aussi, certaines études ont montré des effets de la diversité génétique sur l'herbivorie, similaires, voir même plus forts que ceux apportés par la diversité spécifique [START_REF] Crutsinger | Plant Genotypic Diversity Predicts Community Structure and Governs an Ecosystem Process[END_REF][START_REF] Johnson | Additive and interactive effects of plant genotypic diversity on arthropod communities and plant fitness[END_REF][START_REF] Cook-Patton | A direct comparison of the consequences of plant genotypic and species diversity on communities and ecosystem function[END_REF][START_REF] Crawford | Genetic diversity within a dominant plant outweighs plant species diversity in structuring an arthropod community[END_REF], Moreira et al. 2014a, Barantal et [START_REF] Meurisse | Low temperature tolerance and starvation ability of the oak processionary moth: Implications in a context of increasing epidemics[END_REF][START_REF] Wagenhoff | Temporal synchrony of Thaumetopoea processionea egg hatch and Quercus robur budburst[END_REF]. Une éclosion trop précoce peut néanmoins provoquer une mortalité élevée due à une famine trop longue, au contraire une éclosion trop tardive force les chenilles à faire face à une diminution rapide de la qualité nutritive des feuilles (Feeny 1968a[START_REF] Feeny | Seasonal changes in oak leaf tannins and nutrients as a cause of spring feeding by winter moth caterpillars[END_REF][START_REF] Tikkanen | Phenological variation as protection against defoliating insects: The case of Quercus robur and Operophtera brumata[END_REF][START_REF] Forkner | Feeny revisited: condensed tannins as antiherbivore defences in leaf chewing herbivore communitites of Quercus[END_REF] 1. Pure forests are often seen as more prone to damage by specialist pest insects than mixed forests, and particularly mixed forests associating host and non-host species.

We addressed the effect of tree diversity on oak colonization and defoliation by a major specialist pest, the oak processionary moth (OPM)

2. We quantified OPM abundance and defoliation in pure stands of two oak host species

Introduction

Insect herbivores can cause severe damages to trees and compromise the functioning of forest ecosystems and the delivery of associated services [START_REF] Boyd | The Consequence of Tree Pests and Diseases for Ecosystem Services[END_REF][START_REF] Kautz | Simulating the recent impacts of multiple biotic disturbances on forest carbon cycling across the United States[END_REF]. A long held view in forest ecology is that tree diversity can increase forest resistance to herbivores. Several empirical research and recent meta-analyses support the view that pure forests are more prone to damage by pest insects than mixed forests, which is generally referred to associational resistance [START_REF] Jactel | Tree diversity reduces herbivory by forest insects[END_REF], Castagneyrol et al. 2014c, Barton et al. 2015a[START_REF] Guyot | Tree diversity reduces pest damage in mature forests across Europe[END_REF], Jactel et al. 2017). However, there are several examples of mixed forests being equally, or even more susceptible to insect herbivores than pure stands (i.e., associational susceptibility) [START_REF] Schuldt | Tree diversity promotes insect herbivory in subtropical forests of south-east China[END_REF], 2015[START_REF] Haase | Contrasting effects of tree diversity on young tree growth and resistance to insect herbivores across three biodiversity experiments[END_REF][START_REF] Wein | Tree Species Richness Promotes Invertebrate Herbivory on Congeneric Native and Exotic Tree Saplings in a Young Diversity Experiment[END_REF], Castagneyrol et al. 2018a). Authors having addressed associational effects in forest ecosystems generally compare pure vs. mixed stands for either herbivore abundance or herbivore damage. Yet, herbivore abundance and damage are not always correlated and the underlying processes may differ from one to another [START_REF] Rhainds | Testing the resource concentration hypothesis with tarnished plant bug on strawberry: density of hosts and patch size influence the interaction between abundance of nymphs and incidence of damage[END_REF], which is a likely source of discrepancies among studies, thus calling for the simultaneous consideration of these two aspects of herbivore response to mixed stands.

The resource concentration hypothesis and its further developments [START_REF] Root | Organization of a Plant-Arthropod Association in Simple and Diverse Habitats: The Fauna of Collards (Brassica Oleracea)[END_REF][START_REF] Hambäck | Patch area, population density and the scaling of migration rates: the resource concentration hypothesis revisited[END_REF][START_REF] Hambäck | Effects of plant neighborhoods on plant-herbivore interactions: resource dilution and associational effects[END_REF] state that specialist herbivores are more likely to find, stay, and reproduce in stands where their host plants are more abundant or more frequent than non-host plants. Yet, not only do the absolute and relative abundance of host and nonhost plants contribute to determine what herbivores perceive as suitable feeding resources, but the species identity of host plant also matters. In a meta-analysis of tree diversity effects on insect herbivores, Castagneyrol et al. (2014a) showed that trees were equally likely to be attacked by specialist herbivore species in pure and mixed forests consisting of closely phylogenetically related species, because such mixed forests were perceived as homogeneous resources comparable to pure stands by these herbivores. Conversely, generalist herbivores can cause more damage in mixed forests (associational susceptibility) because they can spill over onto several host species or benefit from a mixing diet [START_REF] Barbosa | Associational Resistance and Associational Susceptibility: Having Right or Wrong Neighbors[END_REF]). However, the same meta-analysis showed that associational susceptibility is rarely observed in mixtures of phylogenetically distant species. Herbivores foraging for food or oviposition sites rely on physical and chemical cues emitted by their host plants, but are also sensitive to cues emitted by non-host plants (Zhang andSchlyter 2004, Andersson et al. 2009). In mixed forests, attractive host cues may thus be diluted or masked by non-host cues, which can decrease the probability of host plant detection by herbivores [START_REF] Zhang | Olfactory recognition and behavioural avoidance of angiosperm nonhost volatiles by conifer-inhabiting bark beetles[END_REF][START_REF] Bruce | Insect host location: a volatile situation[END_REF][START_REF] Barbosa | Associational Resistance and Associational Susceptibility: Having Right or Wrong Neighbors[END_REF][START_REF] Jactel | Non-host volatiles mediate associational resistance to the pine processionary moth[END_REF], Bruce and Pickett 2011b[START_REF] Castagneyrol | Plant apparency, an overlooked driver of associational resistance to insect herbivory[END_REF], 2014c[START_REF] Verschut | Disentangling associational effects: both resource density and resource frequency affect search behaviour in complex environments[END_REF].

Within a forest stand, an herbivore's decision to exploit a given tree may be affected by the perceived quality of this tree, as well as by its perception of the relative quality of its neighbours. For example, [START_REF] Verschut | Disentangling associational effects: both resource density and resource frequency affect search behaviour in complex environments[END_REF] showed that when they offered the choice between two resource types (odour source), the more apparent resource was more likely to be selected by the insect when associated with a less apparent resource. According to the attractant-decoy hypothesis herbivores could be aggregate on more apparent, more attractive or more palatable plants (Atsatt andO'dowd 1976, Hahn and[START_REF] Hahn | Neighbor palatability generates associational effects by altering herbivore foraging behavior[END_REF]. This finding suggests that in a mixed forest associating two species, both used by a particular herbivore but differing in their perceived quality, trees of the preferred species would be more likely to be attacked than in mixed stands (i.e., associational susceptibility), while trees of the less preferred species would be less likely to be attacked than in monocultures (i.e., associational resistance). In this context, associational resistance to oligophagous herbivore insects would be arguably stronger in mixed forests associating host and non-host species.

We investigated associational effects on the preference and performance of the oak processionary moth (OPM) Thaumetopoea processionea (L.). The OPM is a Lepidopteran pest responsible for major defoliations on deciduous oaks in western and central Europe and in part of the Middle East [START_REF] Groenen | Historical distribution of the oak processionary moth Thaumetopoea processionea in Europe suggests recolonization instead of expansion[END_REF]. It is seen as one of the causes of oak decline throughout Europe. The OPM is considered an oligophagous herbivore species feeding mainly on Quercus and occasionally on other Fagaceae (e.g. Fagus sylvatica L.) or

Betulaceae (e.g. Carpinus betulus L.) species. Its life cycle is synchronized with that of its hosts [START_REF] Wagenhoff | Temporal synchrony of Thaumetopoea processionea egg hatch and Quercus robur budburst[END_REF]; larvae emerge at the time of host budburst and feed on host leaves in spring and early summer. The OPM is a gregarious species that can live in very dense populations, with thousands of individuals concentrating on a single tree. From the fourth out of six instar larvae of OPM produce urticating setae responsible for strong allergic reactions in both humans and animals [START_REF] Maier | The oak processionary caterpillar as the cause of an epidemic airborne disease: survey and analysis[END_REF]. Managing forests to increase oak resistance to OPM attacks is therefore of crucial importance, in terms of both forest and public health. Yet, basic knowledge on OPM biology and ecology is lacking.

Here, we quantified male OPM abundance and defoliation in pure stands of two phylogenetically related oak host species: the pedunculate oak Quercus robur (L.) and the sessile oak Q. petraea (Liebl.) and in mixed stands associating the two oak species or each oak species with another non-host broadleaved species. By doing so, we aimed to determine the relative resistance of the two oak host species to OPM attacks and to test the associational resistance hypothesis. We made the following three hypotheses:

1. OPM attacks are reduced in mixtures of oaks and non-host species because of reduced host availability (e.g. resource concentration hypothesis);

2. Mixtures of the two oak species represent intermediate levels of OPM infestation as compared to pure stands of the two oak species;

3. In mixtures of the two oak species, the preferred oak species experience associational susceptibility, while the less preferred experience associational resistance.

By providing a better understanding of OPM preferences for different forest compositions, our study contributes to the development of preventive pest management strategies.

Materials and methods

We conducted two separate experiments. In the first experiment, we used sex pheromone traps to quantify the abundance of male OPM at the level of forest stands consisting in mixtures of Q. robur and Q. petraea or in the corresponding pure stands. In the second experiment, we monitored oak trees for the presence of OPM colonies and quantified OPM defoliation at the level of individual trees in pure stands and in mixtures associating the two oak species or each oak species with another non-host broadleaved species.

Effect of forest stand composition on OPM moth captures

Experimental design -We used pheromone trapping to compare OPM population density in three forest stand types (henceforth, 'triplets') consisting of Q. robur and Q. petraea pure stands and of mixtures of both oak species. According to the definition of the National Forest Inventory of France, a forest stand is considered a pure stand if the dominant tree species represents more than 75% of the canopy cover. In mixed oak stands, both oak species were present in similar proportions. The trial was carried out in France, in 33 forests spread over 30 administrative departments (Fig. 1A). Forest stands were all in the public domain and managed by the French Forest National Office (ONF).

Stands were at least 500 m apart from each other within a triplet to avoid trapping individuals from the same OPM population. Triplets were separated from each other by several tens of kilometres and covered the entire French territory where the two oak species co-occurs.

In each stand, we used two funnel traps located at least 50 m apart from each other. We baited each trap with one different pheromone lure, obtained from Temmen GmbH Biotechnologie (Germany) (lure 1) and Pherobank (the Netherlands) (lure 2). Each trap was positioned in the upper canopy (10 -15 m) to maximize captures [START_REF] Breuer | Field trials with the synthetic sex pheromone of the oak processionary moth <i>Thaumetopoea processionea<i>[END_REF][START_REF] Williams | Monitoring oak processionary moth Thaumetopoea processionea L. using pheromone traps: the influence of pheromone lure source, trap design and height above the ground on capture rates[END_REF]) at the end of June, before the expected flight period. The two traps were removed at the end of August 2018 and moths were returned to the laboratory for identification and counting.

Statistical analysis -We tested the effect of stand composition, pheromone type and their interaction with Generalised Linear Mixed-effects Models (GLMM with Poisson error distribution) using triplet and stand identity as a random factor. We estimated model fit as the proportion of variance explained by fixed effects (marginal R², R m ²) and fixed plus random effects (conditional R², R c ²) [START_REF] Nakagawa | A general and simple method for obtaining R2 from generalized linear mixed-effects models[END_REF].

Effect of forest stand composition on presence of OPM nests and defoliation

Stand selection -We tested the effect of host species and stand species composition on OPM nest density and defoliation in natural mature forests in North Eastern France. We selected 25 forest stands with Q. robur or Q. petraea as the main species with a sampling design consisting in five replicates of the following composition treatments: Q. robur pure stands, Q.

petraea pure stands, mixtures of Q. robur and Q. petraea, mixtures of Q. robur and another broadleaved species (European beech Fagus sylvatica (L.)), mixtures of Q. petraea and another broadleaved species (hornbeam Carpinus betulus (L.)). Sampled forest stands were all in public forests and managed by ONF. In order to standardize pedoclimatic conditions among composition treatments, all stands were selected in the SER (sylvo-eco region) C30

corresponding to "plains and clay depressions of the North-Eastern France" (Inventaire forestier national 2011).

Field survey -We visited each stand in early summer (June-July 2018) before the production of lammas growth (i.e., right before the second leaf flush have obscured initial defoliation rate). We sampled 10 oaks of each species (i.e., 10 oaks in pure stands, 20 oaks in mixtures, 300 sampled oak trees in total) by walking in the stand along a U-shaped transect and selecting the closest oak tree to the transect line, every 10 m. For each individual oak, we measured the diameter at breast height (dbh). We described the tree species composition around five of the focal oaks per species and per stand (i.e. every 20 m along the transect) by identifying and measuring the dbh of every living tree (with dbh > 22.5 cm) in a buffer of 9 m radius (i.e., in a 254 m² buffer). We calculated the basal area of OPM host trees (i.e., summed basal area of Q. robur and Q. petraea) and the basal area of OPM non-host trees in each of these 254 m² plots.

Two observers examined the crown of each sampled oak tree with binoculars for five minutes; they counted the number of OPM larval nests and estimated their size using surface classes. However, the size of OPM nests appeared to vary strongly among oak trees, particularly for the bigger nests, and it was therefore impossible to provide an accurate and reliable estimate of the number of OPM caterpillars per nest. This information was not retained in further analyses. The number of OPM nests could therefore not be used with confidence as a proxy for OPM abundance. Instead, the response variable used for statistical analysis was made binomial, using presence or absence on the sampled tree. We estimated crown defoliation by assigning each tree to a defoliation class, on a scale of 10 % [START_REF] Eichhorn | Part IV: Visual Assessment of Crown Condition and Damaging Agents[END_REF]. Feeding damage by OPM larvae is easily recognized, as the damaged leaves are left with only the veins (skeletonizer like). Defoliation was found to be greater than 10 % only for oaks with at least one OPM larval nest. We therefore assumed that OPM larvae were the main cause of defoliation in surveyed plots.

Statistical analysis -We tested the effect of stand composition on the presence of OPM nests and OPM defoliation at the level of individual focal oak trees by using (Generalised) Linear

Mixed effect Models (G)LMMs with forest stand as a random factor. We tested the effect of oak species and plot composition on OPM using a two-step approach, for presence/absence data and defoliation separately. First, we modelled OPM response to oak species and forest stand composition described as a three-level factor (pure stand, mixtures of oaks, mixture of oak plus another broadleaved species). Second, we replaced the 'composition' three-level factor by the basal area of host and non-host species in plots, to address more explicitly the effect of host density vs host frequency on herbivores. In each model, we used the dbh of focal oak trees as a covariate. For each model, we simplified the initial full model by sequentially removing non-significant fixed effects, starting with the least significant, and estimated model coefficient parameters for the simplified model. We analysed OPM presence/absence data using a GLMM with binomial error distribution. We analysed defoliation with a LMM, using a square root transformation of the response variable to satisfy model assumptions. Defoliation data were analysed on the subset of trees that had been attacked by OPM (i.e., trees with at least one OPM nest, n = 195).

All analyses and figures were performed in R v3.5.1 (R Core Team 2018) with the following packages : multcomp, car, lme4, ggplot2, cowplot, lmerTest, sciplot, MuMIn, dplyr, DHARMa, doBy and tidyr (Hothorn et al. 2008[START_REF] Fox | An {R} Companion to Applied Regression[END_REF][START_REF] Bates | Fitting Linear Mixed-Effects Models using lme4[END_REF], Wickham 2016a[START_REF] Kuznetsova | lmerTest Package: Tests in Linear Mixed Effects Models[END_REF][START_REF] Morales | with code developed by the R Development Core Team, with general advice from the R-help listserv community, and especially Duncan Murdoch[END_REF][START_REF] Wilke | cowplot: Streamlined Plot Theme and Plot Annotations for "ggplot2[END_REF], Barton 2018a[START_REF] Hartig | DHARMa: Residual Diagnostics for Hierarchical (Multi-Level / Mixed) Regression Models[END_REF][START_REF] Højsgaard | doBy: Groupwise Statistics, LSmeans, Linear Contrasts, Utilities[END_REF], Wickham et al. 2018, Wickham and Henry 2018).

Results

Effect of forest stand composition on OPM moth captures

We caught a total of 2,756 OPM males in the 185 pheromone traps. The mean number of captures per trap (± se) was 15 ± 3, ranging from 0 to 388 (median: 2). Captures were more concentrated in North Eastern France (Fig. 3A). Although most of variance was explained by site location (R m ² = 0.02; R c ² = 0.79), there was a significant effect of forest stand species composition on male captures (² = 8.27, Df = 2, P = 0.016) (Fig. 3). Specifically we captured, on average, 1.7 times more OPM in pure Q. petraea stands (15.28 ± 3.44) than in pure Q. robur stands (8.87 ± 2.36). We also found a significant interaction between stand composition and pheromone lure type (² = 38.12, Df = 2, P < 0.001). The number of captures differed between pheromone types only in pure Q. petraea stands (the number of captures was 2.0 times higher with the lure 1) and in mixed stands (the number of captures was 1.3 times higher with the lure 2) (Fig. 3B).

Effect of forest stand composition on presence of OPM nests and defoliation

Among the 300 oak trees sampled in our survey, 195 had at least one OPM nest (i.e., 65 %).

Neither oak species, nor stand composition, nor basal area of neighbours (host or non-host), nor the dbh of focal oaks had a statistically clear effect on the probability of an oak being attacked by OPM ( (B) Effect of stand species composition on captures of male OPM (mean  se). Small letters indicate significant differences in number of male OPM captures between the three composition treatments with the lure 1. Capital letters indicate significant differences in number of male OPM captures between the three composition treatments with the lure 2. Asterisks indicate significant differences in number of male OPM captures between the two lures for each composition treatments (NS = Non-Significant). Q. robur = Q. robur pure stands, Q. petraea + Q. robur = mixture of Q. petraea and Q. robur and Q. petraea = Q. petraea pure stands.

(A) (B)

Discussion

Our results showed that male OPM abundance and attack rate in oaks stands are influenced by host species identity and tree species composition. Specifically, we found that the sessile oak Q. petraea was consistently more attractive to and more defoliated by OPM than the pedunculate oak Q. robur. We also observed that oaks were generally less attacked in mixtures than in pure stands, in particular mixtures associating oaks (OPM hosts) with other (non-host) broadleaved species. Our results are therefore consistent with the finding that mixed forests can be more resistant to specialist pest insects than pure stands [START_REF] Jactel | Tree diversity reduces herbivory by forest insects[END_REF], Castagneyrol et al. 2014a[START_REF] Iason | Community-based biotic effects as determinants of tree resistance to pests and pathogens[END_REF]), but that tree resistance depends primarily on the species composition of mixed forests.

Quercus petraea was the most attractive and defoliated oak species -Captures were more concentrated in North Eastern France following the natural distribution of the OPM [START_REF] Groenen | Historical distribution of the oak processionary moth Thaumetopoea processionea in Europe suggests recolonization instead of expansion[END_REF]. Pheromone traps caught more male moths in Q. petraea than in Q. robur stands, regardless of the pheromone lure. This could be explained by two non-mutually exclusive reasons: i) traps equally attract male OPM in Q. petraea and Q. robur stands but OPM are more abundant in Q. petraea stands; ii) male OPM are equally abundant in both types of stands but trap attraction is stronger in Q. petraea stands.

Plant volatile organic compounds (VOCs) play a key role in attracting insects [START_REF] Reddy | Interactions of insect pheromones and plant semiochemicals[END_REF], Bruce and Pickett 2011b[START_REF] Xu | Plant Volatiles as Mate-Finding Cues for Insects[END_REF] to host plants or repelling insects from non-host plants. VOCs are particularly effective in specialist herbivore species foraging in a highly complex environment mixing very different stimuli [START_REF] Schröder | The Relevance of Background Odor in Resource Location by Insects: A Behavioral Approach[END_REF]. Importantly, plant VOCs have also been shown to act synergistically with sex pheromones to enhance male moth attraction to female pheromones [START_REF] Landolt | Host plant influences on sex pheromone behavior of phytophagous insects[END_REF][START_REF] Reddy | Interactions of insect pheromones and plant semiochemicals[END_REF], Bruce and Pickett 2011b[START_REF] Xu | Plant Volatiles as Mate-Finding Cues for Insects[END_REF]). Thus, we can speculate that Q. petraea released a specific blend of VOCs that improved the response of male OPMs to pheromone lures. The difference in the number of captures among the different stand compositions could be due to Q. petraea releasing more attractive VOCs (in combination with sex pheromones) than Q. robur. Although we cannot exclude the possibility that Q. robur emitted some repellent VOCs, this is unlikely as Q. robur is also a host species for OPM. In support of this view, [START_REF] Williams | The influence of tree species and edge effects on pheromone trap catches of oak processionary moth Thaumetopoea processionea (L.) in the[END_REF] showed that pheromone traps installed in the canopy of Q. robur trees caught more OPM males than pheromone traps installed on six other tree species, including three other deciduous oaks (Q. cerris, Q.

palustris and Q. rubra). These authors suggested that Q. robur trees emitted more attractive isoprenoids than other species. However, their survey did not include Q. petraea.

Surprisingly, in our study, the two pheromone lures seemed to operate differently in stands with different species compositions, at least in respect to their attractivity in presence of Q. petraea. It might be due to complex interactions between the molecules released by the two pheromone lures and the species-specific VOCs emitted by oak trees, or to different environmental conditions in oaks stands differing in their species composition (e.g. microclimate), interfering with the volatility of pheromone attractants. More controlled experiments, using for example olfactometers, would be needed to better understand OPM behavioural response to combinations of pheromone components and oak VOCs.

We cannot either exclude that Q. petraea and Q. robur VOCs were of equal attractiveness and that different levels of capture accounted for different densities of male OPM in the two types of stands. However, a recent study by [START_REF] Straw | The relationship between pheromone trap catch and local population density of the oak processionary moth Thaumetopoea processionea (Lepidoptera: Thaumetopoeidae)[END_REF] reported a positive relationship between OPM population density and the number of males caught in pheromone traps.

Likewise, in another processionary moth species, Thaumetopoea pityocampa (Denis & Schiffermüller), a significant and positive correlation between the abundance of the moth and the level of capture in pheromone traps was found [START_REF] Jactel | Population monitoring of the pine processionary moth (Lepidoptera: Thaumetopoeidae) with pheromone-baited traps[END_REF]). In our study, we used only two traps in each forest stand, which might not be enough to estimate moth density properly. Patrolling moths, originating from neighbouring oak stands, may have been caught

in our traps, leading to overestimation of local density. However, estimating total OPM population density was not the goal of the present study and despite this limitation significant effect of stand composition on male OPM captures remains biologically meaningful.

The greater number of male captures in Q. petraea stands observed in France is consistent with the greater defoliation of Q. petraea trees observed in the current outbreak area, in the North East of the country. Establishing a causal relationship between both observations requires assuming the factors affecting stand selection by OPM males act in the same way on females. Although we are no aware of any study having confirmed this assumption in OPM,

previous study using other model species found that host attractive cues act consistently upon both males and females (García-Robledo and Horvitz 2012, [START_REF] Anderson | Larval host plant experience modulates both mate finding and oviposition choice in a moth[END_REF], Thöming et al. 2013), which facilitates mating. We can therefore speculate that Q. petraea stands were equally attractive to males and females OPM. This would be consistent with the "mother knows best" hypothesis [START_REF] Jaenike | On optimal oviposition behavior in phytophagous insects[END_REF][START_REF] Gripenberg | A meta-analysis of preference-performance relationships in phytophagous insects[END_REF][START_REF] García-Robledo | Parent-offspring conflicts, "optimal bad motherhood" and the "mother knows best" principles in insect herbivores colonizing novel host plants[END_REF] that states that female insectsand in particular specialist herbivore speciestend to lay eggs on plants that confer the best performance to their offspring, thus maximising their own fitness. We caution that the presence of one OPM nest on a tree and subsequent defoliation only indicate that eggs were laid on this tree, but that an absence of nest does not imply that no eggs were laid. The observation that Q. robur was less defoliated than Q. petraea may thus have been driven by differential attractiveness to OPM females, or by differences in oak traits that caused greater early mortality of OPM larvae hatched on Q. robur or a lower consumption on Q. robur. Although, to our knowledge, there is no comparative study on the leaf traits of the two species of oak, several studies have shown that the wood of Q. petraea is characterised by higher levels of volatile compound (such as lactone), but also by lower amounts of ellagitannins compared to Q. robur [START_REF] Mosedale | Variation of heartwood phenolics and oak lactones between the species and phenological types of Quercus petraea and Q. robur[END_REF][START_REF] Feuillat | Variation in the concentration of ellagitannins and cis-and trans-β-methyl-γ-octalactone extracted from oak wood (Quercus robur L., Quercus petraea Liebl.) under model wine cask conditions[END_REF][START_REF] Mosedale | Variability of wood extractives among Quercus robur and Quercus petraea trees from mixed stands and their relation to wood anatomy and leaf morphology[END_REF][START_REF] Prida | Effect of species and ecological conditions on ellagitannin content in oak wood from an even-aged and mixed stand of Quercus robur L. and Quercus petraea Liebl[END_REF], 2007), which is consistent with our results. The difference in defoliation between oak species could be due to a difference in leaf quality leading to a change in consumption rate of OPM larvae. Several studies showed a change of consumption rate with leaf quality of the host [START_REF] Lazarevic | Adaptation of the gypsy moth to an unsuitable host plant[END_REF][START_REF] Foss | Species-specific differences in oak foliage affect preference and performance of gypsy moth caterpillars[END_REF][START_REF] Barbehenn | Hydrolyzable tannins as "quantitative defenses": Limited impact against Lymantria dispar caterpillars on hybrid poplar[END_REF][START_REF] Milanović | Preference and performance of the larvae of Lymantria dispar (Lepidoptera: Lymantriidae) on three species of European oaks[END_REF][START_REF] Damestoy | Oak genotype and phenolic compounds differently affect the performance of two insect herbivores with contrasting diet breadth[END_REF]). However, it remains uncertain whether differences in defoliation among host species result from differences in the abundance of OPM larvae or from differences in their feeding behaviour, whereby larvae could compensate for lower nutritional quality by consuming more leaf material [START_REF] Damestoy | Oak genotype and phenolic compounds differently affect the performance of two insect herbivores with contrasting diet breadth[END_REF]). An unambiguous explanation of the patterns that we revealed would therefore require combining field observations with controlled feeding trials.

Male OPMs captures and defoliation were reduced in mixed stands. -We captured more male OPMs in Q. petraea stands than in mixtures of Q. petraea and Q. robur (with lure 1). It has been shown in other systems that non-host or less preferred host plants can emit repellent VOCs, and have an antagonist effect on male response to pheromone, thus contributing to associational resistance in mixtures [START_REF] Unbehend | Pheromonal Divergence Between Two Strains of Spodoptera frugiperda[END_REF], Thöming et al. 2013[START_REF] Binyameen | Modulation of Reproductive Behaviors by Non-Host Volatiles in the Polyphagous Egyptian Cotton Leafworm, Spodoptera littoralis[END_REF]. Had this been the case for Q. robur, the number of captures would have arguably been lower in Q. robur stands than in mixed stands associating Q. robur with Q. petraea. Yet, the number of males caught in mixtures was comparable with the number of males caught in Q. robur stands. Although our experimental setup does not allow us to address the underlying mechanisms, we interpret the reduced number of OPM captures in mixed Q. robur and Q.

petraea stands as resulting from the dilution of attractive Q. petraea VOCs. We did not try to trap male OPM in mixtures of oaks and non-host species. This would be a more direct test of hypothesis for interpreting the role of Q. petraea VOCs dilution or masking in OPM sex attraction and testing the associational resistance hypothesis, but previous experimental studies conducted on the pine processionary moth showed that the presence of non-host trees in mixed stands could protect the host trees from pest attacks. In this model, non-host trees were found to disrupt host detection by the pine processionary moth by interfering with the visual and chemical cues emitted by the host tree [START_REF] Jactel | Non-host volatiles mediate associational resistance to the pine processionary moth[END_REF][START_REF] Dulaurent | Hide and seek in forests: Colonization by the pine processionary moth is impeded by the presence of nonhost trees[END_REF][START_REF] Castagneyrol | Plant apparency, an overlooked driver of associational resistance to insect herbivory[END_REF], 2014c). The same mechanisms may have operated in the OPM-oak model systems.

OPM defoliation was reduced when oaks were associated with other less preferred species, like beech and hornbeam, regardless of oak species. This pattern is consistent with several studies that reported reduced herbivore défoliation in mixtures than in pure stands. In particular, [START_REF] Guyot | Tree diversity reduces pest damage in mature forests across Europe[END_REF] found similar associational resistance for oak species in mixed broadleaved forests. Reduced herbivory in mixtures is generally explained by the interplay between lower host availability (i.e., reduced concentration, frequency; [START_REF] Root | Organization of a Plant-Arthropod Association in Simple and Diverse Habitats: The Fauna of Collards (Brassica Oleracea)[END_REF][START_REF] Hambäck | Patch area, population density and the scaling of migration rates: the resource concentration hypothesis revisited[END_REF][START_REF] Hambäck | Effects of plant neighborhoods on plant-herbivore interactions: resource dilution and associational effects[END_REF] and "true" associational effects, whereby nonhosts mask host trees or trigger defences [START_REF] Jactel | Tree diversity reduces herbivory by forest insects[END_REF][START_REF] Barbosa | Associational Resistance and Associational Susceptibility: Having Right or Wrong Neighbors[END_REF][START_REF] Jactel | Non-host volatiles mediate associational resistance to the pine processionary moth[END_REF][START_REF] Castagneyrol | Plant apparency, an overlooked driver of associational resistance to insect herbivory[END_REF][START_REF] Alalouni | Does insect herbivory on oak depend on the diversity of tree stands?[END_REF], Jactel et al. 2017). Yet, neither the basal area of host trees nor the basal area of non-host trees around focal oaks explained the amount of OPM defoliation in mixtures. These findings suggest that the lower defoliation observed in mixed stands was not strongly driven by OPM resource concentration;

otherwise we would have detected a positive effect of oak basal area on defoliation. Likewise, if masking or repellent cues are emitted by non-hosts [START_REF] Zhang | Olfactory recognition and behavioural avoidance of angiosperm nonhost volatiles by conifer-inhabiting bark beetles[END_REF][START_REF] Bruce | Insect host location: a volatile situation[END_REF][START_REF] Togni | Odour masking of tomato volatiles by coriander volatiles in host plant selection of Bemisia tabaci biotype B[END_REF][START_REF] Jactel | Non-host volatiles mediate associational resistance to the pine processionary moth[END_REF], Bruce and Pickett 2011b), their effect would have increased with their basal area, which was not the case. At the scale at which we defined species diversity, associational effect might have been mediated by changes in leaf traits relevant to OPM consumption [START_REF] Damestoy | Oak genotype and phenolic compounds differently affect the performance of two insect herbivores with contrasting diet breadth[END_REF], resulting from several mechanismsincluding competition for resources or the emission of volatile compounds by neighbouring plants that change some physical and/or chemical traits of focal plant [START_REF] Arimura | Plant-plant interactions mediated by volatiles emitted from plants infested by spider mites[END_REF][START_REF] Turlings | Exploiting scents of distress: the prospect of manipulating herbivore-induced plant odours to enhance the control of agricultural pests[END_REF], Agrawal et al. 2006[START_REF] Barbosa | Associational Resistance and Associational Susceptibility: Having Right or Wrong Neighbors[END_REF][START_REF] Ballaré | Light Regulation of Plant Defense[END_REF], Kos et al. 2015a, Castagneyrol et al. 2017). Finally, it is also possible that OPM enemies were more abundant and more diverse in mixed stands, thus exerting a stronger top-down control on OPM larvae [START_REF] Root | Organization of a Plant-Arthropod Association in Simple and Diverse Habitats: The Fauna of Collards (Brassica Oleracea)[END_REF][START_REF] Wilby | Natural enemy diversity and pest control: patterns of pest emergence with agricultural intensification[END_REF][START_REF] Sobek | Sapling herbivory, invertebrate herbivores and predators across a natural tree diversity gradient in Germany's largest connected deciduous forest[END_REF][START_REF] Straub | Plant diversity increases herbivore movement and vulnerability to predation[END_REF], Fernandez-Conradi et al. 2018a). Birds and bats are efficient pest regulator in forest and contribute to reduce leaf damages [START_REF] Böhm | Top-Down Control of Herbivory by Birds and Bats in the Canopy of Temperate Broad-Leaved Oaks (Quercus robur)[END_REF]. For instance, [START_REF] Charbonnier | Deciduous trees increase bat diversity at stand and landscape scales in mosaic pine plantations[END_REF] showed that forest composition and in particular, the presence of broad-leaved trees in pine plantation increased activity and modified the community composition of bats which are predators of the pine processionary moth, Thaumetopoea processionea [START_REF] Charbonnier | Numerical and Functional Responses of Forest Bats to a Major Insect Pest in Pine Plantations[END_REF].

In conclusion, our study showed that Q. petraea was more attractive and more defoliated by OPM larvae than Q. robur, suggesting a higher susceptibility of Q. petraea to OPM attacks or a higher density of OPM population resulting from a higher attraction of adults or a better breeding substrate for larvae. In addition and consistent with the literature [START_REF] Jactel | Tree diversity reduces herbivory by forest insects[END_REF][START_REF] Barbosa | Associational Resistance and Associational Susceptibility: Having Right or Wrong Neighbors[END_REF][START_REF] Jactel | Non-host volatiles mediate associational resistance to the pine processionary moth[END_REF][START_REF] Castagneyrol | Plant apparency, an overlooked driver of associational resistance to insect herbivory[END_REF][START_REF] Alalouni | Does insect herbivory on oak depend on the diversity of tree stands?[END_REF]) the lower number of OPM captures and lower defoliation rates in mixed stands suggest that greater resistance of oak to the OPM could be achieved by favouring the presence of non-host species in association with oaks. These results provided a better understanding of the OPM behaviour regarding host species and stand composition. In terms of management, our results call for enhancing tree species diversity to increase the resistance of oak forests to OPM. Further research should be carried out to verify whether such associational resistance process would be more effective by mixing oaks with phylogenetically very distant non-host species, such as conifers. Chapitre 2

Abstract:

The presence of heterospecific neighbours can affect damage caused by pest insects on focal plants. However, how plant neighbours influence herbivore performance is poorly understood. We tested the independent and interactive effects of tree species identity and tree neighbour type (conspecific vs. heterospecific) on the performance of a major oak pest, the oak processionary moth larvae (OPM, Thaumetopoea processionea) fed on Quercus robur and Q. petraea. We performed a factorial greenhouse experiment in which we grew two oak saplings per pot, either from the same species or from both species. We quantified growth and mortality of OPM larvae, leaf phenolic compounds, C:N ratio and bud phenology. OPM larvae performed significantly better on Q. petraea than on Q. robur, regardless of plant neighbour type. Phenolic compounds and C:N, but not phenology, differed between oak species and neighbour species identity. Only bud phenology had a significant effect on OPM performance, which was better when young larvae had access to recently unfolded leaves, regardless of oak species and neighbour identity. Although oak neighbour identity altered the expression of leaf traits, this effect had no measurable consequences on OPM performance.

However, further studies should consider the effect of oak species neighbour on OPM preferences for either Q. robur or Q. petraea, in pure and mixed stands, before translating current results into recommendations for forest management.

Introduction

Plants are embedded in heterogeneous environments where the identity, density and diversity of neighbouring plants can strongly influence their interactions with insect herbivores. Associational resistance theory predicts that plants are less prone to damage by insect herbivores when surrounded by heterospecific neighbours [START_REF] Barbosa | Associational Resistance and Associational Susceptibility: Having Right or Wrong Neighbors[END_REF]).

Associational resistance provided by neighbours is generally attributed to reduced herbivore accessibility to host plants, or to physical or chemical disruption of host searching behaviour of insect herbivores foraging for food or egg laying sites [START_REF] Zhang | Olfactory recognition and behavioural avoidance of angiosperm nonhost volatiles by conifer-inhabiting bark beetles[END_REF][START_REF] Bruce | Insect host location: a volatile situation[END_REF][START_REF] Barbosa | Associational Resistance and Associational Susceptibility: Having Right or Wrong Neighbors[END_REF][START_REF] Jactel | Non-host volatiles mediate associational resistance to the pine processionary moth[END_REF], Bruce and Pickett 2011b[START_REF] Castagneyrol | Plant apparency, an overlooked driver of associational resistance to insect herbivory[END_REF], 2014d[START_REF] Verschut | Disentangling associational effects: both resource density and resource frequency affect search behaviour in complex environments[END_REF]. Accordingly, most studies investigating associational resistance have addressed plant colonization by herbivores or documented damage in various neighbourhood contexts (reviewed by Moreira et al. 2016a). However, plant neighbourhood might also influence performance (e.g. growth rate, survival) of insect herbivores once established on the host plant (e.g. Castagneyrol et al. 2018).

Herbivore performance is mainly driven by host plant traits, and particularly by those determining its nutritional quality. For instance, nitrogen is an important limiting factor for phytophagous insects [START_REF] Mattson | Herbivory in realtion to plant nitrogen content ![END_REF], and low nitrogen content (or high C:N ratio) has been commonly associated with low nutritional plant quality and reduced herbivore performance [START_REF] Mattson | Herbivory in realtion to plant nitrogen content ![END_REF][START_REF] White | The abundance of invertebrate herbivores in relation to the availability of nitrogen in stressed food plants[END_REF]). In addition, secondary metabolites (e.g. phenolic compounds) are commonly considered as effective plant defences against many leaf-feeding herbivores in several tree species [START_REF] Feeny | Plant apparency and chemical defense[END_REF][START_REF] Lill | The effects of leaf quality on herbivore performance and attack from natural enemies[END_REF][START_REF] Forkner | Feeny revisited: condensed tannins as antiherbivore defences in leaf chewing herbivore communitites of Quercus[END_REF]). These compounds are often toxic (Salminen andKaronen 2011, Mithöfer and[START_REF] Mithöfer | Plant Defense Against Herbivores: Chemical Aspects[END_REF] and some have been shown to reduce digestibility in herbivores, hence potentially reducing herbivore damage [START_REF] Feeny | Seasonal changes in oak leaf tannins and nutrients as a cause of spring feeding by winter moth caterpillars[END_REF][START_REF] Roslin | Specialization pays off : contrasting effects of two types of tannins on oak specialist and generalist moth species[END_REF], Abdala-Roberts et al. 2016, Moreira et al. 2018b). For instance, condensed and hydrolysable tannins and flavonoids can reduce plant digestibility by binding digestive enzymes and altering herbivores' digestive tissues through the production of reactive oxygen species [START_REF] Barbehenn | Hydrolyzable tannins as "quantitative defenses": Limited impact against Lymantria dispar caterpillars on hybrid poplar[END_REF][START_REF] Barbehenn | Tannins in plant-herbivore interactions[END_REF][START_REF] Falcone Ferreyra | Flavonoids: biosynthesis, biological functions, and biotechnological applications[END_REF]. Similarly, lignins act as toxic compounds and contribute to increased tissue (leaf or shoot) toughness (Bidlack et al. 1992, Bonawitz and Chapple 2010), a common physical defensive trait [START_REF] Clissold | Gross vs. net income: How plant toughness affects performance of an insect herbivore[END_REF][START_REF] Pearse | The role of leaf defensive traits in oaks on the preference and performance of a polyphagous herbivore, Orgyia vetusta[END_REF][START_REF] Caldwell | Which leaf mechanical traits correlate with insect herbivory among feeding guilds?[END_REF] There is increasing evidence that the identity and diversity of neighbouring plants can indirectly affect herbivore performance on focal plants by modifying plant nutritional and defensive traits. These indirect, trait mediated effects of plant neighbourhood can result from several mechanisms such as competition for resources (e.g. light, water, nutrients), emission of volatile organic compounds by neighbouring plants or plant-soil feedbacks [START_REF] Arimura | Plant-plant interactions mediated by volatiles emitted from plants infested by spider mites[END_REF][START_REF] Turlings | Exploiting scents of distress: the prospect of manipulating herbivore-induced plant odours to enhance the control of agricultural pests[END_REF], Agrawal et al. 2006[START_REF] Barbosa | Associational Resistance and Associational Susceptibility: Having Right or Wrong Neighbors[END_REF][START_REF] Ballaré | Light Regulation of Plant Defense[END_REF], Kos et al. 2015a, 2015b, 2015c, Castagneyrol et al. 2017). For instance, plants growing under the shade of their neighbours tend to be more favourable for herbivores since the allocation of resources to plant defences, such as phenolic compounds and terpenes, is markedly lower [START_REF] Dudt | The influence of light and nutrients on foliar phenolics and insect herbivory[END_REF]Shure 1994, Ballaré 2014). Moreover, the emission of volatile organic compounds by neighbouring plants after a herbivore attack might induce the expression of defensive traits in focal undamaged plants [START_REF] Arimura | Plant-plant interactions mediated by volatiles emitted from plants infested by spider mites[END_REF][START_REF] Turlings | Exploiting scents of distress: the prospect of manipulating herbivore-induced plant odours to enhance the control of agricultural pests[END_REF][START_REF] Barbosa | Associational Resistance and Associational Susceptibility: Having Right or Wrong Neighbors[END_REF][START_REF] Scala | Green leaf volatiles: A plant's multifunctional weapon against herbivores and pathogens[END_REF], Despite these evidences, how neighbour-mediated changes in plant traits influence herbivore performance remains poorly studied and this has precluded a better understanding of the mechanisms underlying associational resistance to insect herbivores.

In the present study, we tested for independent and interactive effects of plant species identity and plant neighbourhood type (conspecific vs. heterospecific) on insect herbivore performance, leaf nutritional and defensive traits and plant phenology of two oak species (pedunculate oak Quercus robur L and sessile oak Q. petraea Liebl.). To this end, we performed a factorial greenhouse experiment in which we grew pedunculate and sessile oak saplings in pots with either conspecific or heterospecific neighbours and quantified growth and mortality of oak processionary moth (OPM, Thaumetopoea processionea L., Lepidoptera) larvae, leaf chemical (phenolic compounds) and nutritional (leaf C:N ratio) traits and plant phenology (bud developmental stage). Overall, this study builds towards a better understanding of the effects of plant neighbourhood composition on insect herbivory, plant defensive traits and the mechanisms underpinning such effects.

Materials and methods

Natural history -The oak processionary moth (OPM), Thaumetopoea processionea L., is a pest responsible for major defoliations on deciduous oaks in western and central Europe and part of the Middle East [START_REF] Groenen | Historical distribution of the oak processionary moth Thaumetopoea processionea in Europe suggests recolonization instead of expansion[END_REF]). The OPM is considered an oligophagous herbivore species feeding mainly on Quercus and occasionally on other Fagaceae (e.g. beech) or Betulaceae (e.g. hornbeam) species [START_REF] Stigter | Thaumetopoea processionea in the Netherlands: present status and management perspectives (Lepidoptera: Notodontidae)[END_REF]. Its life cycle is synchronized with their host trees [START_REF] Wagenhoff | Temporal synchrony of Thaumetopoea processionea egg hatch and Quercus robur budburst[END_REF]; larvae emerge at the time of host budburst and feed on flushing leaves in spring and early summer. OPM caterpillars are gregarious, with sometimes thousands of individuals concentrating on a single tree. From the fourth to the sixth instar, OPM larvae produce urticating setae which are responsible for severe allergic reactions in both humans and animals [START_REF] Maier | The oak processionary caterpillar as the cause of an epidemic airborne disease: survey and analysis[END_REF] (iv) one egg mass on one Q. petraea in a pot with Q. robur kept intact. We used 30 × 15 cm nylon bags with a mesh size of 0.05 × 0.05 cm to prevent the movement of larvae from treated saplings to their intact neighbours. In case of food shortage, larvae and bags were moved to another neighbouring branch of the same sapling. OPM larval growth and mortality -When the first colonies reached the third larval instar (i.e., 26 days after installing egg masses on trees 53 colonies out of 100 had reached the third larval instar), we removed larvae from trees to avoid any risk of urtications (starting at fourth instar). We counted the number of living larvae and the initial larval density (i.e. number of empty eggs from which larvae emerged) in order to estimate mortality rate (i.e., (Number of hatched eggs -Number of living larvae)/ Number of days after hatching, day -1 ). We kept living larvae for 24 hours without food and then weighed them to the closest 10 µg (Balances NewClassic MS semi-micro), giving weight at day j (w j ). In a preliminary trial, we weighted 30 samples of 10 neonate larvae (L1) and found that the average weight (± se) of a neonate was 0.225 ± 0.004 mg. Because of the small variability in neonate weight and because this value was much lower than the mean of w j (5.24 ± 0.50 mg), we considered the neonate weight negligible and thus defined larvae growth rate as GR = w j / Number of days after hatching, mg.day -1 .

Leaf chemical traits -We measured leaf C:N and phenolic content on 5-10 fully expanded intact leaves collected on focal oak trees with larvae. Because larvae emerged before oak budburst, we were able to collect leaves only at the end of the experiment. We were therefore not able to estimate leaf traits before larvae started to feed and potentially induced systemic defences. We considered that phenolic content measured on intact leaves represented constitutive defences before attacks occur (Abdala-Roberts et al. 2016), but we acknowledge that the amount of phenolics may partly reflect systemic induction of oak defences after attacks began. We dried leaves for 48h at 45°C directly after leaf collection and ground dried material to fine powder before further chemical analyses.

First, we extracted phenolic compounds using 20 mg of dry plant tissue with 1 ml of 70% methanol in an ultrasonic bath for 20 min, followed by centrifugation (Moreira et al. 2014b).

We diluted methanolic extracts (1:4 vol:vol) with an extraction solvent and transferred them to chromatographic vials to perform chromatographic analyses. We carried out chromatographic analyses with an Ultra-High-Performance Liquid-Chromatograph (UHPLC Nexera LC-30AD; Shimadzu Corporation, Kyoto, Japan) equipped with a Nexera SIL-30AC

injector and one SPD-M20A UV/VIS photodiode array detector. The UHPLC column was a Kinetex™ 2.6 μm C18 82-102 Å, LC Column 100 × 4.6 mm, protected with a C18 guard cartridge. The flow rate was 0.4 ml min -1 and the oven temperature was set to 25 °C. The mobile phase consisted of two solvents: water-formic acid (0.05%) (A) and acetonitrileformic acid (0.05%) (B), starting with 5% B and using a gradient to obtain 30% B at 4 min, 60% B at 10 min, 80% B at 13 min and 100% B at 15min. The injection volume was 3 μl. We recorded chromatograms at 330 nm and processed data with the LabSolutions software (Shimadzu). We identified four groups of phenolic compounds: flavonoids, ellagitannins and gallic acid derivates ("hydrolysable tannins" hereafter), proanthocyanidins ("condensed tannins" hereafter) and.hydroxycinnamic acid precursors to lignins ("lignins" hereafter). We quantified flavonoids as rutin equivalents, condensed tannins as catechin equivalents, hydrolysable tannins as gallic acid equivalents and lignins as ferulic acid equivalents. We achieved the quantification of these phenolic compounds by external calibration using calibration curves based on chemical equivalent at 0.25, 0.5, 1 and 2 μg ml -1 . Second, we quantified leaf C:N ratio with a gas chromatography in an automatic elemental analyser (FlashEA 1112; Thermo Fisher Scientific Inc.) using 0.006 g of dried leaf powder.

Statistical analysis

Effect of focal and neighbour tree species identity on OPM performance and leaf traits -We tested the effect of focal oak species identity (Focal: Q. petraea vs. Q. robur), neighbour species identity (Neighbour: Conspecific vs. Heterospecific) and their interaction (all fixed factors) on OPM mortality (day -1 , square-root transformed) and growth rates (mg day -1 , logtransformed), as well as on leaf chemical and nutritional traits and plant phenology. We used initial larval density as covariate in models predicting OPM mortality and growth rate. We used Linear Model with Gaussian error distribution and identity link, except for bud developmental stage (scored as and ordinal variable) where we used an Ordinal Logistic

Regression. For the growth rate, we performed the tests only for plants with larvae still alive at the end of the experiment (i.e. n = 86 plants).

Leaf traits associated with effects of focal and neighbour tree species identity on OPM performance -Based on previous analyses, if focal or neighbour tree identities had significant effects on herbivore performance (growth and mortality), we ran the model again (with the same main fixed effects and their interaction), while additionally including leaf traits as covariates potentially associated with effects on leaf herbivory ("mechanistic model"; Abdala-Roberts et al. 2016; Moreira et al. 2017). Rather than including all plant traits as covariates in the herbivore performance models and to reduce issues arising from collinearity among predictors, we only retained traits which were significantly associated with herbivore performance. For this, we ran a separate multiple regression including only leaf traits (concentration of the four phenolic groups, bud development stage and C:N ratio) as predictors of herbivore performance (two models, one for mortality and one for growth). We computed variance inflation factors (VIFs) and verified that correlations among traits (in particular among the concentration of the four types of phenolic compounds) did not inflate coefficient parameter standard error estimates. All VIFs were < 5. However, because there is no clear threshold above which collinearity should be seen as a serious issue (O'brien 2007),

we also ran separate models with the concentration of only one type of phenolic compound at a time. The results were unchanged so we retained the complete model in the final analysis.

Then we ran a second set of models including the above selected traits as co-variates in models testing the effect of Focal and Neighbour species identity on OPM performance. We used sequential decomposition of variance to test the effect of each predictor. We fitted leaf traits before the effect of Focal and Neighbour species identity. With this approach, if leaf traits mediate effects of focal species or neighbourhood on herbivore performance in these mechanistic models, then the significant main effects in the prior models (without covariates)

should turn non-significant after including the covariates.

All analyses and figures were performed in R v3.

Results

Effect of focal and neighbour tree species identity on OPM performance, leaf traits and plant phenology

OPM performance significantly differed between focal oak species (Table 1). In particular, OPM larval mortality was on average twice higher and OPM growth rate was on average twice lower on Q. robur than on Q. petraea (Fig. 5A, 5B), indicating that Q. petraea is a more suitable host for this herbivore species. We did not find any significant effects of neighbour tree species identity nor Focal × Neighbour interaction on OPM larval mortality or growth rate (Fig. 5, Table 2).

Some leaf traits significantly differed among focal oak species (Table 2). In particular, concentrations of hydrolysable tannins and flavonoids were on average 1.7-fold higher and lower (respectively) in Q. petraea than in Q. robur (Fig. 6). We did not find significant effects of neighbour tree species identity on leaf traits (Table 2). However, the Focal × Neighbour interaction significantly affected leaf flavonoid concentration and C:N (Table 2). Specifically, the concentration of flavonoids was on average 1.5 times higher in Q. robur in presence of heterospecific neighbours, whereas the C:N ratio was on average 1.2 time higher in Q.

petraea in presence of conspecific neighbours (Fig. 6). Phenology was not significantly affected by focal or neighbour species (Table 2).

Leaf traits associated with effects of focal and neighbour tree species identity on OPM performance

Bud phenology, but not C:N nor concentrations of any type of phenolic compounds, had a significant effect on OPM growth and mortality rate (Table 3). OPM larvae performed better (lower mortality and better growth) when neonate had access to open buds with expanding leaves (Fig. 7AB).

The significant effect of focal oak species identity on OPM performance (growth and mortality) remained significant after including bud phenology as covariate (Table 4), indicating that phenology did not determine all the observed differences in herbivore performance between the two oak species.

Effects of oak species identity on OPM performance and leaf traits

Our results showed tree species-specific differences in OPM performance. In particular, OPM grew faster and suffered lower mortality rates when feeding on Q. petraea in comparison with Q. robur. Noteworthy, although our study was not designed to survey OPM development time, a greater proportion of Q. petraea than Q. robur seemed to have OPM larvae that had reached the third instar at the end of our experiment (26 days) (Fig. 8). These findings are consistent with two previous studies from our group. In a field experiment with mature oak We also found species-specific differences in oak leaf traits. Contents of hydrolysable tannins were higher in Q. petraea whereas flavonoids were higher in Q. robur. These two oak species have previously been reported to differ in leaf and wood chemistry [START_REF] Thomas | Effects of excess nitrogen and drought on the foliar concentrations of allelochemicals in young oaks (Quercus robur L. and Q. petraea [Matt.] Liebl.)[END_REF][START_REF] Guchu | Influence of the species and geographical location on volatile composition of spanish oak wood (Quercus petraea Liebl. and Quercus robur L.)[END_REF][START_REF] Hu | Drought and air warming affect the speciesspecific levels of stress-related foliar metabolites of three oak species on acidic and calcareous soil[END_REF]). For instance, several studies have shown that the wood of Q. petraea is characterised by lower amounts of ellagitannins compared to Q. robur [START_REF] Mosedale | Variation of heartwood phenolics and oak lactones between the species and phenological types of Quercus petraea and Q. robur[END_REF][START_REF] Feuillat | Variation in the concentration of ellagitannins and cis-and trans-β-methyl-γ-octalactone extracted from oak wood (Quercus robur L., Quercus petraea Liebl.) under model wine cask conditions[END_REF][START_REF] Mosedale | Variability of wood extractives among Quercus robur and Quercus petraea trees from mixed stands and their relation to wood anatomy and leaf morphology[END_REF][START_REF] Prida | Effect of species and ecological conditions on ellagitannin content in oak wood from an even-aged and mixed stand of Quercus robur L. and Quercus petraea Liebl[END_REF], 2007).

Together, these results suggest that intra-specific variability in herbivore performance were independent of plant defences.

Leaf traits associated with effects of focal tree species identity on OPM performance

Our results showed that only leaf phenology had a significant effect on OPM performance.

Specifically, OPM larvae performed better when neonate had access to open buds with expanding leaves. The need for synchrony between herbivore and plant phenology is well documented in the literature (van Asch and Visser 2007, [START_REF] Forkner | Timing is everything? Phenological synchrony and population variability in leaf-chewing herbivores of Quercus[END_REF][START_REF] Wagenhoff | Five years of continuous Thaumetopoea processionea monitoring: tracing population dynamics in an arable landscape of south-western Germany[END_REF], Meurisse et al. 2012[START_REF] Wagenhoff | Temporal synchrony of Thaumetopoea processionea egg hatch and Quercus robur budburst[END_REF]. In oaks, leaf quality for lepidopteran larvae decreases sharply after budburst. Larvae emerging before leaf flush starve because of a lack of food while larvae emerging too late after budburst have to cope with harder, less nutritious and more defended leaves (Feeny 1968b[START_REF] Feeny | Seasonal changes in oak leaf tannins and nutrients as a cause of spring feeding by winter moth caterpillars[END_REF][START_REF] Tikkanen | Phenological variation as protection against defoliating insects: The case of Quercus robur and Operophtera brumata[END_REF][START_REF] Forkner | Feeny revisited: condensed tannins as antiherbivore defences in leaf chewing herbivore communitites of Quercus[END_REF][START_REF] Van Asch | Phenology of Forest Caterpillars and Their Host Trees: The Importance of Synchrony[END_REF][START_REF] Van Asch | Maternal effects in an insect herbivore as a mechanism to adapt to host plant phenology[END_REF]. Several studies showed that OPM neonates are able to survive for a period of up to 2-3 weeks without food [START_REF] Wagenhoff | Five years of continuous Thaumetopoea processionea monitoring: tracing population dynamics in an arable landscape of south-western Germany[END_REF], Meurisse et al. 2012[START_REF] Wagenhoff | Temporal synchrony of Thaumetopoea processionea egg hatch and Quercus robur budburst[END_REF], making less stringent the need for synchrony. In our study, most of the trees had already opening buds when OPM larvae emerged such that only few of them were exposed to a brief starvation period.

However, we conducted this experiment in a greenhouse and larvae were bagged in only one branch. Larvae were therefore subjected to competition for the available resource (i.e. opening buds and young leaves). Food may have been particularly limiting for those larvae that emerged on late-flushing oaks, which could explain why OPM larvae performed better on oaks whose leaves were more developed at the time of egg hatching.

Although bud phenology and herbivore performance were significantly correlated, we found that the effect of focal species identity on OPM performance remained significant after bud phenology was accounted for as a covariate in statistical models. This result indicates that differences in herbivore performance between the two studied oak species depend on other factors than plant phenology. For instance, both oak species might markedly differ in chemical (e.g. volatiles) or physical (e.g. leaf toughness, trichomes) defensive traits [START_REF] Clissold | Gross vs. net income: How plant toughness affects performance of an insect herbivore[END_REF][START_REF] Carmona | Plant traits that predict resistance to herbivores[END_REF][START_REF] Caldwell | Which leaf mechanical traits correlate with insect herbivory among feeding guilds?[END_REF], which could in turn drive herbivore response.

Contrary to our predictions, neither the concentration of phenolic compounds nor C:N ratio had significant effects on OPM performance. These results contradict the common view that phenolic compounds act as chemical anti-herbivore defences [START_REF] Feeny | Plant apparency and chemical defense[END_REF][START_REF] Lill | The effects of leaf quality on herbivore performance and attack from natural enemies[END_REF][START_REF] Forkner | Feeny revisited: condensed tannins as antiherbivore defences in leaf chewing herbivore communitites of Quercus[END_REF]. It is possible that their negative effect on OPM performance were balanced by a compensatory feeding [START_REF] Lazarevic | Adaptation of the gypsy moth to an unsuitable host plant[END_REF][START_REF] Barbehenn | Hydrolyzable tannins as "quantitative defenses": Limited impact against Lymantria dispar caterpillars on hybrid poplar[END_REF][START_REF] Damestoy | Oak genotype and phenolic compounds differently affect the performance of two insect herbivores with contrasting diet breadth[END_REF]. Moreover, most herbivores are generally N-limited [START_REF] Mattson | Herbivory in realtion to plant nitrogen content ![END_REF], resulting in negative relationship between herbivore performance and C:N ratio in consumed plant tissues [START_REF] Mattson | Herbivory in realtion to plant nitrogen content ![END_REF][START_REF] White | The abundance of invertebrate herbivores in relation to the availability of nitrogen in stressed food plants[END_REF]). In our study, C:N ratio was negatively correlated with the concentration of phenolic compounds such that their adverse effect on OPM larvae could have been compensated by the positive effect of nitrogen relative abundance. It is also important to note that because of methodological constraints, we measured chemical traits at the end of the experiment. As a consequence, our measured leaf traits might not perfectly represent the nutritional quality and defences of leaves OPM larvae had access to.

Effects of oak neighbour species identity on OPM performance and leaf traits

Oak neighbour species identity had no significant effects on OPM performance. However, it is important to note these non-significant results should be interpreted with caution for several reasons. First, our experiment was conducted on the short term so we cannot extrapolate our findings to neither OPM complete development time and adult mass or fecundity nor population dynamics. Second, we randomly allocated OPM egg-masses to the different treatments. Yet, in the wild, OPM performances on a particular tree primarily depend on host selection by gravid females foraging for oviposition sites, which may be influenced by neighbours (Damestoy et al. under review;[START_REF] Gripenberg | A meta-analysis of preference-performance relationships in phytophagous insects[END_REF]. For instance, studies conducted on the pine processionary moth showed that the presence of non-host trees in mixed stands could protect the host trees from pest attacks. In this model, non-host trees were found to disrupt host detection by the pine processionary moth by interfering with the visual and chemical cues emitted by the host tree [START_REF] Jactel | Non-host volatiles mediate associational resistance to the pine processionary moth[END_REF][START_REF] Dulaurent | Hide and seek in forests: Colonization by the pine processionary moth is impeded by the presence of nonhost trees[END_REF][START_REF] Castagneyrol | Plant apparency, an overlooked driver of associational resistance to insect herbivory[END_REF], 2014d). Third, because we kept the neighbouring saplings intact, it is possible that the rate of VOCs emission, usually induced by leaf damage [START_REF] Arimura | Plant-plant interactions mediated by volatiles emitted from plants infested by spider mites[END_REF][START_REF] Turlings | Exploiting scents of distress: the prospect of manipulating herbivore-induced plant odours to enhance the control of agricultural pests[END_REF][START_REF] Barbosa | Associational Resistance and Associational Susceptibility: Having Right or Wrong Neighbors[END_REF][START_REF] Scala | Green leaf volatiles: A plant's multifunctional weapon against herbivores and pathogens[END_REF], was too low to induce noticeable change in leaf traits of the focal oak sapling. A better understanding of the effect of oak neighbours on OPM would therefore require the integration of the effects of neighbours on both host selection behaviour and their subsequent consequences for the fitness of OPM offspring.

Our results also showed that oak neighbour species identity did not significantly affect leaf traits. These results contradict previous findings showing changes in leaf traits (physical, nutritional or chemical defence traits) in mixtures e.g., in oak [START_REF] Nickmans | Effects of neighbourhood identity and diversity on the foliar nutrition of sessile oak and beech[END_REF], Castagneyrol et al. 2017), birch (Castagneyrol et al. 2018a[START_REF] Muiruri | Forest diversity effects on insect herbivores: do leaf traits matter?[END_REF], mahogany (Moreira et al. 2014a), common ragwort (Kos et al. 2015b[START_REF] Kostenko | Effects of plant diversity on the concentration of secondary plant metabolites and the density of arthropods on focal plants in the field[END_REF] or velvet grass [START_REF] Walter | How do extreme drought and plant community composition affect host plant metabolites and herbivore performance?[END_REF]. In forests, neighbour-induced changes in leaf traits can be mediated by changes in the abiotic conditions (e.g. light) around focal trees [START_REF] Castagneyrol | Bottom-up and top-down effects of tree species diversity on leaf insect herbivory[END_REF], 2018a[START_REF] Muiruri | Forest diversity effects on insect herbivores: do leaf traits matter?[END_REF]). Here, our oak saplings had the same age and similar size such that they were unlikely affected by the shade of their neighbours. Neighbour-mediated effects on leaf traits can be also mediated by the emission of volatile organic compounds by neighbouring trees [START_REF] Arimura | Plant-plant interactions mediated by volatiles emitted from plants infested by spider mites[END_REF][START_REF] Turlings | Exploiting scents of distress: the prospect of manipulating herbivore-induced plant odours to enhance the control of agricultural pests[END_REF][START_REF] Barbosa | Associational Resistance and Associational Susceptibility: Having Right or Wrong Neighbors[END_REF]).

However, we conducted the experiment in a greenhouse with only few decimetres between adjacent pots, such that volatiles emitted by every oak may have blended within the greenhouse. Finally, neighbour-mediated effects on leaf traits can be also mediated by belowground processes. For example, plants have well documented effects on soil microbes which can in turn influence the growth and defences of their neighbours through plant-soil feedbacks [START_REF] Pineda | Helping plants to deal with insects: The role of beneficial soil-borne microbes[END_REF][START_REF] Van Der Putten | Plant-soil feedbacks: The past, the present and future challenges[END_REF], Kos et al. 2015b, 2015c[START_REF] Correia | Ecological interactions between cork oak (Quercus suber L.) and stone pine (Pinus pinea L.): Results from a pot experiment[END_REF]. Moreover, there is evidence that microbes in the rhizosphere of a plant can influence herbivory on another plant through changes in its leaf chemical content [START_REF] Badri | Potential impact of soil microbiomes on the leaf metabolome and on herbivore feeding behavior[END_REF]). In the condition of our experiment these belowground processes could explain the significant effect of Focal × Neighbour species identity interaction. In particular, Q. robur saplings increased their investment in flavonoids and Q. petraea had lower C:N when they were associated with heterospecific neighbours. Although this research area is too recent as yet to allow further speculation on the mechanisms that drove species-specific differences in oak leaf traits in the presence of conspecific vs. heterospecific neighbours, this would represent a fascinating area for further investigations.

Conclusion

Overall, our study showed that OPM larvae performed better on Q. petraea than Q. robur saplings, suggesting that Q. petraea trees are more susceptible to OPM defoliation. These findings have implications for forest management as they indicate that the pedunculate oak should be preferred over the sessile oak to promote oak forest resistance to OPM. However, we need more insight into host preference mechanisms and particularly how OPM females use volatile organic compounds to differentiate and choose between the two oak species.

We found that synchronization between leaf and larval development is key factor for OPM performances. Because changes in temperature might differentially affect bud burst and egg hatching phenologies more controlled experiments, e.g. in climatic chambers, are needed to predict the effects of climate change on OPM performances and associated defoliation damage.

We could not find clear impact of neighbour identity on OPM performance. However, we used sibling species as models to test diversity effects. We therefore suggest matching oaks with more distant species, such as other non-host deciduous trees or conifers, to identify associative resistance processes as a means of managing OPM.

Quels effets ont le génotype du chêne et les composés phénoliques sur les performances de deux insectes herbivores ?

Damestoy T., Brachi B., Moreira X., Jactel H., Plomion C., Castagneyrol B. (2019) -Oak genotype and phenolic compounds differently affect the performance of two insect herbivores with contrasting diet breadth. Tree physiology, 1-13. doi: 10.1093/treephys/tpy149 Chapitre 3 al. 2006[START_REF] Johnson | Bottom-up effects of plant genotype on aphids, ants, and predators[END_REF][START_REF] Tack | The relative importance of host-plant genetic diversity in structuring the associated herbivore community[END_REF][START_REF] Crawford | Genetic diversity within a dominant plant outweighs plant species diversity in structuring an arthropod community[END_REF]. To understand both the evolution of plant-insect interactions and the effect of plant diversity on arthropod communities, it is crucial to simultaneously (i) investigate the effect of genetic variation on plant-insect interactions, (ii) identify the host traits that influence insect herbivore performance and (iii) understand the impact of this variation on the performance of different functional groups of insect from the community.

Our current understanding of plant genetic effects on herbivores was largely based on studies in crops or non-woody plants comparing plant genotypes producing different phytochemical defenses (but see [START_REF] Rubert-Nason | Vernal freeze damage and genetic variation alter tree growth, chemistry, and insect interactions[END_REF][START_REF] Barker | Genotypic variation in plant traits shapes herbivorous insect and ant communities on a foundation tree species[END_REF][START_REF] Falk | Genetic variation in aspen phytochemical patterns structures windows of opportunity for gypsy moth larvae[END_REF]. In plants, and in trees in particular, defense related phytochemicals are often considered to be key traits driving the effect of within species diversity on arthropod communities [START_REF] Wimp | Plant genetics predicts intra-annual variation in phytochemistry and arthropod community structure[END_REF][START_REF] Richards | Phytochemical diversity drives plant-insect community diversity[END_REF] as well as on the performance of individual herbivore species [START_REF] Slinn | Across Multiple Species, Phytochemical Diversity and Herbivore Diet Breadth Have Cascading Effects on Herbivore Immunity and Parasitism in a Tropical Model System[END_REF]. In particular the specialized metabolism of plants produces hundreds or thousands of different molecules which, considered together, constitute phytochemical profiles that are often highly variable in natural populations [START_REF] Geber | Inheritance and Natural Selection on Functional Traits[END_REF]Griffen 2003, Barbour et al. 2015).

Thus, secondary metabolites are likely a major mechanistic link between genetic variation in trees and levels of herbivory. Among specialized metabolites, phenolic compounds are commonly considered effective plant defences against many herbivores in several tree species [START_REF] Feeny | Plant apparency and chemical defense[END_REF][START_REF] Lill | The effects of leaf quality on herbivore performance and attack from natural enemies[END_REF][START_REF] Forkner | Feeny revisited: condensed tannins as antiherbivore defences in leaf chewing herbivore communitites of Quercus[END_REF]). These compounds are often toxic (Salminen andKaronen 2011, Mithöfer and[START_REF] Mithöfer | Plant Defense Against Herbivores: Chemical Aspects[END_REF] and some have been shown to reduce digestibility in herbivores, hence potentially reducing herbivore damage [START_REF] Feeny | Seasonal changes in oak leaf tannins and nutrients as a cause of spring feeding by winter moth caterpillars[END_REF][START_REF] Roslin | Specialization pays off : contrasting effects of two types of tannins on oak specialist and generalist moth species[END_REF], Abdala-Roberts et al. 2016, Moreira et al. 2017a). For instance, condensed and hydrolysable tannins as well as flavonoids reduce plant digestibility by binding digestive enzymes and altering herbivores' digestive tissues through the production of reactive oxygen species [START_REF] Barbehenn | Hydrolyzable tannins as "quantitative defenses": Limited impact against Lymantria dispar caterpillars on hybrid poplar[END_REF][START_REF] Barbehenn | Tannins in plant-herbivore interactions[END_REF][START_REF] Falcone Ferreyra | Flavonoids: biosynthesis, biological functions, and biotechnological applications[END_REF]). In addition, lignins act as toxic compounds and also contribute to increase leaf toughness [START_REF] Bidlack | Molecular structure and component integration of secondary cell walls in plants[END_REF], Bonawitz and Chapple 2010), a common physical defensive trait [START_REF] Clissold | The gastrointestinal tract as a nutrient-balancing organ[END_REF][START_REF] Pearse | The role of leaf defensive traits in oaks on the preference and performance of a polyphagous herbivore, Orgyia vetusta[END_REF][START_REF] Caldwell | Which leaf mechanical traits correlate with insect herbivory among feeding guilds?[END_REF].

Importantly, the response of herbivores to plant chemical defences has proven variable among herbivore functional groups [START_REF] Slinn | Across Multiple Species, Phytochemical Diversity and Herbivore Diet Breadth Have Cascading Effects on Herbivore Immunity and Parasitism in a Tropical Model System[END_REF]. In particular, leaf phenolic compounds often have contrasting effects on generalist and specialist herbivore species [START_REF] Cornell | Herbivore Responses to Plant Secondary Compounds : A Test of Phytochemical Coevolution Theory[END_REF][START_REF] Lankau | Specialist and generalist herbivores exert opposing selection on a chemical defense[END_REF][START_REF] Bidart-Bouzat | An ecological genomic approach challenging the paradigm of differential plant responses to specialist versus generalist insect herbivores[END_REF][START_REF] Ali | Specialist versus generalist insect herbivores and plant defense[END_REF]. Specialist herbivores are thought to overcome (or even benefit from) low concentration of specific compounds while being more sensitive to molecules produced at high concentration than generalists [START_REF] Karban | Herbivore offense[END_REF], Coley et al. 2006[START_REF] Després | The evolutionary ecology of insect resistance to plant chemicals[END_REF][START_REF] Carmona | Plant traits that predict resistance to herbivores[END_REF]. Beyond the effect of phyto-chemical defenses, other genetically determined plant traits (e.g. physical defenses, phenology, growth rate) may further contribute to resistance to herbivore species differing in their diet breath [START_REF] Barbour | Multiple plant traits shape the genetic basis of herbivore community assembly[END_REF]. Thus, important insights may be gained by directly quantifying the effect of plant genetic variability on the performance of herbivores and the mediating role of phyto-chemical defenses.

We investigated the effect of the genetic and phenotypic variation in pedonculate oak (Quercus robur L.) on the performance of a generalist and a specialist herbivore species. The pedunculate oak is a broadleaved species widely distributed and native to Europe. This tree species supports a large community of insect herbivores, mainly leaf chewers [START_REF] Southwood | The Number of Species of Insect Associated with Various Trees[END_REF] including the gypsy moth Lymantria dispar (GM) and the oak processionary moth Thaumetopoea processionea (OPM). These two herbivores are univoltine and sympatric species native to Europe. Their phenology (egg hatching) is synchronized with oak phenology (bud bursting) [START_REF] Wagenhoff | Temporal synchrony of Thaumetopoea processionea egg hatch and Quercus robur budburst[END_REF]) such that larvae of both species can cause major defoliation in spring and early summer. OPM is considered an oligophagous (specialist) species feeding mainly on Quercus sp. (with preferences for Q. robur, Q. petreae and Q. cerris) and occasionally on closely related Fagaceae species. GM is a highly polyphagous (generalist) species that has been documented to feed on more than 500 host species belonging to different families (including conifers and broadleaved species) [START_REF] Liebhold | Suitability of North American Tree Species to the Gypsy Moth: A Summary of Field and Laboratory Tests[END_REF]. Although the principal host species varies across its geographic range, GM larvae have marked preferences for oaks in south Western Europe where the present study was conducted.

In the present study, we investigated how genetic variation in pedunculate oak affects the feeding behaviour of GM and OPM, including through the production of leaf phenolic compounds (flavonoids, lignins and condensed and hydrolysable tannins), by performing laboratory feeding experiments and assessing the consumption, growth and metabolic efficiency of the two herbivore species feeding on the leaves of 30 oak full-sibs. By doing so, our study builds towards a better understanding of the mechanisms underlying the effect of tree genetic variation on phytophagous insect performances.

Materials and methods

Oak common garden and leaf samples -We used 120 oak trees corresponding to four clonal replicates of 30 full-sib genotypes of a single family obtained from a cross between two parental trees [START_REF] Bodénès | High-density linkage mapping and distribution of segregation distortion regions in the oak genome[END_REF]. Clones were obtained by grafting and established in a common garden in 1998 following a randomized block design (INRA experimental station of Bourran, latitude 44.332492 °N, longitude 0.413993 °E). For this study, we randomly selected 30 genotypes among 207 full-sibs that have been intensively used to study the genetic architecture of plants traits [START_REF] Brendel | Quantitative trait loci controlling water use efficiency and related traits in <I>Quercus robur</I>[END_REF][START_REF] Song | X-ray computed tomography to decipher the genetic architecture of tree branching traits: oak as a case study[END_REF]). In early May 2017, we collected one branch per tree with pole pruners. Branches were ca 1.5 m long and were collected at ca 3-4 m height. Upon collection, we stored branches vertically, dipping in water in plastic bins to avoid desiccation. We then transported the branches back to the laboratory where the bins were placed in a dark cold room at 4 °C. We used leaves from these branches for the laboratory trials described below.

Laboratory feeding trials -OPM larvae were obtained from eggs collected on mature Q.

robur in North Eastern France in November 2016. GM larvae were obtained from a laboratory rearing initiated with eggs collected in Southwestern France in late 2015 (Castagneyrol et al. 2018b). Egg masses of both species were kept in a climatic chamber at 4 °C prior to the experiment. On April 10 th 2017 egg hatching was initiated by transferring eggs into a climatic room at constant temperature (20 °C) and photo-period (12:12 L:D) for four days. Neonates were reared on leaves taken on one single mature Q. robur growing close to the laboratory until they reached the third-instar stage.

We used a feeding trial protocol adapted from [START_REF] Fernandez-Conradi | The effect of tree genetic diversity on insect herbivory varies with insect abundance[END_REF] and Castagneyrol et al. (2018). On the same day we collected branches of the 120 oak trees in the field, we isolated 120 third instar larvae of each defoliator species into individual 354 mL plastic boxes. We kept the larvae without food for 24 h and then weighed them. We also weighed the totality of frass produced during experiment. We randomly assigned each larva to one replicate of one of the 30 oak genotypes. The experiment therefore included 120 GM third instar larvae and 120 OPM third instar larvae, each reared on the leaves of one clone of each oak genotype (i.e. each larva received leaves from a single oak branch).

Every morning, we randomly collected six leaves per branch, choosing leaves with minimum signs of herbivory, disease or senescence. Two leaves were used to feed GM and OPM larvae (i.e., one leaf per larva) while the four remaining leaves were dried for chemical analyses (see below). Each larva received one fresh leaf per day, and partially consumed leaves were removed from rearing boxes and stored for further analysis. The experiment was carried out at room temperature (c.a., 20 °C) and lasted for four days. Leaves introduced into each rearing box were scanned before (day n) and after (day n + 1) to estimate consumption by GM and OPM larvae. Small lost fragments of leaves were carefully isolated from frass and scanned together with the consumed leaves. We scanned leaves using a standard desktop scanner and estimated leaf area before and after consumption with ImageJ software [START_REF] Schneider | NIH Image to ImageJ: 25 years of image analysis[END_REF]). Consumed leaves were dried for 72 h at 45 °C and then weighed in order to establish the relationship between leaf dry weight and leaf area.

At the end of the fourth day, we kept larvae without food for 24 h and weighed them individually. Frass accumulated over the time of the experiment was collected, dried for 72 h at 45°C and then weighed. Larval initial and final weight, frass weight, pre-and postconsumption leaf surface and post-consumption leaf dry weight were used to estimate leaf consumption (mg.day -1 ), larval growth (mg.day -1 ) and the amount of digested leaves (the difference between consumption rate and the daily frass production mg.day -1 ) for each larva fed on leaves from a single oak tree.

Quantification of leaf phenolic compounds -Chemical analyses were conducted at the level of individual branches by pooling all intact leaves collected from the same branch over the course of the four-day experiment. We therefore analysed phenolic content of 120 samples of 16 leaves per branch encompassing the duration of the experiment (4 leaves per branch and per day for 4 days) to capture potential changes in the composition and amount of leaf phenolics over the time of the experiment. We only collected leaves with little or no herbivore damage; hence we considered the levels and composition of phenolics measured as constitutive defences [START_REF] Abdala-Roberts | Effects of plant intraspecific diversity across three trophic levels: Underlying mechanisms and plant traits[END_REF]. After collection, we oven-dried leaves for 48 h at 45 °C, ground them to a thin powder using a Labman high speed grinding station, and stored powder samples at room temperature in individual plastic vials. We extracted phenolic compounds using 20 mg of dry plant tissue with 1 mL of 70 % methanol in an ultrasonic bath for 20 min, followed by centrifugation (Moreira et al. 2014b). We diluted methanolic extracts

(1:4 vol:vol) with an extraction solvent and transferred them to chromatographic vials to perform chromatographic analyses. We carried out chromatographic analyses with an Ultra-High-Performance Liquid-Chromatograph (UHPLC Nexera LC-30AD; Shimadzu) equipped with a Nexera SIL-30AC injector and one SPD-M20A UV/VIS photodiode array detector.

The UHPLC column was a Kinetex™ 2.6 µm C18 82-102 Å, LC Column 100 × 4.6 mm, protected with a C18 guard cartridge. The flow rate was 0.4 mL.min -1 and the oven temperature was set to 25 ºC. The mobile phase consisted of two solvents: water-formic acid (0.05 %) (A) and acetonitrile-formic acid (0.05 %) (B), starting with 5 % B and using a gradient to obtain 30 % B at 4 min, 60 % B at 10 min, 80 % B at 13 min and 100 % B at 15 min. The injection volume was 30 µL. We recorded chromatograms at 330 nm and processed data with the LabSolutions software (Shimadzu). We quantified flavonoids as rutin equivalents, condensed tannins as catechin equivalents, hydrolysable tannins as gallic acid equivalents, and lignins as ferulic acid equivalents. We achieved the quantification of these phenolic compounds by external calibration using calibration curves based on chemical equivalent at 0.25, 0.5, 1 and 2 μg.mL -1 . We expressed phenolic compound concentrations in mg.g -1 tissue on a dry weight basis.

Herbivore performance and nutritional indices -We first estimated leaf biomass consumed by insect larvae using the linear regression of leaf dry weight against leaf area of consumed leaves. We estimated slope and intercept parameters for each genotype separately. We then used parameter estimates of genotype-specific regressions to estimate the amount of consumed biomass from the area of consumed leaves [START_REF] Fernandez-Conradi | The effect of tree genetic diversity on insect herbivory varies with insect abundance[END_REF], Castagneyrol et al. 2018b).

Then, for statistical analysis (see below) we compared larval performance between oak genotypes by using ANCOVA equivalents to Waldbauer's nutritional indexes [START_REF] Waldbauer | The consumption and utilization of food by insects[END_REF][START_REF] Raubenheimer | Analysis of covariance : an alternative to nutritional indices[END_REF][START_REF] Hägele | Dietary mixing in three generalist herbivores: nutrient complementation or toxin dilution?[END_REF]) (Fig. 9):

Larval Relative Growth Rate (RGR) is the ratio between growth rate (G = (w fw t=0 ) / d, mg day -1 , where w t=0 and w f are initial and final larval weights, respectively and d the number of days of the experiment) and initial weight (w t=0 ): RGR = G/w t=0 . It was analysed as ANCOVA equivalent by using growth rate (G) as response variable and initial weight (w t=0 ) as covariate.

Larval Relative Consumption Rate (RCR) is the ratio between leaf biomass consumption (C, mg.day -1 ) and the initial weight: RCR = C/w t=0 . It was analysed by using leaf consumption rate (C) as response variable and initial weight (w t=0 ) as covariate.

Efficiency of Conversion of Ingested food (ECI) is the daily amount of ingested food that is converted into body biomass. ECI is the ratio between growth rate (G) and consumption rate (C) and represents pre-regulatory mechanisms (e.g. consumption). It was analysed by using larval growth rate as response variable (G) and leaf consumption rate (C) as covariate.

Efficiency of Conversion of Digested food (ECD) is the daily amount of digested food (D = C

-F, with F the daily frass production, mg.day -1 ) that is converted into body biomass. ECD is the ratio between growth rate (G) and the amount of digested food (D) and represents postingestive regulatory mechanisms (e.g. enzymatic activity in midgut). It was therefore analysed by using growth rate as response variable (G) and the amount of digested food (D) as covariate.

Approximate Digestibility (AD) represents digestion efficiency. It is the ratio between the amount of digested food (D) and the consumption rate (C): AD = D/C, such that a value close to 1 means a high digestion capacity (small amount of excreted food for a given amount of ingested food). It was analysed by using amount of digested food as response variable (D) and consumption rate (C) as covariate.

We estimated the variation of each nutritional index among oak genotypes by calculating the coefficient of variation (CV) which is the ratio between the overall standard deviation and the overall mean value across all genotypes. Statistical analyses -Third instar GM larvae were larger and consumed much more leaf biomass than third instar OPM larvae. We therefore decided not to compare GM and OPM directly but to analyse their response to oak genotypes and leaf phenolic compounds in separate models.

For each herbivore species, we analysed three response variables: consumption rate (C), growth rate (G) and amount of digested food (D). The results were in terms of nutritional indices by using the appropriate covariate (see above).

We used linear mixed effects models (LMM) where genotype was declared as random factor.

We first estimated proportion of variance in herbivore performance explained by oak genotype using a random effect models:

y i,j = α + g j + ε i,j (1)
where y i,j was the response variable (G, C or D) for clone i and genotype j, α the model intercept, g j the random intercept for genotype j and ε i,j residual errors. g j and ε i,j were assumed to be normally distributed with mean 0 and variance σ g 2 and σ e 2 , respectively. We used 1,000 bootstraps to compute the 95 % confidence interval around σ g 2 . We used the same modelling approach to analyse the genotype effect on the concentration of each group of phenolic compounds.

We tested the effect of leaf phenolic compounds on nutritional indices (RGR, RCR, ECI, ECD, AD) with random intercept models. For both GM and OPM and for each index separately, we first built a full LMM where the numerator of the index (G, C or D) was the response variable, and the denominator (C, D or w t=0 ) was the appropriate covariate (fixed effects). We also included larval initial weight and the concentration of one of the four groups of phenolic compounds (condensed tannins, hydrolysable tannins, flavonoids or lignins, each at a time) as additional fixed effects. The term for oak genotype was a random factor. For instance, the model corresponding to the effect of hydrolysable tannins (HT) conditional to oak genotype on larval growth rate was:

G i,j = α + β 1 w t=0, i, j + β 2 HT i, j + g j + ε i,j (2)
G i,j was the growth rate of larva fed on clone i of genotype j, α the model intercept, β 1 the effect of larval initial weight and w t=0, i, j is the initial weight measured for the larva fed on clone i of genotype j, β 2 the effect of hydrolysable tannins (HT i, j is the concentration of hydrolysable tannins estimated for clone i of genotype j), g j is a random intercept for each tree genotype (N = 30) and ε i,j is the residual term (N = 120) following N(0, σ g 2 ) and N(0, σ e 2 ), respectively.

We then simplified the initial full model by sequentially removing terms with non-significant fixed effects, starting with the least significant, using step function of lmerTest package [START_REF] Kuznetsova | lmerTest Package: Tests in Linear Mixed Effects Models[END_REF]. When needed, we used logarithm (consumption of OPM) or square root (consumption of GM except when HT was a covariate) transformations of response variable to satisfy model assumptions. We estimated model fit by calculating marginal (R m ²)

and conditional (R c ²) R-squares [START_REF] Nakagawa | A general and simple method for obtaining R2 from generalized linear mixed-effects models[END_REF]. R² was interpreted as the amount of variance in the response variable explained by the fixed effects only (R m ²) and by fixed effects conditional to random effects (R c ²).

Finally, we tested whether the effect of oak genotype on herbivore performance was consistent between GM and OPM larvae. We computed average performance traits (G, C and D) per oak genotype for each of the two herbivore species and tested genetic correlations across herbivore species for each performance trait using a Spearman rank sum test. Slopes and regression lines were estimated by performing linear regressions between trait values for GM and OPM.

All analyses were performed in R v3.5.1 (R Core Team 2018) with the following packages:

doBy , sciplot, plyr, ggplot2, cowplot, lme4, lmerTest and MuMIn (Wickham 2011[START_REF] Bates | Fitting Linear Mixed-Effects Models using lme4[END_REF], Wickham 2016b[START_REF] Kuznetsova | lmerTest Package: Tests in Linear Mixed Effects Models[END_REF][START_REF] Morales | with code developed by the R Development Core Team, with general advice from the R-help listserv community, and especially Duncan Murdoch[END_REF][START_REF] Wilke | cowplot: Streamlined Plot Theme and Plot Annotations for "ggplot2[END_REF], Barton 2018b[START_REF] Højsgaard | doBy: Groupwise Statistics, LSmeans, Linear Contrasts, Utilities[END_REF].

Results

Effect of phenolics on performance of GM and OPM larvae

We identified four groups of phenolic compounds: flavonoids (13 compounds), condensed tannin (one compound), hydrolysable tannins (two compounds) and lignins (two compounds).

Tannins, and in particular condensed tannins, were the most abundant phenolic compounds in leaf samples, regardless of oak genotype (Fig. 10). Lignins and flavonoids only represented on average 0.65 % and 8.68 % of total phenolics (Fig. 10). For the four groups of phenolics we observed coefficients of variation greater than 60% (Table 5). There was a significant positive effect of condensed tannins on the growth (G) and consumption rates (C) of GM larvae (Fig. 11ab, Table 6). However, this effect was not significant when consumption (i.e., ECI) or digestion (i.e., ECD) were included as covariates (Table 6). This result suggests that condensed tannins only influenced GM growth rate through pre-ingestive regulatory mechanisms (i.e., increased consumption). Other phenolic compounds (i.e., hydrolysable tannins, flavonoids and lignins) had a significant positive effect on GM growth rate (Fig. 11a, Table 6), even when the amount of digested food (D) was included as a covariate in models (i.e., ECD, Table 6), suggesting that their effect involved post-digestive (i.e., metabolic) regulatory mechanisms.

There was a significant, positive effect of condensed tannins on OPM consumption rate (Fig. 11c, Table 6), and a significant positive effect of consumption on OPM growth rate (Table 6). However, there was no significant effect of phenolic compounds, including condensed tannins, on OPM growth rate, even when consumption or digestion were included as covariates (Table 6).

In both GM and OPM, consumption was significantly and positively affected by initial weight (i.e., RCR, Table 6). Likewise, regardless the phenolic compounds, growth rate of OPM increased with initial weight (i.e., RGR, Table 6), and growth rate of both herbivores increased with consumption (i.e., ECI, Table 6) and digestion (i.e., ECD, Table 6). The amount of food digested by the two herbivore species increased with the amount of ingested food (i.e., AD, Table 6).

Effect of genetic variation in oaks on herbivore performance and oak phenolics

The three herbivore physiological responses (i.e., growth, consumption, digestion) varied greatly in our dataset (CV > 60%), especially growth and digestion (Table 5). The oaks included in our study were full-sibs and shared on average 50 % genetic identity. Despite this high degree of genetic similarity, tree genetics greatly influenced herbivore leaf consumption and digestion. In particular, the tree genotype explained large proportions of variance for consumption (C) and digestion (D) in OPM, as well as digestion in GM (Fig. 12a). Oak genotype did not influence larval growth in both insect species, and the thirty tree genotypes did not display significant differences for the four groups leaf phenolics (95% CI overlapping with 0, Fig. 12b)

Figure 11abc: Effect of oak leaf phenolic compounds (x-axes) on gypsy moth (GM, left panels) growth (a) and consumption (b) and on oak processionary moth (OPM, right panels) consumption (c). Concentrations of phenolic compounds (mg/g

) were standardized to make their effects comparable. For sake of visibility, the figure only represents model predictions (solid lines) and corresponding standard errors (dashed lines) for those polyphenolics that had a significant effect of herbivore growth or consumption. Box and whiskers plots represent the distribution of response (y-axes) and explanatory (x-axes) variables.

Genetic correlations between GM and OPM performance

We found significant positive genetic correlations between performance of GM and OPM for the consumption rate, and the amount of digested food. This suggests that consumption rate or the amount of digested food of the two insect species were impacted by the same or physically linked genetic factors. In other words, oak genotypes that were more consumed or better digested by GM larvae also tended to be more consumed and better digested by OPM larvae (Fig. 13ab).

Discussion

In this study, we showed that two herbivore species with contrasting diet breadth (the generalist GM and the specialist OPM) had different ways to cope with oak phenolic compounds, suggesting different behavioural and physiological responses to plant defences.

We observed that consumption and digestion of herbivores, but not growth, were largely influenced by host genotype, but that leaf phenolic compounds did not contribute to the effect of host genotype on herbivore behavioural and physiological responses. We also detected a significant positive correlation between oak genotype susceptibility to both insect species, suggesting that host resistance to both insects is driven at least in part by a set of common or physically linked loci (genes or genomic regions). Our results shed light on the role of host genotype and associated defences on food processing by insect herbivores and show that genetic variation in trees, even among full-sibs, can influence herbivore responses related to feeding.

Oak genotype influences herbivore consumption and digestion, but not growth

Our results demonstrated that oak genotypes differed in the way they drive behavioural and physiological responses of leaf feeding insects. Some studies have reported a strong influence of plant genotype on damage by insect herbivores (McArt and Thaler 2013). However, how these host genetic effects influence the performance of herbivores has been rarely tested (but see McArt andThaler 2013, Utsumi et al. 2013). Here, we showed that the way in which herbivore response to plant genotype is evaluated does matter [START_REF] Whitlock | Relationships between adaptive and neutral genetic diversity and ecological structure and functioning : a meta-analysis[END_REF]. In particular, we found that the amount of food consumed and digested by the two herbivore species differed between tree genotypes, but that larval growth was, on the contrary, largely independent of oak genotype. This finding partially conflicts with previous studies reporting consistent or stronger effects of host genotype on gypsy moth growth relative to consumption [START_REF] Osier | Effects of genotype, nutrient availability, and defoliation on aspen phytochemistry and insect performance[END_REF]Lindroth 2001, Fernandez-Conradi et al. 2017). This discrepancy could be attributed to the duration of the feeding trials. Although we cannot exclude that oak genotype could have had a stronger effect on herbivore growth should we have prolonged feeding trials, a likely reason for these contrasting results could be that different oak genotypes differed in traits that drove pre-and post-ingestive regulatory processes. Thus, our findings suggest a compensation of lower nutritive quality of some oak genotype by increased consumption and/or efficiency of assimilation, so as to maintain a constant growth regardless of the oak genotype on which they feed [START_REF] Milanović | Preference and performance of the larvae of Lymantria dispar (Lepidoptera: Lymantriidae) on three species of European oaks[END_REF]. Furthermore, the positive genetic correlations between the performances of both herbivores indicated that identical or linked loci impact consumption or digestibility in both the specialist and generalist herbivore. This is consistent with previous work by [START_REF] Reymond | A conserved transcript pattern in response to a specialist and a generalist herbivore[END_REF] who showed an almost identical transcript profile in plant responses to damage by generalist or specialist insect herbivores.

Leaf phenolics have contrasting effects on generalist and specialist herbivore species

Overall, we found that leaf phenolics had increased herbivore growth and consumption, but that these effects differed among the classes of phenolic compounds and, for some of them, between herbivore species. Leaf consumption by both the generalist and the specialist herbivore increased with increasing concentration of condensed tannins in oak leaves. This result is in line with previous studies (e.g., [START_REF] Foss | Species-specific differences in oak foliage affect preference and performance of gypsy moth caterpillars[END_REF] and can be interpreted as a compensatory feeding response of herbivores to plant defences [START_REF] Lazarevic | Adaptation of the gypsy moth to an unsuitable host plant[END_REF][START_REF] Barbehenn | Hydrolyzable tannins as "quantitative defenses": Limited impact against Lymantria dispar caterpillars on hybrid poplar[END_REF]) whereby herbivores increase their consumption to compensate the ingestion of leaves of poorer quality (highly defended). The growth of the generalist herbivore (GM) was enhanced under high concentrations of condensed tannins, but this effect became non-significant when consumption or digestion was accounted for. This result indicates that leaf condensed tannins primarily influenced pre-ingestive regulatory processes in the generalist herbivore [START_REF] Barbehenn | Hydrolyzable tannins as "quantitative defenses": Limited impact against Lymantria dispar caterpillars on hybrid poplar[END_REF]. In contrast, condensed tannins had no effect on the growth of the specialist herbivore (OPM), suggesting that post-ingestive processes (e.g., detoxification) have evolved to avoid the need for overconsumption to maintain growth.

Hydrolysable tannins, lignins and flavonoids had a positive effect on the growth of the generalist herbivore, when digestive processes were accounted for (i.e. ECD), indicating that their effects occurred through post-digestive (i.e., metabolic) regulatory processes. The positive effect of leaf chemical defences on larval growth in the generalist herbivore may have been primarily driven by an increase in gut enzymatic activity [START_REF] Lazarevic | Adaptation of the gypsy moth to an unsuitable host plant[END_REF][START_REF] Lazarevic | Nutritive stress effects on growth and digestive physiology of <I>Lymantria dispar</I> larvae[END_REF][START_REF] Milanović | Behavioural and physiological plasticity of gypsy moth larvae to host plant switching[END_REF]. In contrast, these phenolic compounds had no significant effects on the growth of the specialist herbivore. Insect herbivores, and in particular generalist herbivores [START_REF] Karban | Herbivore offense[END_REF], can adjust the production of gut enzymes in response to the nutritional quality of their diet [START_REF] Clissold | The gastrointestinal tract as a nutrient-balancing organ[END_REF][START_REF] Mrdaković | Testing the adaptive plasticity of gypsy moth digestive enzymes in response to tannic acid using phenotypic selection analysis[END_REF][START_REF] Milanović | Preference and performance of the larvae of Lymantria dispar (Lepidoptera: Lymantriidae) on three species of European oaks[END_REF]). However, this adaptation may result in an metabolic cost [START_REF] Lazarevic | Nutritive stress effects on growth and digestive physiology of <I>Lymantria dispar</I> larvae[END_REF]. Such a cost may be more important for specialist herbivores, which cannot switch onto other host species. They may have then greater difficulties to adapt to changes in the diet quality of the main host [START_REF] Wetzel | Variability in plant nutrients reduces insect herbivore performance[END_REF][START_REF] Pearse | Variation in Plant Defense Suppresses Herbivore Performance[END_REF], as those imposed by our experimental approach. The lack of effect on the growth in the specialist herbivore could be explained by an increased consumption compensating for metabolic costs of enzymatic adjustment [START_REF] Lazarevic | Nutritive stress effects on growth and digestive physiology of <I>Lymantria dispar</I> larvae[END_REF].

Herbivore response to oak genotype is not primarily mediated by leaf phenolics

Previous studies showed that the effect of plant genotype on herbivore damage can be driven by differences in plant secondary metabolites [START_REF] Kersten | Integrated transcriptomics and metabolomics decipher differences in the resistance of pedunculate oak to the herbivore Tortrix viridana L[END_REF][START_REF] Brachi | Coselected genes determine adaptive variation in herbivore resistance throughout the native range of <I>Arabidopsis thaliana<[END_REF] whereby plants having more similar secondary metabolite profiles also tend to be more sensitive to the same herbivore. In contrast, we found that, although leaf phenolic content greatly varied among oak trees, oak genotype only explained ca 6 % of this variability. As such, it is unlikely that the observed effect of oak genotype on the consumption and digestion by the two herbivore species was driven by oak leaf phenolics. However, we cannot exclude the possibility that the amount of phenolic compounds that we measured was affected by induced systemic responses in distant leaves, or, to some extent, by the fact that we used excised leaves in our feeding trials. Thus, phenolic compounds that we measured presumably represented a combination of constitutive defences, plus an unknown level of systemic induction [START_REF] Abdala-Roberts | Effects of plant intraspecific diversity across three trophic levels: Underlying mechanisms and plant traits[END_REF]. Induced defences represent an important part of overall plant defences. We cannot exclude that oak ability to induce specific defences in response to initial herbivores or pathogens attacks is genetically controlled (Arimura et al. 2000b, Agrawal et al. 2002[START_REF] Moreira | Differential allocation of constitutive and induced chemical defenses in pine tree juveniles: A test of the optimal defense theory[END_REF][START_REF] Fürstenberg-Hägg | Plant defense against insect herbivores[END_REF][START_REF] Moreira | Specificity of induced defenses , growth , and reproduction in lima bean (Phaseolus lunatus) in response to multispecies herbivory[END_REF], or that the effect of oak genotype on chemical defences would have been stronger on induced defences than on constitutive defences. Therefore, it is possible that oak genotype did influence herbivore's consumption through induced systemic response that may have remained unnoticed in our experiments. Additionally, it is important to note that secondary metabolites might have lower effects on herbivore performance than other plant defensive traits [START_REF] Carmona | Plant traits that predict resistance to herbivores[END_REF], maybe due to the ability of some herbivores to overcome chemical defences, through detoxification, secretion or degradation of toxins (Karban andAgrawal 2002, Després et al. 2007). We cannot exclude that the 30 oak genotypes of our experiment also differed in other chemical (e.g., terpenes, nutrients, carbohydrates) or physical (e.g., leaf toughness) traits, which could drive herbivores' response to oak genotypes.

For instance, a number of previous studies have reported that low nutrient (e.g. N, P) concentrations in plant tissues drastically reduce herbivore performances [START_REF] Milanović | Preference and performance of the larvae of Lymantria dispar (Lepidoptera: Lymantriidae) on three species of European oaks[END_REF][START_REF] Wetzel | Variability in plant nutrients reduces insect herbivore performance[END_REF]. Similarly, leaf thickness and leaf toughness or hairiness are structural traits which have been reported to negatively affect the performance of leafchewing herbivores [START_REF] Clissold | Gross vs. net income: How plant toughness affects performance of an insect herbivore[END_REF][START_REF] Caldwell | Which leaf mechanical traits correlate with insect herbivory among feeding guilds?[END_REF]. Considering both leaf chemical and structural traits will thus be needed to better characterise the physiological mechanisms linking plant genotype with the performance of feeding herbivores.

Conclusion

Overall, our study builds towards a better understanding of the relationships between host genotype and the performance of associated insect herbivores. In particular, our findings challenge two common views. First, we found that oak leaf phenolic compounds did not reduce the growth of the two herbivore species studied here. In particular, oak leaf phenolic compounds increased herbivore consumption and growth rates, probably as a result of overconsumption. Second, we provide little support to the idea that the effect of tree genotype on associated organisms is driven by tree chemical defences. Yet, we did observe large variability in herbivore performance and production of phenolic compounds among individuals and among genotypes, but these effects were partially independent. The fact that we observed genetic correlations between the performance of both herbivores suggests that other, unmeasured plant defensive traits under genetic control may provide general resistance to herbivores. In conclusion, by addressing the effect of host plant genotype on both the amount of damage (i.e., a plant perspective) and the physiology of two insect herbivores with contrasting diet breadth (i.e., herbivore perspective), we highlight the need for further research on mechanisms driving plant resistance to herbivores.

Dans les forêts, les insectes jouent un rôle écologique majeur. En consommant les plantes, les insectes participent activement à la dynamique des écosystèmes forestiers, en favorisant le recyclage des éléments nutritifs et en créant de nouveaux habitats et de nouvelles ressources pour d'autres espèces animales ou végétales [START_REF] Schowalter | Insect Herbivore Effects on Forest Ecosystem Services[END_REF][START_REF] Bagchi | Pathogens and insect herbivores drive rainforest plant diversity and composition[END_REF][START_REF] Metcalfe | Herbivory makes major contributions to ecosystem carbon and nutrient cycling in tropical forests[END_REF]. Cependant dans un contexte de changements climatiques, l'établissement et le développement des ravageurs sont souvent facilités et compromettent les services écosystémiques que fournissent les forêts [START_REF] Logan | Assessing the impacts of global warming on forest pest dynamics[END_REF]. L'augmentation globale des températures est favorable à l'augmentation des aires de répartition des insectes ravageurs comme la processionnaire du pin, Thaumetopoea pityocampa, en Europe [START_REF] Battisti | Expansion of geographic range in the pine processionary moth caused by increased winter temperatures[END_REF][START_REF] Buffo | Temperature as a predictor of survival of the pine processionary moth in the Italian Alps[END_REF][START_REF] Robinet | Direct impacts of recent climate warming on insect populations[END_REF] ou le dendroctone du pin en Amérique du Nord [START_REF] Carroll | Effects of climate change on range expansion by the mountain pine beetle in British Columbia[END_REF][START_REF] Safranyik | Potential for range expansion of mountain pine beetle into the boreal forest of North America[END_REF] 2007[START_REF] Forkner | Timing is everything? Phenological synchrony and population variability in leaf-chewing herbivores of Quercus[END_REF][START_REF] Wagenhoff | Five years of continuous Thaumetopoea processionea monitoring: tracing population dynamics in an arable landscape of south-western Germany[END_REF], Meurisse et al. 2012[START_REF] Wagenhoff | Temporal synchrony of Thaumetopoea processionea egg hatch and Quercus robur budburst[END_REF]). La concentration de certains métabolites spécialisés dans les feuilles était positivement corrélée au taux de consommation des feuilles par les chenilles, suggérant une compensation alimentaire pour maintenir la croissance sur les feuilles les plus défendues (Chapitre 3). Nous avons montré que l'expression de ces traits était influencée par l'identité des voisins des chênes étudiés (Chapitre 2). Enfin, chez le chêne pédonculé, nous avons mis en évidence l'existence d'un contrôle génétique de l'arbre sur les performances des chenilles (Chapitre 3).

D'un point de vue fondamental, l'originalité de notre travail tient en ce que nous avons abordé l'effet de la diversité des plantes sur les dégâts d'insectes herbivores du point de vue de la plante (traits de défense, quantité de dégâts) et du point de vue de l'herbivore (croissance, survie) de manière simultanée. Cela nous a notamment permis de questionner la vision classique selon laquelle les métabolites spécialisés sont des traits de défenses. Nous avons abordé la résistance des chênes à la chenille processionnaire à différentes échelles spatiales et différents niveaux d'intégration en tâchant de mettre en évidence les processus sous-tendant les patrons observés sur le terrain. Bien qu'il soit difficile d'extrapoler des résultats obtenus dans des conditions extrêmement contrôlées aux conditions naturelles, la complémentarité dans les approches employées apporte des éléments de compréhension aux gestionnaires.

Ainsi, d'un point de vue finalisé, le travail présenté dans cette thèse apporte des éléments de connaissances permettant d'orienter la gestion des forêts pour la résistance à un ravageur majeur en expansion. Toutefois, les résultats que nous avons obtenus soulèvent de nombreuses questionstant sur le plan fondamental que finaliséauxquelles il conviendra de répondre pour replacer les interactions chêne-chenille processionnaire dans le contexte plus large de l'écologie forestière.

Quelle place ont les défenses chimiques dans la résistance aux herbivores ?

5.1.1. Les traits chimiques jouent-ils un rôle dans les défenses de la plante ?

Au travers de nos expériences nous avons mis en évidence un effet significatif de certains traits chimiques de la plante hôte sur les performances de la chenille processionnaire du chêne. Nous avons notamment montré un effet positif de la concentration en tanins condensés sur la consommation (Chapitre 3) mais aucun effet des composés phénoliques sur la croissance de la processionnaire (Chapitre 2 et 3), suggérant une réponse positive de la consommation de l'herbivore à la quantité de défenses pour maintenir une croissance constante. Si nous nous plaçons du point de vue de l'arbre, une augmentation de la consommation signifie donc une augmentation des dégâts sur les feuilles les plus défendues.

Ce paradoxe apparent interroge le véritable rôle des composés phénoliques en tant que défenses efficaces contre les herbivores.

Cependant, toutes les études sur l'effet des métabolites spécialisés sur les herbivores ne vont pas dans ce sens. En effet, plusieurs études ont rapporté un effet négatif des métabolites secondaires sur les performances des insectes herbivores confirmant leur implication dans les défenses de la plante [START_REF] Feeny | Seasonal changes in oak leaf tannins and nutrients as a cause of spring feeding by winter moth caterpillars[END_REF][START_REF] Lill | The effects of leaf quality on herbivore performance and attack from natural enemies[END_REF][START_REF] Agrawal | A role for isothiocyanates in plant resistance against the specialist herbivore Pieris rapae[END_REF][START_REF] Dyer | Synergistic Effects of Amides from Two Piper Species on Generalist and Specialist Herbivores[END_REF][START_REF] Forkner | Feeny revisited: condensed tannins as antiherbivore defences in leaf chewing herbivore communitites of Quercus[END_REF][START_REF] Steppuhn | Nicotine's defensive function in nature[END_REF], Moreira et al. 2017a). Par exemple [START_REF] Lill | The effects of leaf quality on herbivore performance and attack from natural enemies[END_REF] ont montré une corrélation négative entre la masse des chrysalides de Psilocorsis quercicella (Lépidoptères) et la concentration en phénolsnotamment les tannins hydrolysablesdans les feuilles de leur hôte Q. alba. Dans une autre étude, la croissance d'Operophtera brumata (Lépidoptères) était réduite avec l'augmentation de la teneur en tannins des feuilles de chêne, qui réduisait la disponibilité en azote et influençait la palatabilité des feuilles [START_REF] Feeny | Seasonal changes in oak leaf tannins and nutrients as a cause of spring feeding by winter moth caterpillars[END_REF]. [START_REF] Roslin | Specialization pays off : contrasting effects of two types of tannins on oak specialist and generalist moth species[END_REF] [START_REF] Lazarevic | Adaptation of the gypsy moth to an unsuitable host plant[END_REF][START_REF] Lazarevic | Nutritive stress effects on growth and digestive physiology of <I>Lymantria dispar</I> larvae[END_REF][START_REF] Clissold | The gastrointestinal tract as a nutrient-balancing organ[END_REF][START_REF] Mrdaković | Testing the adaptive plasticity of gypsy moth digestive enzymes in response to tannic acid using phenotypic selection analysis[END_REF][START_REF] Milanović | Behavioural and physiological plasticity of gypsy moth larvae to host plant switching[END_REF].

En dépit de l'évidente variabilité dans les conclusions des études ayant évalué le rôle des métabolites spécialisés comme défenses contre les herbivores, il est généralement admis qu'ils jouent un rôle dominant dans l'écologie et l'évolution de la défense des plantes [START_REF] Bennett | Secondary metabolites in plant defence mechanisms[END_REF][START_REF] Berenbaum | Facing the future of plant-insect interaction research: Le retour à la "raison d'être[END_REF][START_REF] Mithöfer | Plant Defense Against Herbivores: Chemical Aspects[END_REF][START_REF] War | Mechanisms of plant defense against insect herbivores[END_REF]. Il est important de considérer le fait que les plantes ont développé au cours de l'évolution un très grand nombre de ces métabolites spécialisés, estimés à plus de 200 000 [START_REF] Dixon | Phytochemistry meets genome analysis, and beyond[END_REF]Strack 2003, Wink 2008). Concernant les composés phénoliques, plus de 9 000 composés ont été décrits [START_REF] Mithöfer | Plant Defense Against Herbivores: Chemical Aspects[END_REF]. Parmi les défenses chimiques on trouve également les terpénoïdes (> 30 000 composés décrit ; [START_REF] Mithöfer | Plant Defense Against Herbivores: Chemical Aspects[END_REF] qui agissent directement sur le métabolisme des insectes (Gershenzon andCroteau 1991, Langenheim 1994), mais aussi en tant que répulsifs des herbivores [START_REF] Aharoni | Terpenoid Metabolism in Wild-Type and Transgenic Arabidopsis Plants[END_REF][START_REF] Laothawornkitkul | Isoprene emissions influence herbivore feeding decisions[END_REF]) et attractif des ennemis naturels des herbivores [START_REF] De Boer | Identification of volatiles that are used in discrimination between plants infested with prey or nonprey herbivores by a predatory mite[END_REF][START_REF] Loivamäki | Isoprene interferes with the attraction of bodyguards by herbaceous plants[END_REF] ; les alkaloïdes (> 12 000 composés décris ; Mithöfer and Boland 2012) comme la nicotine ou la caféine par exemple, sont toxiques pour divers herbivores et agissent sur différents processus métaboliques des herbivores [START_REF] Detzel | Attraction, deterrence or intoxication of bees (Apis mellifera) by plant allelochemicals[END_REF][START_REF] Wink | Modes of Action of Allelochemical Alkaloids: Interaction with Neuroreceptors, DNA, and Other Molecular Targets[END_REF][START_REF] Katoh | Molecular regulation of nicotine biosynthesis[END_REF][START_REF] Hirayama | Differential effects of sugarmimic alkaloids in mulberry latex on sugar metabolism and disaccharidases of Eri and domesticated silkworms: Enzymatic adaptation of Bombyx mori to mulberry defense[END_REF][START_REF] Mithöfer | Plant Defense Against Herbivores: Chemical Aspects[END_REF] ; les glucosinolates sont également toxiques pour de nombreux herbivores incluant les insectes [START_REF] Bones | The myrosinase-glucosinolate system, its organisation and biochemistry[END_REF][START_REF] Borek | Toxicity of rapeseed meal and methyl isothiocyanate to larvae of the black vine weevil (Coleoptera: Curculionidae)[END_REF][START_REF] Lambrix | The Arabidopsis Epithiospecifier Protein Promotes the Hydrolysis of Glucosinolates to Nitriles and Influences Trichoplusia ni Herbivory Author ( s ): Virginia Lambrix[END_REF][START_REF] Hopkins | Role of Glucosinolates in Insect-Plant Relationships and Multitrophic Interactions[END_REF][START_REF] Textor | Herbivore induction of the glucosinolate-myrosinase defense system: Major trends, biochemical bases and ecological significance[END_REF][START_REF] Mithöfer | Plant Defense Against Herbivores: Chemical Aspects[END_REF]. Les métabolites spécialisés sont des molécules complexes qui ont des fonctions protectrices et défensives contre les herbivores ou pathogènes mais également contre des facteurs abiotiques tels que les UV, la sécheresse ou la chaleur [START_REF] Sardans | Ecological metabolomics: Overview of current developments and future challenges[END_REF]. Le rôle des composés spécialisés dans les défenses antiherbivore semble bien admis dans la littérature, cependant il semble être également plus complexe que ce qui est décrit, notamment du fait d'un certain nombre de limites méthodologiques.

Les différentes études sont généralement conduites sur seulement quelques jours du cycle complet des herbivores étudiés. De même, la durée de nos expériences ne couvre pas la totalité du cycle de la processionnaire du chêne, et nous ne pouvons pas exclure le fait que les composés spécialisés des chênes aient des effets sur le temps de développement, le poids final ou la fécondité de la processionnaire du chêne. De plus la concentration des défenses chimiques des plantes a généralement tendance à augmenter au cours de la saison (Feeny 1968a[START_REF] Feeny | Seasonal changes in oak leaf tannins and nutrients as a cause of spring feeding by winter moth caterpillars[END_REF][START_REF] Tikkanen | Phenological variation as protection against defoliating insects: The case of Quercus robur and Operophtera brumata[END_REF][START_REF] Forkner | Feeny revisited: condensed tannins as antiherbivore defences in leaf chewing herbivore communitites of Quercus[END_REF][START_REF] Van Asch | Phenology of Forest Caterpillars and Their Host Trees: The Importance of Synchrony[END_REF][START_REF] Van Asch | Maternal effects in an insect herbivore as a mechanism to adapt to host plant phenology[END_REF] L'ensemble des résultats que nous venons de discuter soulignent deux éléments fondamentaux à prendre en compte dans l'étude de la résistance des plantes aux insectes herbivores : (1) la compréhension des mécanismes de résistance requiert d'aborder ce processus du point de vue de la plante et du point de vue de l'insecte de manière simultanée ;

(2) les mêmes traits pouvant avoir des effets contrastés voire opposés sur différents stades de développement de l'insecte herbivores, il est nécessaire de se donner les moyens d'étudier les mécanismes impliqués sur le temps long, a minima sur l'ensemble du cycle de développement de l'herbivore.

Quelles autres défenses sont impliquées dans la résistance ?

Les traits physiques -Il ne faut pas perdre de vue que si les défenses chimiques des plantes sont une contrainte pour l'insecte herbivore, celui-ci est également contraint par la qualité nutritive ou les traits physiques des tissus qu'il consomme. Ainsi, les effets contrastés des composés spécialisés de défense sur les insectes herbivores pourraient également s'expliquer par l'intervention d'autres traits qui ont un fort effet sur les préférences et les performances des herbivores [START_REF] Nahrung | Larval Gregariousness and Neonate Establishment of the Eucalypt-Feeding Beetle Chrysophtharta agricola (Coleoptera: Chrysomelidae: Paropsini)[END_REF][START_REF] Pérez-Harguindeguy | Leaf traits and herbivore selection in the field and in cafeteria experiments[END_REF][START_REF] Sanson | The biomechanics of browsing and grazing[END_REF][START_REF] Peeters | Leaf biomechanical properties and the densities of herbivorous insect guilds[END_REF][START_REF] Clissold | Gross vs. net income: How plant toughness affects performance of an insect herbivore[END_REF][START_REF] Carmona | Plant traits that predict resistance to herbivores[END_REF][START_REF] Ibanez | Herbivory mediated by coupling between biomechanical traits of plants and grasshoppers[END_REF][START_REF] Malishev | Leaf mechanics and herbivory defence: How tough tissue along the leaf body deters growing insect herbivores[END_REF]. Par exemple, [START_REF] Carmona | Plant traits that predict resistance to herbivores[END_REF] ont montré que les différences de résistances des plantes aux insectes herbivores s'expliquent plus par des différences dans les traits liés au développement (e.g. taux de croissance, phenologie), à la morphologie (e.g. biomasse, hauteur) et aux défenses physiques (e.g. densité de trichome, dureté des feuilles) de la plante, plutôt que les traits chimiques de défenses (e.g. métaboliques spécialisés). Avant l'intervention des traits chimiques de défenses, les traits physiques tels que l'épaisseur et la dureté des tissus, les trichomes ou les épines par exemple, forment donc la première ligne de défense des plantes contre les herbivores et jouent un rôle important dans la résistance de la plantes [START_REF] Hanley | Plant structural traits and their role in anti-herbivore defence[END_REF][START_REF] War | Mechanisms of plant defense against insect herbivores[END_REF]. Ces défenses structurelles comprenant des caractéristiques morphologiques et anatomiques, peuvent conférer un avantage, en termes de fitness, à la plante en empêchant directement les herbivores de se nourrir [START_REF] Agrawal | Plant defense syndromes[END_REF]. Par exemple, le taux d'oviposition de Plutella xylostella (Lépidoptères) était négativement corrélé avec la densité de trichome d'Arabidopsis thaliana [START_REF] Handley | Variation in trichome density and resistance against a specialist insect herbivore in natural populations of Arabidopsis thaliana[END_REF]. Il a également été montré une réduction de la défoliation avec la densité de trichome sur des feuilles [START_REF] Traw | Reduced Performance of Two Specialist Herbivores (Lepidoptera: Pieridae, Coleoptera: Chrysomelidae) on New Leaves of Damaged Black Mustard Plants[END_REF]Dawson 2002, Dalin and[START_REF] Dalin | Adult beetle grazing induces willow trichome defence against subsequent larval feeding[END_REF]) et une augmentation de la densité de trichomes sur les nouvelles feuilles suite à une attaque par un herbivore [START_REF] Agrawal | Induced responses to herbivory in wild radish: Effects on several herbivores and plant fitness[END_REF][START_REF] Traw | Is induction response negatively correlated with constitutive resistance in black mustard?[END_REF][START_REF] Dalin | Adult beetle grazing induces willow trichome defence against subsequent larval feeding[END_REF][START_REF] Björkman | Influence of leaf trichome density on the efficiency of two polyphagous insect predators[END_REF].

Aussi, les dégâts des herbivores semblent fortement liés à la résistance et la dureté des feuilles [START_REF] Caldwell | Which leaf mechanical traits correlate with insect herbivory among feeding guilds?[END_REF]. [START_REF] Clissold | Gross vs. net income: How plant toughness affects performance of an insect herbivore[END_REF] [START_REF] Hochuli | Insect herbivory and ontogeny: How do growth and development influence feeding behaviour., morphology and host use?[END_REF][START_REF] Nahrung | Larval Gregariousness and Neonate Establishment of the Eucalypt-Feeding Beetle Chrysophtharta agricola (Coleoptera: Chrysomelidae: Paropsini)[END_REF][START_REF] Malishev | Leaf mechanics and herbivory defence: How tough tissue along the leaf body deters growing insect herbivores[END_REF] dont la force des mandibules augmente avec la masse de la tête [START_REF] Clissold | The Biomechanics of Chewing and Plant Fracture: Mechanisms and Implications[END_REF].

Les composés organiques volatiles et les ennemis naturels -Les développements récents de l'écologie chimique ont permis de mettre en évidence l'importance des composés organiques émis par les plantes dans les interactions plantes-herbivores. Ces composés volatils agissent non seulement sur l'attractivité des herbivores [START_REF] Bernasconi | Herbivore-induced emissions of maize volatiles repel the corn leaf aphid, Rhopalosiphum maidis[END_REF][START_REF] Aharoni | Terpenoid Metabolism in Wild-Type and Transgenic Arabidopsis Plants[END_REF][START_REF] Laothawornkitkul | Isoprene emissions influence herbivore feeding decisions[END_REF][START_REF] Unsicker | Protective perfumes: the role of vegetative volatiles in plant defense against herbivores[END_REF]) mais aussi sur l'induction de défense [START_REF] Farmer | Surface-to-air signals[END_REF][START_REF] Frost | Within-plant signalling via volatiles overcomes vascular constraints on systemic signalling and primes responses against herbivores[END_REF], 2008) Les dégâts d'insecteset de la processionnairesont donc déterminés par un ensemble de mécanismes intrinsèquement liés à la plante et à sa physiologie, mais également à l'environnement biotique de la plante, notamment par l'intervention des ennemis naturels des herbivores. Cependant, des effets isolés et bien documentés par des expériences en laboratoire sont difficiles à mettre en évidence dans des conditions naturelles, où une multitude de facteurs biotiques et abiotiques interagissent.

Quel est le rôle du voisinage dans les préférences et performances des insectes herbivores ?

L'identité des arbres voisins peut avoir un effet indirect sur les performances des herbivores par la médiation des traits de défenses des arbres hôtes, ceci pouvant être la conséquence de différents mécanismes, tel que la compétition pour les ressources ou l'émission de composés volatiles par les voisins qui vont induire la modification des traits de défense des plantes hôtes [START_REF] Arimura | Plant-plant interactions mediated by volatiles emitted from plants infested by spider mites[END_REF][START_REF] Turlings | Exploiting scents of distress: the prospect of manipulating herbivore-induced plant odours to enhance the control of agricultural pests[END_REF], Agrawal et al. 2006[START_REF] Barbosa | Associational Resistance and Associational Susceptibility: Having Right or Wrong Neighbors[END_REF][START_REF] Ballaré | Light Regulation of Plant Defense[END_REF], Kos et al. 2015a, 2015b, 2015c, Castagneyrol et al. 2017) [START_REF] Arimura | Plant-plant interactions mediated by volatiles emitted from plants infested by spider mites[END_REF][START_REF] Turlings | Exploiting scents of distress: the prospect of manipulating herbivore-induced plant odours to enhance the control of agricultural pests[END_REF][START_REF] Barbosa | Associational Resistance and Associational Susceptibility: Having Right or Wrong Neighbors[END_REF][START_REF] Scala | Green leaf volatiles: A plant's multifunctional weapon against herbivores and pathogens[END_REF]) ait été trop faible pour induire des changements notables dans les traits foliaires de l'arbre attaqué. Les interactions entre plantes au niveau racinaire pourraient également influencer la croissance des arbres et leurs défenses [START_REF] Pineda | Helping plants to deal with insects: The role of beneficial soil-borne microbes[END_REF][START_REF] Van Der Putten | Plant-soil feedbacks: The past, the present and future challenges[END_REF], Kos et al. 2015b, 2015c[START_REF] Correia | Ecological interactions between cork oak (Quercus suber L.) and stone pine (Pinus pinea L.): Results from a pot experiment[END_REF].

Les micro-organismes de la rhizosphère d'une plante peuvent influencer les niveaux de défenses et la qualité nutritive de la plante et agir sur les herbivores [START_REF] Badri | Potential impact of soil microbiomes on the leaf metabolome and on herbivore feeding behavior[END_REF]. Dans les conditions de notre expérience ces processus pourraient expliquer les différences des traits foliaires observées en présence d'un voisin héterospécifique. Toutefois, l'importance de ces effets sur le contrôle des dégâts d'herbivores dans les conditions naturelles reste peu connue.

Dans notre étude nous ne montrons aucun effet clair du voisinage sur les performances de la processionnaire (Chapitre 2). Cependant notre expérience a été réalisé seulement sur les 3 premiers stades larvaires de la CPC, il est donc difficile d'extrapoler les résultats aux temps de développement, à la masse de la chrysalide ou des adultes, ou au taux de fécondité. Aussi, l'identité des voisins peut avoir un effet sur la sélection de l'hôte par la femelle (Chapitre 1 ; [START_REF] Gripenberg | A meta-analysis of preference-performance relationships in phytophagous insects[END_REF]. Par exemple, des études sur la chenille processionnaire du pin (Thaumetopoea pytiocampa) ont montré que la présence de voisins non hôtes pourrait perturber la détection des arbres hôtes par les papillons [START_REF] Jactel | Non-host volatiles mediate associational resistance to the pine processionary moth[END_REF][START_REF] Dulaurent | Hide and seek in forests: Colonization by the pine processionary moth is impeded by the presence of nonhost trees[END_REF][START_REF] Castagneyrol | Plant apparency, an overlooked driver of associational resistance to insect herbivory[END_REF], 2014c). Une meilleure compréhension de l'effet des voisins sur les performances de la processionnaire du chêne nécessiterait donc l'intégration des effets des voisins sur le comportement de sélection de l'hôte par les adultes et ses conséquences sur le développement des larves.

Comment les plantes font elles face à la grande diversité d'ennemies ?

Notre étude se base sur l'interaction entre la chenille de la processionnaire du chêne qui est un 2003[START_REF] Agrawal | Plant defense syndromes[END_REF][START_REF] Rasmann | Plant defense against herbivory : progress in identifying synergism , redundancy , and antagonism between resistance traits[END_REF][START_REF] Carmona | Plant traits that predict resistance to herbivores[END_REF][START_REF] War | Mechanisms of plant defense against insect herbivores[END_REF]. [START_REF] Agrawal | Plant defense syndromes[END_REF] ont proposé le concept de « syndromes de défense » (Plant defense syndromes) qu'ils définissent comme un ensemble de traits regroupant ceux liés à la qualité nutritionnelle de la plante (teneur en eau ou en azote par exemple), à sa toxicité (e.g. métabolites spécialisés), sa phénologie, sa tolérance à l'herbivorie, ses caractéristiques physiques (e.g. épines, trichomes ou dureté) et ses défenses indirectes (e.g. composés volatiles). L'ensemble de ces traits agirait de manière synergique contre les herbivores et offrirait à la plante un niveau de défense supérieur à celui qui serait possible si tous ces traits étaient pris individuellement. Les syndromes de défenses réunissent des traits le plus souvent covariants, et dépendent fortement de l'environnement biotique et abiotique de la plante [START_REF] Agrawal | Plant defense syndromes[END_REF]. Il se peut que les plantes qui partagent les mêmes guildes d'herbivores aient développé la même manière de se défendre [START_REF] Kursar | Convergence in defense syndromes of young leaves in tropical rainforests[END_REF]. Ceci suggère donc que la plante utilise simultanément plusieurs traits de défense en réponse aux différentes attaques.

Dans cette étude nous nous sommes intéressés à un nombre restreint de traits potentiellement liés aux défenses des chênes. Toutefois, il ne faut pas perdre de vue l'implication potentielle d'autres traits que nous n'avons pas pris en compte ( § 1.1.2). Nous avons également montré une réponse différente de la processionnaire du chêne par rapport au bombyx disparate vis-àvis de ces défenses chimiques mesurées (Chapitre 3), ceci impliquant des effets différents sur les dégâts liés au niveau de spécialisation de l'insecte herbivore, suggérant donc l'implication de mécanismes de résistance spécifique. Cependant nous avons également mis en évidence une corrélation génétique entre les performances de la processionnaire et du bombyx disparate (les génotypes conférant des plus grandes performances à la processionnaire, conféraient également de plus grandes performances au bombyx, Chapitre 3), suggérant l'implication d'un ensemble de traits de résistance également efficaces contre ces deux défoliateurs. Nos résultats suggèrent donc un contrôle génétique de la résistance aux herbivores et également l'implication de locus identiques ou liés qui auraient un impact sur les performances de ces deux herbivores défoliateurs. Cela ne signifie pas qu'il y aurait une combinaison de gènes susceptibles de créer un « super-chêne » résistant à tous les agresseurs, mais que certains caractères de résistance aux herbivores seraient assez généraux pour armer la plante face à plusieurs espèces de ravageur.

La résistance aux herbivores est-elle contrôlée génétiquement ?

On trouve dans la littérature, une implication de régions génomiques (locus de caractères quantitatifs ; QTLs) dans la résistance des plantes aux insectes et/ou pathogène ravageurs [START_REF] Risterucci | Identification of QTLs related to cocoa resistance to three species of Phytophthora[END_REF][START_REF] Frei | QTL Mapping of Resistance to Karny in Common Bean[END_REF][START_REF] Fujita | Molecular mapping of a novel gene, Grh5, conferring resistance to green rice leafhopper (Nephotettix cincticeps Uhler) in rice, Oryza sativa L[END_REF][START_REF] Duan | Quantitative Trait Loci Mapping of Resistance to Laodelphax striatellus (Homoptera: Delphacidae) in Rice Using Recombinant Inbred Lines[END_REF][START_REF] Stoeckli | QTL analysis for aphid resistance and growth traits in apple[END_REF][START_REF] Vosman | QTL mapping of insect resistance components of Solanum galapagense[END_REF]. La résistance de la plante hôtes a été classée en trois types de mécanismes : l'antixénose qui permet aux plantes d'empêcher ou de réduire la colonisation par les insectes en perturbant leur comportement, l'antibiose qui agit sur la croissance, le développement, la survie et la reproduction des insectes, et la tolérance qui agit sur la capacité de la plante à supporter les dégâts [START_REF] Painter | Insect resistance in crop plants[END_REF] [START_REF] Agrawal | Plant defense syndromes[END_REF]. L'identification de plusieurs QTL impliqués dans la résistance aux différents herbivores offre la possibilité d'améliorer la durabilité de la résistance des chênes par une possible accumulation de nombreux gènes de résistance différents situés dans différentes régions chromosomiques.

Cependant, dans l'étude de la résistance des plantes aux ravageurs, il semble également important de considérer le compromis entre le développement de la plante et l'allocation des ressources aux défenses [START_REF] Herms | The Dilemma of Plants : To Grow or Defend[END_REF][START_REF] Fine | The growth-defense trade-off and habitat specialization by plants in Amazonian forests[END_REF][START_REF] Huot | Growth-defense tradeoffs in plants: A balancing act to optimize fitness[END_REF][START_REF] Züst | Trade-Offs Between Plant Growth and Defense Against Insect Herbivory: An Emerging Mechanistic Synthesis[END_REF][START_REF] Ballaré | Recalculating growth and defense strategies under competition: key roles of photoreceptors and jasmonates[END_REF], la mise en place de défense impose une demande en énergie et en ressource importante, ce qui suggère une réduction de la croissance.

Un arbre « résistant » serait avantagé par rapport aux autres en cas de pullulation de ravageur, mais en l'absence de pullulation les arbres les plus « résistants » pourraient être défavorisés [START_REF] Ghirardo | Function of defensive volatiles in pedunculate oak (Quercus robur) is tricked by the moth Tortrix viridana[END_REF]. Ces résultats sont en adéquation avec plusieurs études qui ont rapporté une réduction des dégâts par les herbivores dans les mélanges, par rapport aux peuplements purs [START_REF] Jactel | Tree diversity reduces herbivory by forest insects[END_REF], Castagneyrol et al. 2014c[START_REF] Guyot | Tree diversity reduces pest damage in mature forests across Europe[END_REF], Jactel et al. 2017). Cet effet d'association peut s'expliquer par une réduction de la concentration de la ressource (i.e.

l'espèce hôte) dans les peuplements mélangés [START_REF] Root | Organization of a Plant-Arthropod Association in Simple and Diverse Habitats: The Fauna of Collards (Brassica Oleracea)[END_REF][START_REF] Hambäck | Patch area, population density and the scaling of migration rates: the resource concentration hypothesis revisited[END_REF][START_REF] Hambäck | Effects of plant neighborhoods on plant-herbivore interactions: resource dilution and associational effects[END_REF]) et par une interaction hôtenon-hôte directe, telle que les espèces non hôtes masquent physiquement et olfactivement les arbres hôtes ou déclenchent des défenses directes contre les herbivores [START_REF] Zhang | Olfactory recognition and behavioural avoidance of angiosperm nonhost volatiles by conifer-inhabiting bark beetles[END_REF][START_REF] Bruce | Insect host location: a volatile situation[END_REF][START_REF] Jactel | Tree diversity reduces herbivory by forest insects[END_REF][START_REF] Barbosa | Associational Resistance and Associational Susceptibility: Having Right or Wrong Neighbors[END_REF][START_REF] Togni | Odour masking of tomato volatiles by coriander volatiles in host plant selection of Bemisia tabaci biotype B[END_REF], Bruce and Pickett 2011a[START_REF] Jactel | Non-host volatiles mediate associational resistance to the pine processionary moth[END_REF][START_REF] Castagneyrol | Plant apparency, an overlooked driver of associational resistance to insect herbivory[END_REF][START_REF] Alalouni | Does insect herbivory on oak depend on the diversity of tree stands?[END_REF], Jactel et al. 2017). Il est également possible que les ennemis naturels des herbivores soient plus abondants et plus diversifiés dans les peuplements mélangés, exerçant ainsi un plus fort contrôle des populations d'herbivore [START_REF] Root | Organization of a Plant-Arthropod Association in Simple and Diverse Habitats: The Fauna of Collards (Brassica Oleracea)[END_REF][START_REF] Wilby | Natural enemy diversity and pest control: patterns of pest emergence with agricultural intensification[END_REF][START_REF] Sobek | Sapling herbivory, invertebrate herbivores and predators across a natural tree diversity gradient in Germany's largest connected deciduous forest[END_REF][START_REF] Straub | Plant diversity increases herbivore movement and vulnerability to predation[END_REF], Fernandez-Conradi et al. 2018a). Par exemple les oiseaux et les chauves-souris exercent un contrôle biologique des herbivores très efficaces dans les forêts et contribuent à réduire les dégâts des herbivores [START_REF] Böhm | Top-Down Control of Herbivory by Birds and Bats in the Canopy of Temperate Broad-Leaved Oaks (Quercus robur)[END_REF][START_REF] Charbonnier | Deciduous trees increase bat diversity at stand and landscape scales in mosaic pine plantations[END_REF] ont montré que la composition des forêts et en particulier la présence d'essences feuillues (en l'occurrence, non-hôte) dans les peuplements de pins maritimes, augmentait l'activité des chauves-souris et modifiait la composition de la communauté de chauves-souris qui sont prédateurs des papillons de la processionnaire du pin. Les prédateurs et parasitoïdes seraient donc plus abondants et plus efficaces dans les peuplements mélangés. Il est vraisemblable que l'ensemble de ces mécanismes interviennent également dans la plus grande résistance des peuplements mixtes vis-à-vis de la chenille processionnaire du chêne. Toutefois, leur importance relative reste à évaluer pour ce modèle particulier.

Bien que dans la majorité des travaux publiés les auteurs aient documenté une réduction des dégâts d'insectes dans les peuplements mélangés, on ne peut pas occulter les exemples du phénomène inverse [START_REF] Schuldt | Tree diversity promotes insect herbivory in subtropical forests of south-east China[END_REF], 2015[START_REF] Haase | Contrasting effects of tree diversity on young tree growth and resistance to insect herbivores across three biodiversity experiments[END_REF][START_REF] Wein | Tree Species Richness Promotes Invertebrate Herbivory on Congeneric Native and Exotic Tree Saplings in a Young Diversity Experiment[END_REF], Castagneyrol et al. 2018a). Par exemple, [START_REF] Grossman | Neighborhood diversity simultaneously increased and decreased susceptibility to contrasting herbivores in an early stage forest diversity experiment[END_REF] (Lépidoptères) dont les dégâts seraient plus importants en l'absence H. sulawesi [START_REF] Wielgoss | A minor pest reduces yield losses by a major pest: Plant-mediated herbivore interactions in Indonesian cacao[END_REF]. L'efficacité des mesures de lutte contre les ravageurs ne doit pas seulement être mesurée sur l'herbivore étudié, mais doit également prendre en compte les effets sur les espèces susceptibles de modifier directement les traits de la plante hôte et les autres espèces d'herbivores. La diversité des forêts jouerait un rôle important dans la résistance des chênes à la chenille processionnaire, cependant au vu des résultats tirés de la littérature, nous ne pouvons pas tirer de conclusions hâtives quant à la gestion des peuplements forestiers vis-àvis des attaques par la processionnaire du chêne sans intégrer dans les études les différents facteurs biotiques potentiellement en interaction avec la processionnaire.

Les programmes de gestion des chênaies doivent-ils favoriser le chêne pédonculé au détriment du chêne sessile ?

Dans notre étude nous avons mis en évidence un effet de l'espèce hôte et de la composition du peuplement sur les préférences et performances de la chenille processionnaire du chêne.

Nous avons notamment montré une plus grande susceptibilité du chêne sessile, caractérisée par une plus grande attractivité pour les papillons du mâle de la CPC, par un niveau de défoliation plus élevé sur chêne sessile et également par une plus grande survie et prise de poids des larves sur cette essence. Le chêne sessile semble donc être une espèce particulièrement sensible aux attaques de la processionnaire, et donc non préconisée dans les programmes de gestion des chênaies pour une meilleure résistance à la processionnaire.

Ces différences pourraient potentiellement s'expliquer par des différences de traits des deux espèces. Bien que, à notre connaissance, il n'existe pas d'étude comparative des traits foliaires de défense de ces deux espèces de chênes, plusieurs études ont montré que le bois du chêne sessile se caractérisait par des taux plus élevés de composés volatiles, mais également par une quantité plus faible d'ellagitannins par rapport au chêne pédonculé [START_REF] Mosedale | Variation of heartwood phenolics and oak lactones between the species and phenological types of Quercus petraea and Q. robur[END_REF][START_REF] Feuillat | Variation in the concentration of ellagitannins and cis-and trans-β-methyl-γ-octalactone extracted from oak wood (Quercus robur L., Quercus petraea Liebl.) under model wine cask conditions[END_REF][START_REF] Mosedale | Variability of wood extractives among Quercus robur and Quercus petraea trees from mixed stands and their relation to wood anatomy and leaf morphology[END_REF][START_REF] Prida | Effect of species and ecological conditions on ellagitannin content in oak wood from an even-aged and mixed stand of Quercus robur L. and Quercus petraea Liebl[END_REF], 2007). Sous réserve que ces observations faites dans les tissus ligneux puissent être extrapolées aux feuilles, elles sont en adéquation avec nos résultats. La différence de dégâts entre les espèces de chênes pourrait également être due à une différence de qualité des feuilles entraînant une modification du taux de consommation de larves de la CPC. Plusieurs études ont montré une modification du taux de consommation avec la qualité foliaire de l'hôte [START_REF] Lazarevic | Adaptation of the gypsy moth to an unsuitable host plant[END_REF][START_REF] Foss | Species-specific differences in oak foliage affect preference and performance of gypsy moth caterpillars[END_REF][START_REF] Barbehenn | Hydrolyzable tannins as "quantitative defenses": Limited impact against Lymantria dispar caterpillars on hybrid poplar[END_REF], Milanović et al. 2014, Chapitre 3). Cependant, il n'est pas certain que les différences d'attraction des adultes et de défoliation par les larves entre les espèces hôtes résultent de différences dans l'abondance de la processionnaire ou de différences dans leur comportement de recherche et alimentaire. Des études supplémentaires sont indispensables pour identifier les traits de défenses impliqués dans la résistance à la processionnaire et dans la différence entre les essences, et améliorer notre compréhension de l'interaction chêne-chenille processionnaire.

Au vu de ces résultats, il semblerait donc raisonnable, s'il s'agit de prévenir en partie les attaques de la processionnaire du chêne, de favoriser le chêne pédonculé dans les programmes de gestion des chênaies au détriment du chêne sessile qui est plus susceptible aux attaques de ce ravageur. Cependant, face aux changements globaux actuels, notamment caractérisés par une augmentation de la fréquence et de l'intensité des événements extrêmes telles que les sécheresses ou les tempêtes (IPCC 2014), les gestionnaires des écosystèmes forestiers doivent faire face à des conditions abiotiques changeantes pouvant affecter directement le développement des arbres. De nombreuses études suggèrent que les modifications des conditions abiotiques (e.g. températures et précipitations) modifient la répartition des arbres [START_REF] Pucko | Species shifts in response to climate change: Individual or shared responses?[END_REF][START_REF] Ruiz-Labourdette | Changes in tree species composition in Mediterranean mountains under climate change: Indicators for conservation planning[END_REF][START_REF] Ozolinčius | Lithuanian forests and climate change: Possible effects on tree species composition[END_REF]. Bien que les chênes pédonculés et les chênes sessiles puissent coexister en mélange, leurs différences écologiques sont telles qu'ils sont généralement trouvés dans des stations différentes. Le chêne pédonculé est une espèce qui a une demande en eau et en nutriments plus importante que le chêne sessile et tolère les inondations périodiques [START_REF] Eaton | Quercus robur and Quercus petraea in Europe: distribution, habitat, usage and threats[END_REF]. Le chêne sessile est quant à lui plus tolérant aux événements de sécheresse et pousse préférentiellement sur des sols bien drainés [START_REF] Eaton | Quercus robur and Quercus petraea in Europe: distribution, habitat, usage and threats[END_REF]. [START_REF] Friedrichs | Complex climate controls on 20th century oak growth in Central-West Germany[END_REF]) et Becker et al. (1994) ont montré par analyse de dendrochronologie que le chêne sessile maintenait une plus forte croissance que le chêne pédonculé, particulièrement pendant les années sèches. Au vu des contraintes écologique des deux espèces et de leur niveau de tolérance aux événements de sécheresse, les scénarios prévoient un déclin progressif du chêne pédonculé dans les forêts tempérées au profit du chêne sessile, plus résistant au déficit hydrique [START_REF] Durand | Le dépérissement du C hêne pédonculé dans les Pyrénées-Atlantiques[END_REF][START_REF] Lévy | A comparison of the ecology of pedunculate and sessile oaks: Radial growth in the centre and northwest of France[END_REF][START_REF] Langenheim | Higher plant terpenoids: A phytocentric overview of their ecological roles[END_REF][START_REF] Arend | Seasonal photosynthetic responses of European oaks to drought and elevated daytime temperature[END_REF].

Il semblerait donc que la prise en compte de la problématique de la processionnaire du chêne dans les programmes de gestion des chênaies ne fasse que rajouter un élément de complexité à la gestion de la résistance des forêts face à des perturbations multiples en interaction.

Toutefois, notre analyse n'a pas pris en compte les contraintes abiotiques auxquelles peuvent être soumis les chênes. Le chêne pédonculé semble être plus résistant vis-à-vis de la chenille processionnaire du chêne, mais en condition de stress hydrique et dans des conditions environnementales défavorables à son développement, le chêne pédonculé pourrait être défavorisé. Ces contraintes abiotiques, comme le stress hydrique et les vagues de chaleur ont des effets directs et indirects bien documentés sur les performances des insectes herbivores [START_REF] Walter | How do extreme drought and plant community composition affect host plant metabolites and herbivore performance?[END_REF], Castagneyrol et al. 2018b, 2018a[START_REF] Netherer | Acute Drought Is an Important Driver of Bark Beetle Infestation in Austrian Norway Spruce Stands[END_REF]. La gestion des forêts de chêne devra prendre en compte les stresses multiples en interaction, et ne pas se focaliser uniquement sur la seule problématique de la chenille processionnaire du chêne.

Conclusion

L'ensemble de nos résultats ont permis de mieux comprendre les processus impliqués dans les interactions plantes-insectes, à la fois du point de vue de l'herbivore et de celui de la plante,

"

  Ah fair enough!"… Et bien je me lance alors ! Pour commencer je remercie mes financeurs qui ont permis le bon déroulement de mes travaux de thèse, l'INRA, l'ONF et le DSF. Merci à Bastien et Christophe de m'avoir fait confiance quand je leur ai déposé ma candidature pour ce sujet de thèse pour le moins urticant (il fallait que je la fasse, on s'en débarrasse tout de suite).

  Merci à Véronique, Chantal, Florence, Marie et Loïc pour leur soutien administratif et informatique et sans lesquelles j'aurai été bien embêté ! Merci aux collègues de Pierroton et de l'Université pour avoir supporté mes « discrètes » interventions dans les bureaux et les couloirs. Mention spécial pour Jennifer, cette machine de m**** n'a pas réussi à nous achever. Myriam G. merci pour les cours de lancer de babouche, et ENFIN on va pouvoir se consacrer pleinement à « Wesh !! » ET SURTOUT !!!! Merci à Elena, ma colocataire de bureau, je n'ai aucun défaut à te donner (et dieu sait que je peux trouver des défauts aux gens). Merci de m'avoir soutenu, d'avoir supporté mes sautes d'humeur, mes chants et mes chansons pourries, mes danses endiablées et mes crises artistiques (les papillons resteront à jamais dans ce bureau). Merci d'avoir soutenu activement mon côté asocial (je voulais aller chercher de l'eau mais y a des gens en bas … tant pis j'attends). MERCI POUR TOUT, NE CHANGE RIEN TU ES PARFAITE !!!!! (« Ouille, Ouille, Ouille !!! ») Merci à ma famille, mes parents, mon frère et ma soeur, Joséphine et Juliane pour leur soutien inconditionnel ! VRAIMENT, MERCI !!!!! En bref, merci à tous ceux qui ont activement ou passivement participé au bon déroulement de cette thèse !! Merci !!! SOMMAIRE 1. Introduction ...................................................................................................................... 15 1.1. Les insectes ravageurs : une problématique écologique et économique ................... 16 1.2. Traits foliaires impliqués dans la résistance des arbres aux insectes défoliateurs ..... 17 1.2.1. Qualité nutritive et métabolites spécialisés : des défenses contre les herbivores défoliateurs ....................................................................................................................... 18 1.3. Spécialiste vs généraliste : deux stratégies alimentaires ............................................ 19 1.4. Résistance et susceptibilité par association dans les peuplements mélangés : patterns et mécanismes ...................................................................................................................... 20 1.4.1. Effet de la dilution de la ressource sur la susceptibilité vis-à-vis des herbivores 21 1.

2. 4 .

 4 Discussion .................................................................................................................. 41 3. Quels effets ont l'espèce hôte et le voisinage sur les traits foliaires et les performances de la processionnaire ? .................................................................................................................. 47 3.1. Introduction ............................................................................................................... 48 3.2. Materials and methods ............................................................................................... 50 3.3. Results ....................................................................................................................... 54 3.3.1. Effect of focal and neighbour tree species identity on OPM performance, leaf traits and plant phenology ................................................................................................ 54 3.3.2. Leaf traits associated with effects of focal and neighbour tree species identity on OPM performance ............................................................................................................ 57 3.4. Discussion .................................................................................................................. 59 3.4.1. Effects of oak species identity on OPM performance and leaf traits ................. 59

Figure 3AB :

 3AB Figure 3AB: (A) Geographical distribution and number of male OPM moths captured in 33 departments of France. Pie size is proportional to the total number of catches in the triplet.

Figure 4 :

 4 Figure 4: Effects of tree species identity and stand species composition on OPM defoliation (mean  se). Asterisks indicate significant differences in defoliation between the two oak species (all P < 0.001). Letters indicate significant differences in defoliation by OPM between the three composition treatments. Oak = Oak pure stands, Q. petraea + Q. robur = mixture of Q. petraea and Q. robur and Oak + Broadleaved = mixture of one oak species with another broadleaved species).

3 .

 3 Quels effets ont l'espèce hôte et le voisinage sur les traits foliaires et les performances de la processionnaire ? Damestoy T., Moreira X., Jactel H., Valdes-Correcher E., Plomion C., Castagneyrol B. -Growth and mortality of the oak processionary moth, Thaumetopoea processionea L., on two oak species: direct and trait-mediated effects of host and neighbour species identity -Journal of Pest Science -Editor Assigned -03/10/19

  Oak and OPM phenology -We checked egg masses every day from 4 to 30 April 2018 and noted OPM developmental stage (unhatched eggs, L1, L2, L3), in order to estimate proportion of each larval instar at the end of our experiment. Within OPM colonies, individual development is synchronized because hatching and moulting are synchronous among larvae from the same egg mass. We recorded the developmental stage of the terminal bud of each oak at the time OPM eggs hatched, using a seven-level ordinal scale from 0 to 6 (0 = dormant bud, 1 = bud swollen, 2 = bud open, 3 = beginning of leaf expansion, 4 = one leaf free, 5 = internodes are elongating, 6 = fully expanded leaves)[START_REF] Ducousso | Latitudinal and altitudinal variation of bud burst in western populations of sessile oak (Quercus petraea (Matt) Liebl)[END_REF][START_REF] Derory | Contrasting relations between diversity of candidate genes and variation of bud burst in natural and segregating populations of European oaks[END_REF]).

Figure 5AB :Figure 6 :

 5AB6 Figure 5AB: Effect of focal (symbols) and neighbour (colour) tree species identity on OPM mortality (A) and growth rates (B). Small symbols represent raw data. Large symbols and vertical error bars represent raw means ± SE. Letters above dots represent outputs of posthoc tests. Different letters indicate statistically significant differences between treatments

Figure 7AB :

 7AB Figure 7AB: Effect of bud developmental stage (measured with a seven-levels ordinal scale from 0 [closed apical bud] to 6 [fully expanded leaves]) at the time OPM hatched on OPM growth rate (A) and mortality (B). Symbols and vertical error bars represent raw means ± SE, black squares are for Q. robur and white triangles for Q. petraea. Letters above dots represent outputs post-hoc tests. Different letters indicate statistically significant differences between bud developmental stages

  trees,(Damestoy et al. under review) found that Q. petraea was consistently more attractive to OPM moths (i.e. more captures of moths in Q. petraea stands by pheromone trapping) and more defoliated than Q. robur(Damestoy et al. under review). Similarly, in a greenhouse experiment with one-year-old oak saplingsMoreira et al. (2018a) found that leaf damage by gypsy moth larvae (Lymantria dispar) was significantly greater on Q. petraea than on Q. robur.

Figure 8 :

 8 Figure 8: Proportion of plant with each of larval instar in each treatment at the end of the experiment. L1 = light grey, L2 = mid grey, L3 = dark grey.

Figure 9 :

 9 Figure 9: Schematic representation of nutritional indices and their ANCOVA equivalents. For each nutritional index (AD: Approximate Digestibility, ECI: Efficiency of Conversion of Ingested food, ECD: Efficiency of Conversion of Digested food, RGR: Relative Growth Rate, RCR: Relative Consumption Rate), formula and ANCOVA equivalents are shown. For instance, the ANCOVA equivalent of RGR takes Growth Rate (G) as a response variable and Initial Weight (w t=0 ) as a covariate such that RGR is given by (G/w t=0 ). G: Larval Growth (w fw t=0 ), C: Leaf Biomass Consumption, F: Frass Production, D: Leaf Biomass Digestion (C -F).

Figure 10 :

 10 Figure 10: Oak phenolic profiles. Average percentage of the four groups of phenolic compounds in the leaf samples of each of the 30 pedunculate oak genotypes. Numbers on the x-axis correspond to oak genotype label.

Figure 12ab :

 12ab Figure 12ab: Percentage of variance explained by oak genotype on herbivore physiological traits (G, C and D) (a) and leaf phenolic compounds (b). Histograms represent the percentage of variance explained by oak genotype for each trait in intercept only LMM. Error bars represent 95 % confidence interval (estimated with bootstraps).

Figure 13ab :

 13ab Figure 13ab: Genetic correlations between consumption (a), and digestion (b) of both herbivore species. Dots represent genotype-specific means. Regression line and grey areas represent predictions from linear models and corresponding SE. Spearman's correlation coefficients, rho, and the equation of regression line and their p-values are indicated in each panel. GM = Gypsy Moth; OPM = Oak Processionary Moth.

  qui sont deux exemples marquants de cette réponse au changement climatique. Nos résultats contribuent à la meilleure compréhension de l'interaction chênechenille processionnaire et des mécanismes sous-jacents. Nous avons mis en évidence une plus grande susceptibilité du chêne sessile, caractérisée par une plus grande attractivité pour les papillons de la CPC (Chapitre 1), par un niveau de défoliation plus élevé (Chapitre 1) et également par une plus grande survie et prise de poids des larves sur cette essence (Chapitre 2). Nous avons aussi montré une résistance par association apportée par les essences non hôtes dans les peuplements mélangés (Chapitre 1). En mesurant les traits foliaires (notamment : métabolites spécialisés, C:N, et stade phénologique des bourgeons à l'éclosion des larves), nous avons cherché à caractériser les mécanismes conférant une plus grande résistance au chêne. Nous avons montré que la synchronisation entre le développement des bourgeons et celui des larves est un facteur clé déterminant les performances de la CPC (Chapitre 2, van Asch and Visser

  ont montré une corrélation négative entre le taux de croissance d'Australian plague (Orthoptères) et la dureté des feuilles d'Astrebla lappacea et une corrélation positive entre le temps de développement et la dureté des feuilles. Les différents stades larvaires peuvent également avoir des comportements différents (Johnson and Zalucki 2007) notamment vis-à-vis des traits physiques. Par exemple chez les insectes masticateurs les stades les plus jeunes ont plus de difficulté à se nourrir sur des feuilles dures que les stades larvaires les plus avancés

L 5 . 5 .

 55 'étude des interactions plante-ravageurs à des fins de gestion et de contrôle des populations doit donc prendre le problème dans son ensemble et étudier les différentes interactions possiblement liées à la dynamique de population de l'insecte ravageur étudié : l'ensemble de traits potentiellement lié à la défense de la plante (directe et indirecte) et les interactions avec les autres niveaux trophiques. Au-delà d'un contrôle génétique des traits de défenses, les arbres vivent dans un environnement biotique et abiotique complexe et changeant, ainsi les interactions plantes insectes ne se limitent pas à une simple interaction entre deux organismes mais intègrent une variété d'interactions entre ravageur mais aussi entre les plantes qui peuvent influencer les relations plantes insectes. Le voisinage des arbres hôtes pourrait avoir un effet direct sur les traits de défenses de l'arbre hôte modifiant les préférences et les performances des herbivores. Implications pour la gestion des chênaies pour la résistance à la chenille processionnaire 5.5.1. La diversité est-elle une barrière face aux ravageurs ? Nous avons mis en évidence que les peuplements mélangés associant des chênes à d'autres essences feuillues non consommées par la processionnaire du chêne subissent en moyenne moins d'attaques par ce ravageur (Chapitre 1). Ceci suggère donc un effet positif de la diversité des essences forestières sur la résistance des chênes à la chenille processionnaire. Il est notable que cet effet bénéfique du mélange d'essences était comparable pour le chêne sessile et le chêne pédonculé. D'après ces résultats, la gestion de la composition spécifique des peuplements forestiers apparaît donc comme un levier potentiel pour augmenter la résistance des chênes à la chenille processionnaire.

  ont rappelé les effets contrastés de la diversité des arbres selon l'espèce hôte étudiée. La diversité augmentait les dégâts chez certaines essences et les diminuait chez d'autres : les chênes étaient plus résistants aux insectes mineurs dans un environnement diversifié, tandis que les bouleaux y étaient plus susceptibles. De même,[START_REF] Setiawan | The effects of local neighbourhood diversity on pest and disease damage of trees in a young experimental forest[END_REF] ont montré que le niveau des dégâts des herbivores défoliateurs sur Q. robur était plus élevé en monoculture qu'en mélange avec le bouleau Betula pendula, probablement dû au fait que les bouleaux étaient plus haut que les chênes. Inversement, ils montrent un plus haut niveau de dégât par les défoliateurs sur Q. petraea dans les mélanges avec le hêtre, Fagus sylvatica.[START_REF] Alalouni | Does insect herbivory on oak depend on the diversity of tree stands?[END_REF] ont montré que les dégâts sur les chênes diminuaient avec l'augmentation de la diversité des peuplements. Au contraire,[START_REF] Wein | Tree Species Richness Promotes Invertebrate Herbivory on Congeneric Native and Exotic Tree Saplings in a Young Diversity Experiment[END_REF] n'ont montré aucun effet de la diversité sur les dégâts par les herbivores sur les chênes pédonculés. Dans une méta-analyse,[START_REF] Vehviläinen | Tree species diversity influences herbivore abundance and damage: meta-analysis of long-term forest experiments[END_REF] ont mis en évidence une défoliation des chênes plus élevée dans les peuplements en mélange. Selon l'identité des essences associées au chêne, celui-ci peut subir plus ou moins de dégâts. Dans le cas de la processionnaire, les travaux que nous avons menés sur le terrain plaident en faveur de l'association chêne-charme ou chêne-hêtre comme moyen de réduire les niveaux d'attaques par la processionnaire. La chenille processionnaire du chêne étant relativement spécialisée sur le genre Quercus, il est vraisemblable que cet herbivore causerait moins de dégâts quelles que soient les essences qui lui seraient associées(Castagneyrol et al. 2014b), mais cela reste une extrapolation de cas d'études sur d'autres modèles biologiques.Toutefois, plusieurs points de vigilance sont à rappeler avant que de ne considérer les mélanges d'essences comme une panacée contre la chenille processionnaireou tout autre ravageurs. Il convient notamment de rappeler que l'augmentation de la diversité des arbres dans les forêts se fait nécessairement au détriment de la densité de chaque espèce : plus d'espèces, c'est proportionnellement moins de chêne, ou moins de place pour le chêne. Toute stratégie de gestion visant à promouvoir la diversité des essences en forêt pour réduire l'impact des herbivores se heurte donc potentiellement à d'autres objectifs de gestion comme la production de telle ou telle essence. De plus, on gardera à l'esprit d'une part que pour une même essence hôte tous les herbivores ne répondent pas de la même manière à la composition spécifique d'un peuplement mélangé et d'autre part que les herbivores ou les pathogènes s'attaquant simultanément ou séquentiellement à un même arbre sont en interaction[START_REF] Kaplan | Interspecific interactions in phytophagous insects revisited: A quantitative assessment of competition theory[END_REF][START_REF] Wielgoss | A minor pest reduces yield losses by a major pest: Plant-mediated herbivore interactions in Indonesian cacao[END_REF], Fernandez-Conradi et al. 2018b). Les dégâts par la processionnaire du chêne semblent être réduits dans les peuplements mélangés, cependant il faudrait ajouter à cette étude l'effet de la diversité des peuplements sur d'autres ravageurs susceptibles d'interagir avec elle comme le bombyx disparate ou l'oïdium du chêne par exemple. Les effets d'association sur le chêne pourraient être positifs vis-à-vis de la processionnaire du chêne mais pourraient également avoir des conséquences positives ou négatives sur d'autres ravageurs ou pathogènes. Par exemple il a été montré dans des plantations de cacao que l'interaction entre deux insectes ravageurs du cacaoyer, Theobroma cacao, a eu un effet positif sur le rendement par rapport à une attaque par un seul des deux insectes. L'utilisation d'insecticide comme moyen de gestion contre Helopeltis sulawesi (Hémiptères) serait contre-productive et profiterait à Conopomorpha cramerella

  en prenant en compte différentes échelles spatiales et niveaux d'intégration (de la forêt à l'arbre). Nous avons montré une résistance par association apportée par la diversité des arbres en forêts, en accord avec la littérature et se traduisant par une réduction des dégâts sur les arbres hôtes. Nos résultats ont permis également de mettre en évidence des différences de susceptibilité de différentes essences hôtes aux attaques d'un même ravageur, se traduisant par une différence dans les préférences (i.e. attractivité des adultes) et les performances (i.e. consommation, croissance, survie) des herbivores. En mesurant les traits foliaires de l'hôte nous avons pu caractériser les mécanismes conférant une plus grande résistance aux plantes sur le terrain comme en laboratoire. Nous avons montré que la synchronisation entre le développement des bourgeons et celui des larves était un facteur clé déterminant les performances de l'herbivore et que la concentration en métabolites spécialisés (notamment les tannins condensés) était positivement corrélée au taux de consommation des feuilles par les insectes suggérant une surconsommation des feuilles les plus défendues pour maintenir une croissance stable. Ces résultats en particulier, remettent en cause l'efficacité des composés phénoliques des feuilles en tant que défenses chimiques de la plante et soulignent l'importance de prendre également en compte la réponse comportementale des herbivores à la variation des composés phénoliques de leur hôte. Nous avons mis en évidence que l'expression de certains de ces traits était influencée par l'identité des voisins des chênes étudiés sans pour autant montrer d'effet de ces voisins sur les performances des insectes. Inversement, nous soutenons peu l'idée que le génotype de la plante hôte ait dicté la variabilité dans les traits chimiques de l'hôte mais nous avons observé que la variabilité dans les performances des herbivores était fortement expliquée par le génotype de l'hôte. Ces résultats mettent en évidence un contrôle au moins partiel des traits des plantes par leurs voisins de même qu'un contrôle génétique de l'hôte sur les performances des herbivores et d'autres traits de défenses non mesurés sous contrôle génétique qui pourraient conférer une résistance contre plusieurs types d'herbivores.L'ensemble de ces résultats permet également d'apporter des éléments de connaissances permettant d'orienter la gestion des peuplements de chênes pour une meilleure résistance à la chenille processionnaire du chêne. Favoriser les peuplements mélangés permettrait de réduire les dégâts sur les chênes, et il semblerait préférable de favoriser le chêne pédonculé dans les peuplements gérés, pour sa moindre attractivité des adultes mâles et sa plus grande résistance qui se traduisait par une plus faible survie et croissance des larves, sur le terrain comme en laboratoire. Cette proposition contredit les préconisations d'utilisation du chêne sessile pour adapter les chênaies au risque de déficit hydrique et ajoute un élément de complexité à la gestion de la résistance des forêts face à des perturbations multiples en interaction. De plus, à l'aune de la mise en évidence de l'effet du génotype du chêne et de ses traits foliaires dans le développement de la chenille processionnaire du chêne, la caractérisation de régions génomiques impliquées dans la résistance aux herbivores apporterait des connaissances supplémentaires sur l'implication des phénotypes de défense des chênes contre la processionnaire et offrirait la possibilité d'améliorer la durabilité de la résistance des chênes à la chenille processionnaire, en identifiant les réponses spécifiques, potentielles, des chênes aux attaques par ce ravageur spécialiste. L'étude de l'effet des traits de défense de l'arbre, incluant les traits physiques et chimiques, sur l'ensemble du cycle de développement de la processionnaire du chêne, incluant les préférences d'hôtes par les femelles gravides et le développement complet des larves, apporterait des éléments de compréhension complémentaires à la caractérisation de la résistance des chênes établie dans cette étude, avant de se placer à une plus grande échelle spatiale, faisant intervenir un environnement biotique et abiotique changeant pouvant interagir avec cet insecte ravageur pour le moins urticant.

Effet de la dilution de la ressource sur la susceptibilité vis- à-vis des herbivores

  des forêts pures et dans des forêts mélangées composées d'espèces étroitement apparentées, car ces forêts mélangées étaient perçues comme une ressource homogène comparable à des peuplements purs. Au sein d'un peuplement forestier, la décision d'un herbivore d'exploiter telle ou telle ressource peut être affectée par la qualité perçue de cette ressource, ainsi que par la perception de la qualité relative des voisins.

	Verschut et al. (2016) ont montré que lorsqu'ils offraient le choix entre deux types ressources,
	la ressource la plus apparente avait plus de chances d'être sélectionnée par l'insecte
	lorsqu'elle était associée à une ressource moins apparente. Selon l'hypothèse
	« attractant -decoy », les herbivores pourraient s'agréger sur les hôtes les plus apparents, plus
	attrayants ou plus appétissants (Peter R. Atsatt and Dennis J. O'Dowd 1976, Hahn and Orrock

les herbivores généralistes pourraient causer plus de dégâts dans les mélanges d'espèces (i.e. susceptibilité par association) en se nourrissant sur un plus large spectre d'espèces hôtes et ainsi bénéficier d'un régime alimentaire complémentaire (Barbosa et al. 2009). 1.4.1. L'abondance absolue et relative des plantes hôtes et des plantes non hôtes contribue donc à déterminer ce que les herbivores perçoivent comme ressources appropriées ou non. Dans une méta-analyse des effets de la diversité des arbres sur les insectes herbivores, Castagneyrol et al. (2014a) ont montré que les arbres étaient autant susceptibles d'être attaqués par des herbivores spécialistes dans 2016) et ainsi épargner des voisins potentiellement consommables, mais moins attractifs. Ceci suggère que dans une forêt mélangée associant plusieurs plantes hôtes utilisées par un herbivore mais dont la qualité perçue diffère, les hôtes préférés pourraient être plus susceptibles d'être attaqués que dans les peuplements purs (i.e. susceptibilité par association), alors que les hôtes les moins préférés seraient moins susceptibles d'être attaqués (i.e. résistance par association).

  ont montré que les composés volatiles émis par des plants infestés de haricot de Lima (Phaseolus lunatus L.) activaient la transcription de gènes de défenses chez les plants intacts. A l'échelle plus large d'un dispositif expérimental manipulant la richesse

spécifique des arbres,

[START_REF] Castagneyrol | Bottom-up and top-down effects of tree species diversity on leaf insect herbivory[END_REF]

ont montré que l'épaisseur et la dureté des feuilles de chêne étaient moins importantes en présence de pin maritime. Le voisinage joue donc un rôle crucial dans le choix de la plante hôte par l'insecte et dans les performances de ce dernier en agissant sur les traits physiques et chimiques de la plantes hôte.

1.6. Problématique et système d'étude

	arbres : la composition spécifique du peuplement, les effets de voisinage et les traits foliaires
	potentiellement liés à la défense.
	al.
	2019). La plupart de ces études rapportent des effets de la diversité génétique sur les
	communautés d'arthropodes (herbivores et prédateurs) avec une augmentation de la diversité
	et de l'abondance des herbivores et des prédateurs en fonction du niveau de diversité des
	plantes hôtes, pouvant directement être liées aux dégâts par les insectes herbivores. Par
	ailleurs, Koricheva and Hayes (2018) ont montré dans leur méta-analyse, que les dégâts
	causés par les herbivores généralistes étaient réduits par la diversité génétique alors que ceux
	causés par les herbivores spécialistes n'étaient pas affectés.
	La diversité, qu'elle soit génétique ou spécifique, joue donc un rôle fondamental dans la
	structuration des communautés, en agissant (i) directement sur les mécanismes de
	reconnaissance par les insectes et de protection de la plante hôte et (ii) indirectement par la
	modulation des traits liés aux défenses des plantes hôtes agissant sur les performances de
	l'insecte.
	Les insectes herbivores doivent donc surmonter plusieurs barrières pour pouvoir (i)
	sélectionner la plante hôte, (ii) se nourrir, (iii) se développer et (iv) se reproduire. La sélection
	de la plante hôte est le premier mécanisme agissant sur les interactions plantes insectes, avec
	l'intervention de trait d'attraction et/ou de répulsion émis à la fois par la plante hôte et par les
	voisins. Une fois la plante hôte trouvée, l'insecte herbivore doit faire face à tout un arsenal de
	mécanismes de défenses physiques ou chimiques dont la variabilité dépend à la fois de la
	plante hôte et de l'identité de ses voisins (environnement biotique) (Fig. 1). Dans cette étude,
	nous nous sommes focalisés sur la question de la résistance des forêts de chênes à un ravageur
	défoliateur, la chenille processionnaire du chêne (CPC), Thaumetopoea processionea L. Au
	travers d'une série d'observations et d'expériences menées à plusieurs niveaux d'intégration
	(depuis le génotype de l'arbre jusqu'au peuplement forestier), nous nous sommes intéressés à
	la réponse de cet insecte à différents facteurs écologiques intervenant dans la résistance des

Figure 1 : Schéma récapitulatif des processus affectant les préférences et performances des insectes herbivores.

1.6.1. La chenille processionnaire du chêne : problème de santé des forêts et humaine

	avant le débourrement des chênes. Une forte synchronisation entre éclosion et débourrement a
	été observée dans plusieurs études, où l'éclosion des oeufs est antérieure de quelques jours
	La chenille processionnaire du chêne est un des principaux défoliateurs des forêts voire de quelques semaines au débourrement des chênes (van Asch and Visser 2007,
	Européennes (Wagenhoff and Veit 2011). Présente principalement en Europe Centrale et au Wagenhoff and Veit 2011, Meurisse et al. 2012, Wagenhoff et al. 2014). Les néonates sont
	Moyen-Orient (Groenen and Meurisse 2012), la France n'en est pas moins épargnée, les capables de survivre à une famine d'environ 2-3 semaines (Wagenhoff and Veit 2011,
	départements du Nord Est enregistrant les plus fortes pullulations du pays (DSF 2006). En
	plus de réduire la croissance des chênes qu'elle défolie, la chenille processionnaire occasionne
	de fortes réactions allergiques chez les usagers des forêts (Lamy and Novak 1987, Maier et al.
	2003, Mindlin et al. 2012). Les soies urticantes libérées à partir du 4ème stade larvaire sont
	présentes en abondance dans les nids et sont très facilement transportées par le vent. La
	réduction des attaques de la chenille sur les peuplements forestiers est donc un enjeu sanitaire
	important, non seulement pour la santé des forêts, mais également pour les problèmes de santé
	humaine qu'elle occasionne.
	La chenille processionnaire du chêne est l'un des ravageurs du genre Quercus, principalement
	Q. robur, Q. petraea et Q. cerris en Europe. En cas de forte pullulation, les larves peuvent se
	diriger vers d'autres essences de feuillus, comme le charme, le bouleau ou le hêtre par
	exemple, cependant il semble peu probable qu'elle puisse y achever son cycle (Stigter et al.
	1997). La processionnaire du chêne est une espèce univoltine, les adultes, de couleur grise et
	mesurant 2-3 cm, émergent de leur chrysalide au cours de l'été. Ces papillons de nuit ont une
	courte durée de vie, ils ne se nourrissent pas et dépendent des réserves accumulées pendant la
	phase larvaire ; ils survivent quelques jours pour la reproduction et la ponte. Les mâles ont
	une capacité de dispersion supérieure aux femelles et meurent une fois accouplés, les femelles
	quant à elles effectuent un dernier vol pour pondre leurs oeufs (Stigter et al. 1997, Groenen
	and Meurisse 2012).
	Les pontes sont déposées principalement à l'extrémité des rameaux de 3-10 mm de diamètre
	au niveau des branches terminales au sommet des chênes, mais des pontes peuvent
	occasionnellement être retrouvées sur des branches maîtresses. Elles sont recouvertes
	d'écailles protectrices produites par la femelle. Les larves sont déjà parfaitement formées à
	l'intérieur des oeufs dès la fin de l'automne, avant l'entrée en diapause, sans pour autant être
	viables à ce stade (Wagenhoff et al. 2014). En hiver les oeufs entrent en diapause, ce qui leur
	confère une résistance aux fortes températures négatives. Meurisse et al. (2012) ont montré
	pour les oeufs de la processionnaire un point de surfusion à -28 °C où la plupart des oeufs ont
	gelé. L'éclosion, fortement dépendante des températures se fait au printemps, en avril juste

Figure 2 : Cycle de la processionnaire du chêne sur une année. © Damestoy T. Pour beaucoup d'espèces de lépidoptères forestiers, la dynamique des populations est

  Thaumetopoea qui effectuent leur nymphose dans le sol, la processionnaire du chêne forme un tissage individuel contenant la chrysalide directement dans le nid. Les adultes émergent des chrysalides 30 à 40 jours après leur formation. L'ensemble du cycle du chêne est représenté dans la figure 2.

	, van
	Asch and Visser 2007, Van Asch et al. 2010). Après s'être nourries les famines sont peu
	surmontables. En effet, Meurisse et al. (2012) ont montré que la processionnaire était capable
	de surmonter une famine pendant 9-14 jours suivant les conditions environnementales, suite à
	un gel de printemps par exemple. La chenille processionnaire peut néanmoins supporter des
	températures négatives allant jusqu'à une température létale avoisinant les -14°C (point de
	surfusion montré par Meurisse et al. (2012). Elles ont principalement une activité
	crépusculaire voire nocturne, se nourrissant la nuit et se rassemblant le jour dans un nid
	constitué de soies, de poils, de fèces et d'exuvies, tissés sur les feuilles et rameaux de l'hôte.
	Ces nids représentent un tissage très léger et sont abandonnés après chaque mue. A l'instar de
	T. pityocampa, elles ont un comportement grégaire et se déplacent en procession sous forme
	de ruban, laissant derrière elles un réseau de fils. Les chenilles processionnaires du chêne se
	distinguent par une large bande noire et de longues soies sur le dos. Elles se nourrissent
	pendant 2-3 mois jusqu'à la mi-juin (début Juillet), période pendant laquelle se succèdent 6
	phases larvaires. Les chenilles L1 sont brunes avec une tête noire, et mesurent 2 mm environ.
	Suite à la première mue au début du mois de mai, les L2 s'éclaircissent légèrement et restent
	relativement petites avec une taille inférieure à 10 mm. C'est à partir du 4 ème stade que les chenilles acquièrent leur caractère urticant suite à la production de soies au niveau de plaques dorsales de leur abdomen, appelées « miroirs ». La thaumétopoéine est la molécule présente dans ces soies et est responsable de leur caractère urticant. Ces soies microscopiques s'accrochent dans les muqueuses et l'épiderme des personnes ou animaux, libérant par frottements la molécule urticante. Pendant le 5 ème stade (Juin), les chenilles tissent un nid plus résistant et plus gros sur le tronc et les branches maîtresses des chênes. C'est au 6 ème stade larvaire (de juin à début juillet) que la chenille prend une couleur grise et une tête noire et atteint sa taille maximale avoisinant une trentaine de millimètres. Contrairement à de naturels, climats, modifications des ressources disponibles, sont autant de facteurs influençant directement ou indirectement la fécondité, la croissance, la mortalité ou la capacité de migration des insectes (Berryman 1996). Bien que la dynamique de population et les facteurs susceptibles d'expliquer les fluctuations des populations de processionnaire du chêne soient mal connus (voir Baker et al. 2009, Wagenhoff and Veit 2011), les températures semblent être un facteur important affectant l'éclosion et le développement de la processionnaire. Wagenhoff and Veit (2011) ont également observé que le déclin de population de nombreuses espèces du genre conditionnée par des facteurs environnementaux biotiques et abiotiques changeant : ennemies processionnaire coïncidait avec un retard dans le débourrement des chênes.

1.6.2. Problématique et hypothèses

	dernière étude a permis de mieux comprendre les mécanismes sous-jacents aux effets de la
	Par une série d'observations et d'expérimentations sur le terrain et en laboratoire, nous avons variabilité génétique des arbres sur les performances d'insectes phytophages.
	mis en évidence les effets d'association d'essences dans les peuplements forestiers et la
	manière dont les traits foliaires des chênes conditionnent les performances de la
	processionnaire. Enfin nous nous sommes également interrogés sur les causes génétiques et
	environnementales de la variabilité interindividuelle dans l'expression de ces traits, en
	recherchant l'implication d'un contrôle génétique dans la résistance des chênes à la
	processionnaire, et une modification du phénotype des chênes par la présence de voisins con-
	ou hétérospécifiques. Avec ces expériences, nous émettons les hypothèses que le mélange
	dans les peuplements forestiers affecterait les pullulations de la processionnaire du chêne, en
	réduisant la détection des chênes hôtes par les adultes. La variabilité inter et intra-spécifique
	dans les traits foliaires liés à l'herbivorie agirait non seulement sur le choix de l'hôte par les
	adultes mais également sur les performances des larves. Les effets d'association pourraient
	être également dus à une expression différentielle des traits foliaires affectés par l'identité du
	voisin, entraînant des effets indirects liés aux traits du voisin sur les performances de la
	processionnaire.
	Nous avons, premièrement, étudié les effets d'association dans des peuplements naturels purs
	de deux espèces de chêne phylogénétiquement proche : le chêne pédonculé, Quercus robur et
	le chêne sessile, Quercus petraea et des peuplements mélangés associant les deux chênes ou
	chaque chêne avec d'autres espèces de feuillues non hôtes de la CPC. Cette première étude a
	permis de mieux comprendre les effets d'association sur le comportement de la CPC. Ensuite,
	nous nous somme focalisés sur l'effet de l'identité de la plante hôte et celle de ses voisins sur
	la variation dans les traits foliaires et ses conséquences sur les performances de la CPC. Dans
	une expérience en serre, nous avons testé les effets de la présence d'un voisin conspécifique
	ou hétérospécifique. Nous avons quantifié les défenses chimiques et la qualité nutritionnelle
	des feuilles de la plante hôte et mesuré les performances de la CPC. Cette seconde étude a
	permis de de mieux comprendre les effets indirects du voisin sur la variabilité des traits
	foliaires et des performances de la CPC. Enfin nous nous sommes intéressés aux effets du
	génotype du chêne pédonculé et de sa variation phénotypique des traits foliaires de défenses
	sur les performances de la CPC. En effectuant des tests d'alimentation en laboratoire, nous
	avons évalué les performances des chenilles se nourrissant des feuilles de différents génotypes
	de chêne pédonculé dont la concentration en métabolite secondaire a été quantifiée. Cette

2. Quels effets ont l'espèce hôte et la composition sur les captures des adultes et les défoliations de la processionnaire ?

	Damestoy T., Jactel H., Belouard T., Schmuck H., Christophe P., Castagneyrol B. -

Tree species identity and forest composition affect the number of captures and defoliation of the oak processionary moth Thaumetopoea processionea -Agricultural and Forest

	Chapitre 1 Abstract:
	Entomology -Accepted -02/11/19

Table 1

 1

). By contrast, stand location (i.e. forest identity) explained 86 % of variability in presence of OPM nests on oaks. Defoliation of oak trees by OPM larvae was on average (± se) 23.36 ± 1.79 %. Quercus petraea was significantly more defoliated (31.78 ± 3.37 %) than Q. robur (16.78 ± 1.62 %) (Fig 4, Table 1). OPM defoliation was significantly higher in pure stands (31.21 ± 3.83 %) than in mixed stands with other broadleaved species (15.00 ± 1.85 %) (Fig 4, Table 1). Neither basal areas of neighbours (host or non-host) nor dbh of focal tree significantly explained the percentage of defoliation (Table 1). Stand location (i.e., forest identity) explained 34 % of variability in OPM defoliation.

Table 1 :

 1 Summary of (Generalised) Linear Mixed effect Models first testing the effect of host species and stand composition (upper section) and then testing the effect of host species, basal area of host trees and non-host trees (lower section), on the presence of nests on trees and the percentage of defoliation. R² are given for the simplified model, R²m is the marginal R² associated with the fixed effects and R²c is the conditional R² associated with the fixed effects plus the random effects. Significant coefficients (p-value < 0.05) are in bold.

		Response Variables
		Presence of nests	Defoliation
	Predictors	² (

df) p-value R² m (R² c ) F value (df) p-value Rm² (Rc²)

	Oak species	3.04 (1)	0.081	0 (0.66)	13.48 (1, 67.7)	< 0.001 0.13 (0.56)
	Stand composition	2.41 (2)	0.299		4.51 (2, 13.84)	0.031
	Oak dbh	0.11 (0)	0.741		3.59 (1, 112.1)	0.061
	Oak species × Stand composition	3.22 (3)	0.199		0.25 (2, 20.4)	0.780
	Oak species	3.04 (1)	0.081	0 (0.66)

9.82 (1, 108.27) 0.002 0.05 (0.49) Basal area of host trees 0.24 (1) 0.622 0.59 (1, 184.6) 0.444 Basal area of non-host trees 0.93 (1) 0.333 0.57 (1, 188.7) 0.449 Oak dbh 0.01 (1) 0.939 2.82 (1, 153.3) 0.095 Oak species × Basal area of host trees 0.23 (1) 0.629 < 0.01 (1, 183.5) 0.982 Oak species × Basal area of nonhost trees 0.78 (1) 0.377 2.52 (1, 186.9) 0.114

  . Managing to increase oak forest resistance to OPM attacks is therefore of crucial importance, in terms of both forest and public health.Study design -We established a greenhouse experiment at INRA forest station in Cestas (Southwestern France, GPS: 44°44'10.32" N, 0°46' 26.21" W), with potted trees. In January 2017, we planted 100 two-year-old oak saplings in 5L pots. In total, we prepared 25 pots with two Q. petraea individuals, 25 pots with two Q. robur individuals, and 50 pots with one individual of each species. Saplings of the two oak species were similar in size. We kept pots in a greenhouse with ambient temperature, humidity and light and watered as needed for one year before the start of the experiment.In December 2017, we collected OPM egg masses in mature oak forests in North-Eastern France. In April 2018, before oak budburst, we installed one egg mass (about 100 eggs) on

one oak tree per pot. We established two 'focal species' treatments (Q. petraea vs. Q. robur) crossed with two 'neighbour species' treatments (conspecific vs heterospecific) resulting in the four following experimental treatments with 25 replicates each: (i) one egg mass on one Q. robur in a pot with Q. petraea kept intact; (ii) one egg mass on one Q. robur in a pot with Q. robur kept intact; (iii) one egg mass on one Q. petraea in a pot with Q. petraea kept intact;

Table 2 :

 2 Summary statistics of models testing the effects of Focal and Neighbour species identities and their interaction on OPM larval performances (mortality and growth rate), leaf traits and leaf phenology. Linear models were used to test the effects of Focal and Neighbour species identity on OPM growth and mortality and all leaf traits. For Bud phenology we used Ordinal logistic regression. Significant coefficients (P < 0.05) are in bold

				Predictors
		Focal species	Neighbour species	F × N
	Response Variables	F numdf, dendf	P-value	F numdf, dendf	P-value	F numdf, dendf	P-value
	OPM mortality	17.46 1, 91	< 0.001	17.83 1,90	0.214	0.08 1,89	0.776
	OPM growth	8.99 1, 84	0.004	2.79 1, 82	0.099	0.34 1, 81	0.563
	Flavonoids	16.16 1, 90	< 0.001	1.36 1, 90	0.245	5.41 1, 90	0.022
	Lignins	0.78 1, 91	0.379	2.03 1, 92	0.157	2.67 1, 90	0.105
	Condensed tannins	1.29 1, 92	0.258	0.00 1, 91	0.962	1.48 1, 90	0.227
	Hydrolysable tannins	14.57 1, 92	< 0.001	0.95 1, 91	0.331	0.76 1, 90	0.387
	C:N	2.32 1, 90	0.131	0.94 1, 90	0.334	8.86 1, 90	0.004

LR Chisq (Df) P-value LR Chisq (Df) P-value LR Chi-sq (Df) P-value

	Bud phenology	2.40 (1)	0.122	1.07 (1)	0.300	0.44 (6)	0.508

Table 3 :

 3 Summary statistics of linear models testing the effects of leaf traits on OPM performance (mortality and growth rates). Significant coefficients (P < 0.05) are in bold. Traits with significant effect were selected and incorporated as covariate in mechanistic models (see Table2).

			Response Variables
		OPM mortality	OPM growth
	Predictors	F numdf, dendf	p-value	F numdf, dendf	p value
	Condensed tannins	0.03 1, 81	0.870	0.07 1, 73	0.797
	Hydrolysable tannins	1.90 1, 83	0.171	1.48 1, 77	0.227
	Lignins	0.09 1, 82	0.768	0.12 1, 74	0.732
	Flavonoids	0.98 1, 84	0.324	1.00 1, 76	0.320
	C:N	3.01 1, 85	0.086	0.15 1, 75	0.695
	Bud phenology	5.78 6, 86	< 0.001	3.72 6, 79	0.003
	Initial density	20.58 1, 86	< 0.001	3.03 1, 78	0.086

Table 4 :

 4 Summary statistics of mechanistic models testing the effects of Focal and Neighbour species identities and their interaction on OPM performance (mortality and growth rate). For both models, we included bud phenology as a covariate to test if the effect of focal species identity on herbivore performance was determined by this factor. We also include initial larval density as covariate. Significant coefficients (P < 0.05) are in bold

		OPM mortality	OPM growth
	Predictors	F numdf, dendf	P-value	F numdf, dendf	P-value
	Focal	9.47 1, 85	0.002	9.54 1, 77	0.003
	Neighbour	2.34 1, 84	0.130	3.63 1, 76	0.061
	Focal × Neighbour	1.47 1, 83	0.229	0.24 1, 75	0.629
	Bud phenology	6.28 6, 85	< 0.001	4.28 6, 77	< 0.001
	Initial density	24.97 1, 85	< 0.001	4.40 1, 77	0.039

Table 5 :

 5 Summary

	Organisms	Traits	Min	Max	Overall Mean	CV
			(mean ± sd)	(mean ± sd)	(mean ± sd)
	Oak	Total phenolics (mg.g -1 )	2.44 ± 0.63	8.36 ± 6.90	4.68 ± 3.61	0.77
		Condensed tannins (mg.g -1 )	1.18 ± 0.81	5.88 ± 5.37	2.79 ± 2.39	0.86
		Hydrolysable tannins (mg.g -1 )	0.66 ± 0.14	3.10 ± 2.57	1.58 ± 1.35	0.86
		Flavonoids (mg.g -1 )	0.17 ± 0.03	0.48 ± 0.32	0.29 ± 0.17	0.60
		Lignins (mg.g -1 )	0.01 ± 0.002	0.04 ± 0.04	0.02 ± 0.02	0.83
	GM	Growth (mg.day -1 )	13.50 ± 14.22	45.84 ± 19.03	27.56 ± 16.77	0.61
		Consumption (mg.day -1 )	15.34 ± 12.17	52.91 ± 21.28	35.10 ± 20.24	0.57
		Digested food (mg.day -1 )	-3.43 ± 7.91	25.50 ± 14.88	12.42 ± 11.14	0.90
	OPM	Growth (mg.day -1 )	-0.04 ± 0.55	1.24 ± 0.38	0.53 ± 0.78	1.48
		Consumption (mg.day -1 )	1.15 ± 0.50	5.3 ± 1.20	3.08 ± 1.67	0.54
		Digested food (mg.day -1 )	0.42 ± 0.67	4.56 ± 1.38	2.23 ± 1.36	0.60

of oak and herbivore trait means and variances (n = 30). Min and Max are the mean of the minimum and maximum values calculated across clones of the same genotype (i.e., n = 4 replicates). Overall Mean represents the mean calculated across all clones. The coefficient of variation (CV) is the ratio between the standard deviation and Mean.

Table 6 :

 6 Summary of linear mixed effect models testing the effects of oak phenolic compounds on nutritional indices for gypsy moth (GM) and oak processionary moth (OPM) larvae. R² aregiven for the simplified model, R²m is the marginal R² associated with the fixed effects and R²c is the conditional R² associated with the fixed effects plus the random effects. Model coefficient parameter estimates are reported for significant effects only (Estimate, se).

	Gypsy moth	Oak processionary moth
	(Lymantria dispar)	(Thaumetopoea processionea)

  ont mis en évidence un effet contrasté des tannins sur les performances des insectes, les tannins condensés n'avaient aucun effet tandis que les tannins hydrolysables réduisaient significativement la croissance des deux insectes généralistes (Acronicta psi et Amphipyra pyramidea ; Lépidoptères) mais n'agissaient pas sur la croissance des deux insectes spécialistes du chêne (Dichonia aprilina et Catocala sponsa ; Lépidoptères). Dans une méta-analyse sur les traits des plantes impliquées dans la résistance aux herbivores, Carmona et al. (2011) ont montré que les métabolites secondaires ne prédisaient pas significativement la résistance à l'herbivorie. Foss and Rieske (2003) ont documenté des taux de consommation et de croissance de Lymantria dispar (Lépidoptère) plus élevés lorsque les larves étaient nourries sur des feuilles de chêne des marais (Q. palustris) présentant un haut niveau de tannins. Barbehenn et al. (2009) ont également montré une augmentation du taux de consommation de L. dispar lorsqu'elles

	étaient nourries sur des feuilles de peuplier traitées avec des tannins. Ces réponses inattendues
	des herbivores aux métabolites secondaires pourraient être dues à une réponse adaptative de
	l'insecte en ajustant son activité enzymatique en réponse à la qualité nutritionnelle des tissus
	consommés

  , ceci pouvant potentiellement impacter négativement les derniers stades de développement de l'insecte. Il semble donc difficile d'extrapoler les résultats de nos expériences, et la croissance des chenilles sur quelques jours, à une perte de fécondité par exemple. De même nous avons mesuré seulement les composés phénoliques des feuilles, nous ne pouvons pas exclure l'hypothèse que ces traits, ou d'autres, soient impliqués dans la résistance des chênes à d'autres étapes du cycle de développement de la processionnaire du chêne ou sur l'ensemble du cycle. C'est notamment le cas des traits mécaniques et des traits impliqués dans la reconnaissance et la sélection de l'arbre hôte par la femelle de la CPC. Par exemple, Malishev and Sanson (2015) ont montré que les tissus d'Eucalyptus sp. les plus durs affectaient différemment les différents stades larvaires d'Extatosoma tiaratum et limitaient leur consommation. Ghirardo et al. (2012) ont montré un taux de consommation plus élevé de la larve de Tortix viridana sur les chênes pédonculés « résistants » que sur les chênes « sensibles », et des taux de croissances et de mortalité identiques quel que soit le phénotype du chêne. Cependant, les chênes « résistants » avaient des concentrations en polyphénols plus élevées (induisant probablement une augmentation de la consommation sur les feuilles les plus défendus, Chapitre 3), mais également les composés volatiles émis par les deux types de phénotypes étaient différents. Les chênes sensibles émettaient des composés volatiles attractifs pour la femelle, tandis que les chênes résistants émettaient plus de composés répulsifs (e.g. sesquiterpènes).De plus, il est important de dissocier les défenses constitutives, qui s'expriment même en l'absence d'herbivores, des défenses induites qui sont mobilisées par la plante à la suite d'une agression par un herbivore[START_REF] Karban | The ecology and evolution of induced resistance against herbivores[END_REF][START_REF] Moreira | Specificity of induced defenses , growth , and reproduction in lima bean (Phaseolus lunatus) in response to multispecies herbivory[END_REF]. Ce sont ces défenses induites qui sont les plus spécifiques et qui agissent en réponse à une attaque[START_REF] Agrawal | Tradeoffs and Negative Correlationsin Evolutionary Ecology BT -Evolution Since Darwin[END_REF][START_REF] Karban | The ecology and evolution of induced resistance against herbivores[END_REF][START_REF] Xiao | Herbivore-specific induction of indirect and direct defensive responses in leaves and roots[END_REF]. Elles reposent sur les capacités de la plante à reconnaître le type d'herbivore et à mobiliser les réponses nécessaires contre l'agresseur. Cependant, nos analyses ne prennent pas en compte les défenses induites par la processionnaire du chêne.Nous ne pouvons donc pas exclure l'éventualité que les défenses induites aient eu des effets différents que les défenses mesurés pendant nos expériences. L'induction des défenses chez le chêne pédonculé et le chêne sessile suite aux attaques par la processionnaire du chêne reste à déterminer, ainsi que leur effet direct ou indirect sur les préférences et performance de la CPC.

  et l'attraction des ennemis naturels[START_REF] Van Poecke | Indirect defence of plants against herbivores: Using Arabidopsis thaliana as a model plant[END_REF][START_REF] Heil | Indirect defence via tritrophic interactions[END_REF][START_REF] Unsicker | Protective perfumes: the role of vegetative volatiles in plant defense against herbivores[END_REF][START_REF] Mumm | Variation in natural plant products and the attraction of bodyguards involved in indirect plant defense[END_REF][START_REF] Hagenbucher | Resistance Mechanisms Against Arthropod Herbivores in Cotton and Their Interactions with Natural Enemies[END_REF][START_REF] Kaplan | Indirect plant-parasitoid interactions mediated by changes in herbivore physiology[END_REF][START_REF] Aljbory | Indirect plant defense against insect herbivores: a review[END_REF][START_REF] Gasmi | Can herbivore-induced volatiles protect plants by increasing the herbivores' susceptibility to natural pathogens?[END_REF] en réponse à une attaque par les herbivores. Ces composés volatiles, constitutifs ou induits par une attaque, peuvent par exemple indiquer aux herbivores que la plante a initié la production de composés toxiques ou la présence d'autres insectes sur la plante et donc de potentiel compétiteurs. Les composés volatiles émis par les plants de tabac Nicotiana tabacum réduisaient le taux d'oviposition des femelles de Heliothis virescens (Lépidoptères) (De Moraes et al. 2001) et agissait également comme répulsif contre les attaques de Frankliniella occidentalis (Thysanoptères) (Delphia et al. 2007). Ghirardo et al. (2012) ont montré une différence d'attractivité des femelles de Tortix viridana suivant les composés volatiles émis par les chênes pédonculés.Ils peuvent également induire de manière systémique la production de défenses antiherbivores dans les tissus de la plante pas encore attaqués en prévention d'une éventuelle attaque. Par exemple[START_REF] Frost | Within-plant signalling via volatiles overcomes vascular constraints on systemic signalling and primes responses against herbivores[END_REF] ont montré que les feuilles de peuplier qui n'avaient pas subi de dégât mais qui avaient été exposées à des composés volatiles émis par des feuilles de la même tige, endommagée par L. dispar, avaient une réponse défensive plus élevée par rapport aux feuilles qui n'avaient pas été exposées aux composés volatiles. de mâles de la CPC dans les peuplements de chêne sessile que dans les peuplements de chêne pédonculé, suggérant une plus forte attractivité du chêne sessile par rapport au chêne pédonculé.[START_REF] Williams | The influence of tree species and edge effects on pheromone trap catches of oak processionary moth Thaumetopoea processionea (L.) in the[END_REF] montrent que les pièges à phéromone installés dans la canopée de chênes pédonculés capturaient plus de mâles de la CPC que les pièges installés sur six autres essences, incluant trois autres chênes (Q. cerris, Q. palustris et Q. rubra). Ces auteurs suggèrent que le chêne pédonculé émettrait plus de composés volatiles attractifs, notamment des terpénoïdes, que les autres espèces. Le chêne sessile n'était cependant pas inclus dans leur étude. Bien que, à notre connaissance, il n'y ait pas d'étude comparative sur les traits foliaires attractifs des deux espèces de chêne étudiées, plusieurs études ont montré que le bois du chêne sessile était caractérisé par un plus haut niveau de composés volatils (comme les lactones) que le chêne pédonculé[START_REF] Mosedale | Variation of heartwood phenolics and oak lactones between the species and phenological types of Quercus petraea and Q. robur[END_REF][START_REF] Feuillat | Variation in the concentration of ellagitannins and cis-and trans-β-methyl-γ-octalactone extracted from oak wood (Quercus robur L., Quercus petraea Liebl.) under model wine cask conditions[END_REF][START_REF] Mosedale | Variability of wood extractives among Quercus robur and Quercus petraea trees from mixed stands and their relation to wood anatomy and leaf morphology[END_REF][START_REF] Prida | Variation in wood volatile compounds in a mixed oak stand: strong species and spatial differentiation in whisky-lactone content[END_REF]. Des expériences plus contrôlées, seraient nécessaires pour mieux comprendre la réponse comportementale de la CPC à des combinaisons de composés volatils des chênes. De plus nous ne disposons que de peu d'informations sur les effets des ennemis naturels sur les populations de la processionnaire. Les oeufs, les larves et les chrysalides de la CPC peuvent être parasités et

	Les ennemis naturels des herbivores jouent également un rôle dans la régulation des
	populations d'insectes, réduisant ainsi les dégâts causés par les herbivores (Mooney et al.
	2010, Böhm et al. 2011, Giffard et al. 2012). Par exemple Bruinsma et al. (2009) ont montré
	que trois espèces de guêpes parasitoïdes (Hyménoptères) étaient significativement plus
	attirées par le mélange de composés volatils émis par les plants de Brassica oleracea attaqués
	par des défoliateurs. Aussi la quantité d'attaque des chenilles par les oiseaux était directement
	corrélée à l'émission de composés volatiles (e.g. terpènes) de Betula pubescens (Mäntylä et
	al. 2008) (voir aussi Amo et al. 2013).
	A ce jour, aucune étude publiée n'a spécifiquement mis en évidence l'intervention des
	signaux chimiques volatils émis par les chênes dans leur reconnaissance et leur colonisation
	par les papillons de CPC. Cependant, nous montrons dans le Chapitre 1, que les pièges à
	phéromones capturaient plus

prédatés par divers invertébrés ou vertébrés.

[START_REF] Sobczyk | Der Eichenprozessionsspinner in Deutschland[END_REF]

a établi une liste des ennemis naturels potentiels de la processionnaire du chêne, incluant des parasitoïdes Hémiptères et Diptères

[START_REF] Stigter | Thaumetopoea processionea in the Netherlands: present status and management perspectives (Lepidoptera: Notodontidae)[END_REF][START_REF] Zwakhals | Pimpla processioneae and P. rufipes: specialist versus generalist ( Hymeno-ptera : Ichneumonidae , Pimplinae )[END_REF][START_REF] Kitson | Detecting host-parasitoid interactions in an invasive Lepidopteran using nested tagging DNA metabarcoding[END_REF]

, des prédateurs Coléoptères et Hétéroptères

[START_REF] Ferrero | Un auxiliaire précieux de la forêt: le Calosome sycophante[END_REF][START_REF] Stigter | Thaumetopoea processionea in the Netherlands: present status and management perspectives (Lepidoptera: Notodontidae)[END_REF][START_REF] Baker | Evaluation of a pest risk analysis on Thaumetopoea processionea L., the oak processionary moth, prepared by the UK and extension of its scope to the EU territory[END_REF]

), mais également des oiseaux et des chauves-souris. Certains de ces ennemis naturels (comme Parus major, Picromerus bidens ou Calosoma sycophanta) qui sont connus comme agents de biocontrôle contre d'autres ravageurs (e.g.

[START_REF] Mahdian | Effects of temperature on predation by the stinkbugs Picromerus bidens and Podisus maculiventris (Heteroptera: Pentatomidae) on noctuid caterpillars[END_REF][START_REF] Mols | Great tits (Parus major) reduce caterpillar damage in commercial apple orchards[END_REF][START_REF] Kanat | The effect of Calosoma sycophanta L. (Coleoptera: Carabidae) feeding on the pine processionary moth, Thaumetopoea pityocampa (Denis & Schiffermüller) (Lepidoptera: Thaumetopoeidae), in the laboratory[END_REF][START_REF] Sands | Carcelia Iliaca ( Diptera : Tachinidae ), a Specific Parasitoid of the Oak Processionary Moth ( Lepidoptera : Thaumetopoeidae ), New To Great Britain[END_REF][START_REF] Kitson | Detecting host-parasitoid interactions in an invasive Lepidopteran using nested tagging DNA metabarcoding[END_REF]

, pourraient potentiellement agir contre la processionnaire du chêne. Par exemple,

[START_REF] Kitson | Detecting host-parasitoid interactions in an invasive Lepidopteran using nested tagging DNA metabarcoding[END_REF]

ont montré par metabarcoding, que presque la moitié des larves de la CPC analysées (n = 1 000 ; récoltées à Londres) était parasitée par C. illiaca, interrogeant son rôle dans le biocontrôle des populations de la CPC.

  . Malgré le fait que nous n'ayons pas montré d'effet du voisin sur les performances de la CPC, nous avons montré un effet du voisin hétérospécifique sur la concentration en flavonoïdes du chêne pédonculé et sur le ratio C:N du chêne sessile. Plusieurs études ont montré des changements significatifs des traits foliaires des plantes en présence de voisins hétérospécifiques : sur le chêne[START_REF] Nickmans | Effects of neighbourhood identity and diversity on the foliar nutrition of sessile oak and beech[END_REF], Castagneyrol et al. 2017), le bouleau(Castagneyrol et al. 2018a[START_REF] Muiruri | Forest diversity effects on insect herbivores: do leaf traits matter?[END_REF], l'acajou(Moreira et al. 2014a), le séneçon commun(Kos et al. 2015b,

[START_REF] Kostenko | Effects of plant diversity on the concentration of secondary plant metabolites and the density of arthropods on focal plants in the field[END_REF]

) ou la houlque laineuse

[START_REF] Walter | How do extreme drought and plant community composition affect host plant metabolites and herbivore performance?[END_REF]

) par exemple. La production de composés volatiles par les plantes voisines, induite par les dégâts des herbivores peut également provoquer une modification des traits de défense chez les plantes voisines encore intactes

(Arimura et al. 2000a, 2001[START_REF] Ruther | Plant-plant signaling: Ethylene synergizes volatile emission in Zea mays induced by exposure to (Z)-3-hexen-1-ol[END_REF][START_REF] Karban | Plant behaviour and communication[END_REF][START_REF] Heil | Herbivore-induced plant volatiles: targets, perception and unanswered questions[END_REF][START_REF] Karban | Volatile communication between plants that affects herbivory: A meta-analysis[END_REF], Moreira et al. 2016b

). Par exemple,

[START_REF] Arimura | Plant-plant interactions mediated by volatiles emitted from plants infested by spider mites[END_REF]

ont montré que les composés volatiles émis par des plants infestés de haricot de Lima (Phaseolus lunatus L.) activaient la transcription de gènes de défense chez les plants intacts. Dans notre expérience (Chapitre 2) les voisins sont gardés intacts, il est donc possible que l'émission de composés volatiles, habituellement induit par les attaques des herbivores

  Cette vision restreinte du problème ne considère pas la plante dans son environnement biotique complet où une multitude d'organismes interagissent ensemble affectant non seulement les plantes mais aussi les organismes eux-mêmes. La plante doit donc produire des défenses efficaces contre les herbivores tout en considérant la diversité au sein des différents groupes.

	Du fait de la complexité des interactions entre les plantes et leurs herbivores et du grand nombre
	d'herbivores susceptibles d'attaquer simultanément ou séquentiellement une plante, la résistance d'une
	plante ne peut en aucun cas dépendre d'un seul trait de défense mais d'un ensemble de traits agissant

défoliateur spécialiste et le chêne. Les plantes doivent cependant faire face à une grande diversité d'herbivore variant dans leur niveau de spécialisation et dans leur stratégie alimentaire et ayant différentes réponses aux moyens de défense des plantes. Qu'ils soient piqueurs suceurs, défoliateurs ou endophytes, généralistes ou spécialistes, tous ces types d'insectes herbivores sont potentiellement en interaction avec le même hôte, de sorte qu'ils interagissent entre eux et évoluent dans un environnement complexe. La plupart des études discutent cependant de chaque association individuellement les unes des autres, ce qui réduit la complexité des interactions.

sur l'ensemble de la communauté de bioagresseurs interagissant avec la plante

(Kursar and Coley

  . Cependant, le phénotype de résistance résulte de plusieurs mécanismes simultanés difficiles à distinguer. Ainsi,[START_REF] Duan | Quantitative Trait Loci Mapping of Resistance to Laodelphax striatellus (Homoptera: Delphacidae) in Rice Using Recombinant Inbred Lines[END_REF] identifient trois QTLs contrôlant l'antixénose contre Laodelphax striatellus (Hémiptères) sur les chromosomes 3, 4 et 11 du riz, Oryza sativa et deux QTLs exprimant une antibiose (effet sur le taux de survie) vis-à-vis de L. striatellus sur les chromosomes 3 et 11. ont montré l'implication de QTL du chêne pédonculé dans la résistance aux herbivores. Ils ont identifié une région du génome impliquée dans la synthèse de composés phénoliques de défense dont la concentration est négativement corrélée à la taille des galeries creusées par les mineuses dans les feuilles de chêne pédonculé. indispensable pour mieux comprendre les mécanismes sous-jacents. De plus, la corrélation génétique entre les deux herbivores, que nous montrons dans le Chapitre 3, suppose que les deux herbivores pourraient être affectés par les mêmes traits de résistance des chênes[START_REF] Strauss | Ecological and evolutionary consequences of multispecies plant-animal interactions[END_REF][START_REF] Leimu | A meta-analysis of genetic correlations between plant resistances to multiple enemies[END_REF][START_REF] Wise | Evolution of resistance to a multiple-herbivore community: Genetic Correlations, diffuse coevolution, and constraints on the plant's response to selection[END_REF] et supporte la théorie du syndrome de défense

	Le contrôle génétique de la résistance des plantes aux herbivores d'une part (i.e., l'héritabilité
	de ce caractère) et la variabilité phénotypique de ces traits dans les populations naturelles sont

Vosman et al. (2019) ont montré une forte corrélation entre la survie de deux insectes ravageurs (Bemisia tabaci et Trialeurodes vaporariorum ; Hémiptères) et un QTL présent sur le chromosome 2 de Solanum galapagense et impliqué dans la synthèse de composé de défense et dans la présence de trichomes glandulaires, négativement corrélée avec la survie des insectes. Kroymann et al. (2003) mettent en évidence chez Arabidopsis taliana, un QTL de résistance à Spodoptera exigua (Lepidoptres) lié à la biosynthèse des glucosinolates. Concernant les chênes, Gailing (2008) a mis en évidence que les caractères morphologiques des feuilles du chêne pédonculé, notamment le nombre et le pourcentage de nervures intercalaires par exemple, étaient sous contrôle polygénique et identifie deux QTLs. Saintagne et al. (

2004

) ont montré que les traits morphologiques des feuilles (notamment le nombre de nervures intercalaires ou la pubescence par exemple) impliqués dans la différenciation interspécifique des chênes pédonculés et des chênes sessiles sont sous contrôle polygénique.

[START_REF] Malishev | Leaf mechanics and herbivory defence: How tough tissue along the leaf body deters growing insect herbivores[END_REF]

ont montré une implication des nervures des feuilles dans la résistance des feuilles à l'herbivorie, notamment en augmentant la dureté de celle-ci.

Brachi et al. (in prep)

le matériau de base de la sélection naturelle et artificielle. Principalement étudiés en contexte agronomique, l'utilisation de plantes « résistantes » constitue un moyen de lutte majeur contre les ravageurs et un substitut aux insecticides. L'identification de gènes liés à la résistance des arbres semble

Remerciements

Abstract:

Research on plant-herbivore interactions has long recognized that plant genetic variation plays a central role in driving insect abundance and herbivory, as well as in determining plant defence. However, how plant genes influence herbivore feeding performances, and which plant defensive traits mediate these effects remain poorly understood. Here we investigated the feeding performances of two insect leaf chewers with contrasting diet breadth (the generalist Lymantria dispar and the specialist Thaumetopoea processionea) on different genotypes of pedunculate oak (Quercus robur) and tested the role of leaf phenolics. We used leaves from four clones of 30 Q. robur full-sibs grown in a common garden to estimate the performance of both herbivores in laboratory feeding trials and to quantify the concentration of constitutive chemical defences (phenolic compounds). We found that tree genetics influenced leaf consumption by T. processionea but not by L. dispar. However genetic variation among trees did not explain growth rate variation in either herbivore nor in leaf phenolics. Interestingly, all phenolic compounds displayed a positive relationship with L. dispar growth rate and, leaf consumption by both herbivores displayed a positive relationship with the concentrations of condensed tannins, suggesting that highly-defended leaves could induce a compensatory feeding response. While genetic variation in oaks did not explain herbivore growth rate, we found positive genetic correlations between the two herbivores for leaf consumption and digestion. Overall, we found that oak genotype and phenolic compounds partly and independently contribute to variability in herbivore performance. We challenged the current view of plant-insect interaction and provided little support to the idea that the effect of plant genotype on associated organisms is driven by plant defences. Together, our results point to the existence of genetically determined resistance traits in oaks whose effects differ between herbivores and motivate further research on mechanisms governing oak-herbivore interactions.

Key words: Genetic variability; Chemical defences; Herbivory; Plant-insect interactions; Oak

Introduction

Over the past decades, numerous studies reported that genetically determine phenotypic variation within host plant species influences plant-insect interactions, some leading to coevolutionary processes [START_REF] Crutsinger | Plant Genotypic Diversity Predicts Community Structure and Governs an Ecosystem Process[END_REF][START_REF] Johnson | An emerging synthesis between community ecology and evolutionary biology[END_REF][START_REF] Hughes | Ecological consequences of genetic diversity[END_REF][START_REF] Bidart-Bouzat | An ecological genomic approach challenging the paradigm of differential plant responses to specialist versus generalist insect herbivores[END_REF][START_REF] Mcart | Plant genotypic diversity reduces the rate of consumer resource utilization[END_REF] and that within population genetic diversity in plants influences insect community composition (Crutsinger et