Alain Ratnadass
email: alain.ratnadass@cirad.fr

Jacques Avelino

Paula Fernandes

Philippe Letourmy

Régis Babin

Péninna Deberdt

Jean-Philippe Deguine

Isabelle Grechi

Krishna Naudin

Béatrice Rhino

Fabrice Declerck

Hamé Abdou

Kadi Kadi

Raymond Mahob

Bodovololona Rabary

Lala Sahondra Rafarasoa

Françoise Lescourret

Johnnie Van

Den Berg

Synergies and tradeoffs in natural regulation of crop pests and diseases under plant species diversification

Keywords: Agroforestry, Companion plant, Conservation agriculture, Cover crop, Green manure, Horticulture, Life history trait, Market gardening, Modelling, Spatial scale

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Farmers in the tropics are faced with considerable plant protection-related risks. The outcomes of pest management decisions, whether these are to take no action, to apply pesticides, or to implement other management strategies, impact widely the whole food production chain. Pest and disease damage to crops, as well as pesticide applications per se, can cause (i) food insecurity and lost income, (ii) adverse effects on human and environmental health (which also affects other actors in crop value chains e.g. agricultural workers and laborers, and consumers), and (iii) export restrictions due to quarantine pests and diseases and maximum limits on pesticide residues [START_REF] Sikora | The big giveaway -farmers and biological constraints[END_REF].

This high vulnerability of crops to damage by pests and diseases is mostly observed in intensive monocrop production systems which have been overly simplified [START_REF] Tilman | Agricultural sustainability and intensive production practices[END_REF]. In these systems, companion agrobiodiversity is low, and thus unable to provide supporting ecosystem services such as pest and disease regulation. It has been hypothesized that the resilience of intensive cropping systems to pest and disease outbreaks could be obtained through diversified plantings that approach the structure and trait composition of proximate natural ecosystems, similar to those found in many traditional agroecosystems [START_REF] Dawson | Agriculture in nature's image[END_REF][START_REF] Jackson | Natural systems agriculture: a truly radical alternative[END_REF][START_REF] Lewis | A total system approach to sustainable pest management[END_REF][START_REF] Malézieux | Designing cropping systems from nature[END_REF]. This hypothesis originated in observations that pest and disease outbreaks are typically low in natural systems, and therefore, that designing agroecosystems to include high functional biodiversity may preserve the ecological functions that keep pest and disease incidence low [START_REF] Hooper | Effects of biodiversity on ecosystem functioning: A consensus of current knowledge[END_REF][START_REF] Soliveres | Biodiversity at multiple trophic levels is needed for ecosystem multifunctionality[END_REF][START_REF] Tscharntke | Landscape perspectives on agricultural intensification and biodiversity -ecosystem service management[END_REF]. In this context, strategic conservation of plant diversity or increased plant diversity at multiple spatial and temporal scales are important [START_REF] Gaba | Multiple cropping systems as drivers for providing multiple ecosystem services: from concepts to design[END_REF][START_REF] Malézieux | Mixing plant species in cropping systems: concepts, tools and models[END_REF][START_REF] Tittonell | Ecological intensification of agriculture -sustainable by nature[END_REF]. In addition to agronomic benefits, e.g. reduced soil erosion, and improved water and nutrient use by crop plants [START_REF] Malézieux | Mixing plant species in cropping systems: concepts, tools and models[END_REF], increasing agroecosystem plant diversity may enhance ecosystem services [START_REF] Bommarco | Ecological intensification: harnessing ecosystem services for food security[END_REF][START_REF] Isbell | Benefits of increasing plant diversity in sustainable agroecosystems[END_REF], notably pest and disease regulation [START_REF] Deguine | Protection des cultures: de l'agrochimie à l'agroécologie[END_REF][START_REF] Ferron | Vers une conception agroécologique de la protection des cultures[END_REF][START_REF] Nicholls | Agroecological bases of ecological engineering for pest management[END_REF]. Increased agroecosystem diversity may also mitigate or alleviate risks in the face of disturbances such as climatic events and price fluctuations.

Between 2008 and 2012, CIRAD ("Agricultural Research for Development") and its partners implemented the "Optimization of ecological mechanisms of pest and disease management for sustainable improvement of agrosystem productivity" (Optimisation des Mécanismes Ecologiques de Gestion des bioAgresseurs pour l'Amélioration durable de la productivité des Agrosystèmes, or Omega3) project [START_REF] Ratnadass | Designing ecologically intensive cropping systems for regulating pests and diseases via plant species diversification-based suppressive processes in the tropics[END_REF]. This project examined the impacts of increased plant species diversity (PSD) on pest and pathogen population densities, and on crops. It included specific studies that considered both temporal (permanent plantings and rotations) and spatial (field and farmscape) interventions. Farmscape in this context refers to a small area of the landscape which has been defined

as "an area comprising the farm plus a 1.5 km buffer area" (Estrada-Carmona et al., 2019). The project had two specific objectives: (i) to gain knowledge on ecological processes that regulate pest and pathogen populations and how these could be mobilized to improve pest and disease management, and (ii) to generate tools and methods for the design and evaluation of novel diversified cropping systems and land use arrangements that prevent pest and pathogen outbreaks.

Among the eight major pathways of pest and disease regulation via increased PSD that were reviewed prior to the Omega3 project implementation (Ratnadass et al., 2012a), we selected eleven biological models, distributed over six case studies, representing a range of spatial scales of PSD deployment, and of pest or pathogen dispersal ability and host specificity (Table 1). Amongst pest and pathogen life history traits, dispersal ability and host specificity are the most amenable to manipulation via PSD.

This was done with a view to robustness and capacity to generalize results.

At the field scale, we studied the sanitizing effects of green manure, cover crops and companion plants (GMCCPs) on two pest and pathogen complexes. Firstly, we studied the soil borne pathogen Ralstonia solanacearum (Burkholderiaceae) which causes bacterial wilt of tomato (Solanum lycopersicum) (Solanaceae) in Martinique. Secondly, we studied the effects of GMCCP on scarab beetles (white grubs and black beetles Heteronychus spp.) (Coleoptera: Scarabaeidae) and witchweed (Striga asiatica) (Orobanchaceae) which impact upland rice (Oryza sativa) (Poaceae) in Madagascar. [START_REF] Leston | The flight behaviour of cocoa-capsids (Hemiptera: Miridae)[END_REF]; 13 Opoku et al. (2002); 14 Ristaino and Gumpertz (2000); 15 McCook (2006); 16 Bowden et al. (1971);

The pest attracting effects of trap plants at field level was studied in combination with other pest management pathways. These were (i) the barrier effects and conservation biological control of tomato fruitworms (Helicoverpa zea and H. armigera) (Lepidoptera: Noctuidae), respectively on tomato in Martinique and okra (Abelmoschus esculentus) (Malvaceae) in Niger (and to a lesser extent whitefly (Bemisia tabaci) (Hemiptera: Aleyrodidae) on okra in Niger) and, (ii) a food attractant mixed with a biological insecticide on cucurbit fruit flies (Diptera: Tephritidae) in Réunion. We also studied the effect of distinct intercropping arrangements of cocoa (Theobroma cacao) (Malvaceae) and companion perennials on the cocoa plant bug Sahlbergella singularis (Hemiptera: Miridae) and

Phytophthora megakarya (Peronosporaceae), black pod rot disease in Cameroon. Similarly, we studied the effect of distinct intercropping arrangements of coffee (Coffea spp.) (Rubiaceae) and companion perennials on Hemileia vastatrix (Pucciniales), the causative agent of the coffee leaf rust, in Costa Rica. At the farmscape scale, we studied the effects of density and arrangement of various land uses on the incidence of leaf rust, and the abundance of the coffee berry borer (Hypothenemus hampei) (Coleoptera: Curculionidae) in coffee in Costa Rica.

Here, we present the major results of the above-mentioned case studies, emphasizing the major lessons learned. These results particularly emphasize that while PSD can have important effects on pest and disease regulation, different contexts can produce conflicting effects, either between pest or disease regulation and agronomic performance, or among individual pest and/or pathogen species within pest and pathogen complexes. A need therefore exists to manage tradeoffs or exploit synergies that underlie PSD-based regulatory processes. We also discuss how modeling could be used as a tool for designing and developing PSD-based cropping systems that are resilient to pest and disease outbreaks. We focus on the way to optimize systems that can have antagonistic effects on pests and diseases, and how, if need be, pest and disease regulating effects can be enhanced. We also consider inclusion of "mimics" of PSD-based regulatory processes to address a particular pest or disease that cannot be controlled by PSD per se.

Major outcomes of the Omega3 project case studies

References to case studies that were developed during this project and which can be consulted for more details are provided in Table 2. 1)

Ralstonia solanacearum, a quarantine organism, causes bacterial wilt that affects many crops worldwide. This disease is particularly damaging to tomato in tropical and subtropical environments.

Since chemical control is largely ineffective and not a sustainable option for disease control [START_REF] Enfinger | Evaluation of chemicals and application methods for control of bacterial wilt of tomato transplants[END_REF] in these regions, alternative management measures based on ecological processes are needed. In Martinique, the importance of bacterial wilt has increased dramatically since 1999, with the emergence and rapid spread of a new genotype of R. solanacearum, phylotype IIB/4NPB, throughout the island [START_REF] Deberdt | Diversity of Ralstonia solanacearum in French Guiana expands knowledge of the "emerging ecotype[END_REF][START_REF] Wicker | Ralstonia solanacearum strains from Martinique (French West Indies) exhibiting a new pathogenic potential[END_REF]. This phylotype infects tomato cultivars previously considered resistant, resulting in significant yield losses. To address this, crop species that were previously used as nematode-sanitizing crops were investigated as a potential alternative management method.

The (undesirable) ability of the emergent population of R. solanacearum to persist in planta and in the rhizosphere of eight candidate crop cultivars was first evaluated under controlled conditions (growth chambers) and the incidence of bacterial wilt assessed in tomato plants subsequently grown in the same soil media. These candidate crops were in the families Brassicaceae (Raphanus sativus cv Melody and R. sativus cv Karacter), Asteraceae (Tagetes erecta cv Sunset and Tagetes patula cv Bonita) and Fabaceae (Mucuna deeringiana from Singapore and M. deeringiana cv Mucuna aña;

Crotalaria juncea cv IAC-1 and Crotalaria spectabilis cv Comùn). Results showed that all assessed cultivars hosted R. solanacearum latently, but that the concentration of this pathogen in the rhizosphere differed between cultivars within the same species and between species within the same genus. Among the Brassicaceae and Asteraceae, the highest concentration of R. solanacearum was found in planta and in the rhizosphere of T. erecta. The concentration of the R. solanacearum population in the rhizosphere of R. sativus cv. Karacter was significantly higher than in that of R. sativus cv. Melody. In Fabaceae, the in planta concentration of R. solanacearum was statistically similar in all species. The concentration of the R. solanacearum population in the rhizosphere of C. juncea cv. IAC-1 was significantly higher than that in C. spectabilis cv. Comùn. Melody can be used in ecological management strategies for bacterial wilt, even though no drastic suppression of R. solanacearum population inside stem tissues and in the rhizosphere of these two GMCCPs may occur [START_REF] Deberdt | Crotalaria spectabilis and Raphanus sativus as previous crops show promise for the control of bacterial wilt of tomato without reducing bacterial populations[END_REF].

Greenhouse pot experiments were then conducted to investigate the effect of the most promising plant species identified from those mentioned above on soil microbial communities and their ability to suppress bacterial wilt in a naturally infected soil. These plant species were: M. deeringiana, C. spectabilis, C. juncea, Allium fistulosum (Liliaceae), R. sativus and T. patula. Each was cultivated as sanitizing plants at three planting densities (bare soil control, field density and twice the field density)

followed by planting the same pot with a tomato crop. Tomatoes were grown in each pot as a bioindicator of bacterial wilt, after the preceding crop plants were either removed or uprooted and chopped with their residues incorporated into the soil for 10 days of initial decomposition, prior to planting of tomato seedlings. The incidence of plants exhibiting tomato bacterial wilt was reduced by 51% and 61% respectively in pots previously cultivated with C. juncea and C. spectabilis. For all the plant species, bacterial wilt incidence correlated negatively with the incidence of GramN and GramP bacteria and actinomycetes suggesting an antagonistic interaction between these microbial communities and R. solanacearum populations. The only treatment that significantly reduced the incidence of bacterial wilt was that with C. juncea which resulted in a 66% reduction in the incidence of bacterial wilt compared to the bare soil control. Across all sanitizing plant treatments, however, bacterial wilt incidence was negatively correlated with soil NH4 + level, GramN bacteria and actinomycetes densities, plant root biomass and the bacteria/fungi ratio, thus suggesting significant soil ecosystem impacts on disease control.

Overall results suggested that C. juncea and C. spectabilis have potentially important biological control properties as rotational crops. Their use may result in favorable changes in microbial communities that suppress tomato bacterial wilt disease [START_REF] Diédhiou | Impact of biocontrol plants on bacterial wilt and non-targeted soil microbial communities on a naturally infested soil[END_REF]. However, most of the plant species showing promise under growth chamber and greenhouse conditions lost their sanitizing effect once their residues were ploughed into the soil. This could possibly be ascribed to a change in the relative concentrations of microbial groups, despite an overall increase in soil microbial community biomass.

The potential of the three Fabaceae species (M. deeringiana, C. juncea and C. spectabilis) to control bacterial wilt was then evaluated under field conditions, without incorporation of plant residues into the soil, but leaving the residues as a mulch on the soil surface. The best control was obtained with C. juncea and C. spectabilis with a decrease of bacterial wilt incidence on tomato by 71% and 58%, respectively. These results thus suggest that C. juncea and C. spectabilis could be used as sanitizing plants, placed as a mulch but not incorporated, to contribute to bacterial wilt control under field conditions in agroecological crop protection strategies [START_REF] Deberdt | The use of biocontrol plants to manage bacterial wilt of tomato in the tropics[END_REF].

Allelopathic effects of cover crops vis-à-vis upland rice pests in conservation agriculture

systems in Madagascar (case study 2, Table 1)

Heteronychus spp. are important pests of upland rice in the Central Highlands of Madagascar. Both the adults (black beetles) and larvae (white grubs) are particularly damaging in some conservation agriculture (no-till) systems. Certain species of cover crops used in conservation agriculture systems may have suppressive effects on white grub and adult black beetle populations. It is thought that the mechanism of suppression is via alteration of soil macrofauna communities or changes in status (pest or beneficial) of some white grub/black beetle species, through changes in feeding strategies from rhizophagous to saprophagous [START_REF] Ratnadass | Interaction entre le système de culture et le statut (ravageur ou auxiliaire) des vers blancs (Coleoptera : Scarabaeoidea) sur le riz pluvial[END_REF]2017).

Diverse cover crops were evaluated for suppression effects on white grubs within upland rice cropping systems. These cultivation systems were: no-till rice with a cover crop, no-till rice without a cover crop, and conventional tillage rice. The plants used as cover crops in two-year rice and cover crop(s) rotations with rice were: hairy vetch (Vicia villosa) and rattlebox (Crotalaria grahamiana) (Fabaceae); fodder radish (R. sativus) (Brassicaceae); Brachiaria ruziziensis x Brachiaria brizantha (var. Mulato) (Poaceae); and a mixture of Cleome hirta (Capparaceae), Tagetes minuta and Cosmos caudatus (Asteraceae). Fodder radish emerged as a promising tool for white grub pest management. In addition, it did not reduce the diversity and abundance of soil macrofauna [START_REF] Rabary | White grubs, Scarabaeidae larvae (Insecta, Coleoptera) control by plants in conservation agriculture: effects on macrofauna diversity[END_REF].

We also conducted a laboratory study on the possible effects of adding dried plant mulch (hereafter 'residues') to the soil (1% of dried plant mulch + 1% of rice straw) on Heteronychus bituberculatus larvae and adults. The residues of ten cover crop species were tested. Pigeon pea Cajanus cajan (Fabaceae), green leaf desmodium Desmodium intortum (Fabaceae) and finger millet Eleusine coracana (Poaceae) were used in addition to the seven cover crops mentioned above. Adding residues of C. grahamiana, E. coracana and C. caudatus resulted in significantly higher white grub mortality than in the control treatments (with 2% of rice straw), while residues of R. sativus had a marginally significant effect. Adding residues of C. grahamiana, C. caudatus, C. hirta, T. minuta and R. sativus significantly reduced damage caused by black beetles to rice plants [START_REF] Rafarasoa | Effect of cover crop residues on white grubs and adults of Heteronychus bituberculatus (Coleoptera: Dynastidae) and on the damage they cause to upland rice[END_REF].

In the mid-West of Madagascar, witchweed infestation on hillsides often results in farmers having to abandon cultivation of cereals such as rice or corn (Zea mays) (Poaceae). Various conservation agriculture cropping systems were therefore tested for their witchweed-alleviating potential and compared with conventional tillage practices. Most of the evaluated systems were based on the twoyear rice and corn rotation cycles favored by farmers. The common rice and corn rotation system with conventional tillage was compared with: i) rice+Stylosanthes and corn+Stylosanthes rotation, with

Stylosanthes guianensis (Fabaceae) cv CIAT 184 used as dry mulching material for direct seeding; ii) rice and corn+B. ruziziensis+C. cajan rotation, with Brachiaria and pigeon pea used as dry mulch; iii) rice and corn+cowpea rotation, with cowpea Vigna unguiculata (Fabaceae) used as dry mulch; iv) rice+perennial Arachis and corn+perennial Arachis rotation, with Arachis pintoi or A. repens (Fabaceae). In the latter treatment, Arachis plants were suppressed (i.e. not killed but having aboveground part desiccated by using low herbicide rates) before direct rice or corn seeding.

Evaluation of the suppressive effect on witchweed was based on the number of witchweed seeds remaining in the soil, and number of plants parasitizing cereals at the end of the season. The best control was obtained with perennial Arachis (A. pintoi or A. repens) and S. guianensis. Systems based on S. guianensis always produced more rice than other systems. The second-best treatment was cowpea rotated with corn. Surprisingly, the perennial Arachis treatment, which was most efficient at controlling witchweed, did not result in higher yields than those recorded with the conventional tillage systems. This can be ascribed to competition for water between the main crop and Arachis plants, especially during dry years. Because this system requires a specific management knowledge, and because Arachis seeds are difficult to source, this cropping system is seldom adopted by farmers.

Conversely, the high adaptability and practicality of S. guianensis systems make them more amenable to adoption by farmers [START_REF] Michellon | Striga asiatica: a driving-force for dissemination of conservation agriculture systems based on Stylosanthes guianensis in Madagascar[END_REF]. These results suggest that R. sativus, C. grahamiana and C. caudatus on the one hand, and S. guianensis on the other hand, are promising pest-suppressive cover crops against respectively scarab beetles in the Central Highlands and witchweed in the midwest of Madagascar.

2.3. Attracting and barrier effects of trap plants on Helicoverpa spp. fruitworms and whitefly in market-gardening systems of Martinique and Niger (case study 3, Table 1)

The corn earworm, H. zea, is a major insect pest of sweet corn, cotton (Gossypium hirsutum)

(Malvaceae), and vegetable crops, particularly tomato in North and South America, including the West Indies. In West Africa, the tomato fruitworm, H. armigera, is considered to be the most destructive pest of okra [START_REF] Kumar | Okra (Abelmoschus spp.) in West and Central Africa: Potential and progress on its improvement[END_REF]. As an environmentally friendly alternative management practice to synthetic insecticides, we (i) tested the potential of sweet corn varieties as border trap crops, and (ii) determined the optimal corn planting time so as to concentrate H. zea on this trap crop and divert the pest away from tomato crops in Martinique. The potential of using trap crops for the management of the tomato fruitworm H. armigera, was also evaluated in Niger on okra. In these studies, short and extra-short growing season varieties of pigeon pea, sorghum (Sorghum bicolor) and cotton were used as perimeter trap crops, and crop yield compared to those of insecticide-sprayed and unsprayed plots.

In Martinique, H. zea infestation was lower in tomato fields that had a corn plant border, and when corn silk emergence was synchronized with tomato flowering compared to fields without corn borders.

The Sugar Jean and Java Sweet corn varieties were suitable trap crops because few larvae survived on the silks and larval growth and development was poor. These two varieties, which therefore show "dead-end" trap cropping properties [START_REF] Shelton | Dead-end trap cropping: a technique to improve management of the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae)[END_REF], could reduce the risk of H. zea development and dispersion from corn plant borders into the tomato crop [START_REF] Rhino | Corn as trap crop to control Helicoverpa zea in tomato fields: importance of phenological synchronization and choice of cultivar[END_REF].

In Niger, however, only a slight regulating effect on tomato fruitworm was observed in okra with an extra-short growing season pigeon pea cultivar (cv ICPL 85010) planted as trap crop. This was however not ascribed to the trapping function of pigeon pea but to increased top-down regulation by generalist predatory spiders. Colonization by the latter was indeed significantly higher on the unsprayed okra crop with extra-short growing season pigeon pea cultivar borders than on both sprayed and unsprayed okra monocrops in the absence of extra-short growing season pigeon pea cultivar borders [START_REF] Ratnadass | Potential of pigeon pea as a trap crop for control of fruit worm infestation and damage to okra[END_REF]. Early-season establishment of spiders in okra plots surrounded by extra-short growing season pigeon pea cultivars was itself ascribed to higher early infestation by prey (leafhoppers: Empoasca sp.), which was most likely due to increased attractiveness of nitrogen-rich okra plants associated with a legume (namely pigeon pea).

Besides the above-mentioned experimental efforts, a modeling approach was used to determine how the deployment modalities of trap crops and commercial crops affect trap cropping efficacy. In this respect, Individual Based Models (IBMs) are suitable tools to study the interplay between factors that influence successful diversification strategies. IBMs are spatially explicit models with a strong emphasis on the behavior of individual organisms, that have been used to study pest management efficacy of agro-ecosystem diversification strategies [START_REF] Fenoglio | Evaluating trap cropping strategies for insect pest control through simulation models[END_REF][START_REF] Potting | Insect behavioural ecology and other factors affecting the control efficacy of agro-ecosystem diversification strategies[END_REF][START_REF] Vinatier | From IBM to IPM: Using individual-based models to design the spatial arrangement of traps and crops in integrated pest management strategies[END_REF]. Such a modeling approach was thus applied to the management of H. zea by means of sweet corn as a trap crop [START_REF] Grechi | An individual-based modeling approach to assess trap cropping management of Helicoverpa zea in tomato field in Martinique[END_REF] (Box 1). The results of the case study on tomato in Martinique, supported by the modelling approach, suggested that highly attractive sweet corn varieties with dead-end trap crop properties (e.g. Sugar Jean and Java) could be used as border trap crops to control H. zea, via bottom-up effects. On the other hand, ICPL 85010, an extra-short growing season pigeon pea cultivar, could be used as perimeter trap crop to control H. armigera on okra in Niger, via top-down effects.

2.4. Assisted trap cropping for cucurbit fruit fly management in Réunion (case study 4, Table 1)

In Réunion, fruit fly species belonging to the tribe Dacini of the family Tephritidae (Diptera) have been reported as the major cause of damage to cucurbit crops for many decades, notably: Zeugodacus cucurbitae, Dacus ciliatus and Dacus demmerezi [START_REF] Ryckewaert | Fruit flies (Diptera: Tephritidae) on vegetable crops in Reunion Island: state of knowledge, control methods and prospects for management[END_REF]. In order to develop environmentally-friendly alternatives to the use of synthetic insecticides for tephritid management, we compared the attractiveness of corn and Napier grass (Pennisetum purpureum) (Poaceae) as potential trap plants onto which baits could be applied. This study was conducted with only two of the fly species and results showed that corn, since it provided roosting sites, was more attractive to adults of Z. cucurbitae and D. demmerezi than Napier grass, irrespective of fruit fly sex and sexual maturity status. This study therefore showed the relevance of using corn as a trap plant onto which to apply bait, e.g. GF-120® Syneïs Appât ® , a spinosad-based fruit fly bait (Dow AgroSciences LLC, Indianapolis, IN) for the management of cucurbit fruit fly populations [START_REF] Atiama-Nurbel | Maize more attractive than Napier grass as nonhost plants for Bactrocera cucurbitae and Dacus demmerezi[END_REF].

In order to study the characteristics (seasonal fluctuation, relative abundance and sex ratio) of the communities of tephritid species that damage cucurbits in Réunion, the numbers of adult flies roosting on corn planted within or around fields of cultivated cucurbits were recorded. Adults of the three fly species spent most of their time on corn plants and their numbers on the cucurbit crop (zucchini:

Cucurbita pepo) were very low, except for a short period every day, depending on the species. While males remained roosting on corn plants throughout the day, only gravid females migrated to zucchini plants to lay eggs. Hourly observations showed that the three species had circadian rhythms which varied according to the species. On the other hand, the three different spatial arrangements of corn plantings (borders around the field, or patches, or strips within the field) were all found to be effective in concentrating cucurbit fruit fly populations onto corn plants [START_REF] Bonnet | Interactions entre les Mouches des Cucurbitacées (Diptera, Tephritidae), une plante hôte (courgette) et une plante piège (maïs), disposé en bandes et en patches intra-parcellaires à La Réunion[END_REF][START_REF] Deguine | Agroecological cucurbit-infesting fruit fly management. A review[END_REF].

Duhautois (2010) also analyzed the structure of the Diptera communities to assess the impact of corn on non-target fly species. She focused on six families: Calliphoridae, Muscidae, Otitidae, Sarcophagidae, Sepsidae and Syrphidae and found that at the regional scale, the community structure was influenced by the locality, while at the field scale it was influenced by corn phenology. Beyond concentrating pest fly species, corn plants harbored beneficial species such as hoverflies (Diptera:

Syrphidae), which are well-known pollinators and bio-indicators of agroecosystem health, with larvae of some species being efficient predators of plant-sucking pests [START_REF] Sarthou | Landscape parameters explain the distribution and abundance of Episyrphus balteatus (Diptera: Syrphidae)[END_REF]. Populations of the dominant hoverfly species, Melanostoma annulipes, a predator of many pest species [START_REF] Schmutterer | Ökologische Untersuchungen an entomophagen Syrphiden und ihren Parasiten im Hochland von Kenia (Ostafrika)[END_REF], peaked in late March, corresponding to the flowering period of corn. Their abundance was also the highest at 08:30 a.m. which is also the time of maximal corn pollen shed [START_REF] Duhautois | Structuration des communautés de Diptères sur le maïs, Zea mays, utilisé comme plante piège à La Réunion[END_REF].

Although it was found that application of GF-120® bait resulted in attraction and mortality of dipteran taxa other than tephritid flies, this was not quantified for these species [START_REF] Duhautois | Structuration des communautés de Diptères sur le maïs, Zea mays, utilisé comme plante piège à La Réunion[END_REF].

These results suggest that corn is more suitable than Napier grass as a trap plant onto which apply the adulticide bait GF-120® for cucurbit fruit fly management, either as borders around the field, or patches, or strips within the field. However, a potential indirect negative effect on natural biological control of other pest guilds, due to the attractiveness of GF-120® for predatory flies, was also highlighted.

2.5. Intercropping cocoa to create barriers for the management of cocoa mirid bugs and black pod rot disease in Cameroon (case study 5, Table 1)

The brown cocoa mirid bug, S. singularis, is the most harmful insect pest of cocoa in Cameroon [START_REF] Mahob | Field assessment of the impact of farmers' practices and cacao growing environment on mirid abundance and their damage under unshaded conditions in the Southern Cameroon[END_REF], while black pod rot, caused by P. megakarya, is the major disease affecting this crop [START_REF] Ndoungue | The impact of soil treatments on black pod rot (caused by Phytophthora megakarya) of cacao in Cameroon[END_REF]. Unshaded monocrop cocoa orchards are a common practice in the cocoa belt of West Africa, which accounts for more than 70% of the world cocoa production. However, fullsun orchards are often highly damaged by mirid bugs that feed on developing vegetative parts such as green shoots, buds and young leaves [START_REF] Mahob | Field assessment of the impact of farmers' practices and cacao growing environment on mirid abundance and their damage under unshaded conditions in the Southern Cameroon[END_REF]. Mirid bug damage lesions lead to the desiccation of branch tips, cankers on branches, and premature ageing of trees which then quickly become unproductive. Consequently, unshaded cocoa orchards, especially the young developing trees, require intensive use of insecticides for mirid control [START_REF] Mahob | Pesticides use in cocoa sector in Cameroon: characterization of supply source, nature of actives ingredients, fashion and reasons for their utilization[END_REF]. A study was conducted to assess the impact of cocoa-fruit tree intercropping on mirid bug infestation. The study focused on the potential effect of using fruit trees as physical barriers to limit mirid bug infestation on cocoa during the first years of plantation, when fruit trees are not yet developed enough to provide cocoa with shade. Previous studies have shown that mirid bugs may use visual cues while flying in search of their host [START_REF] Leston | The flight behaviour of cocoa-capsids (Hemiptera: Miridae)[END_REF] and we hypothesized that intercropping with fruit trees (which are known not to be cocoa mirid bug plant hosts) may disturb mirid bug flight behavior.

Over two consecutive years, mirid infestation was assessed in seven four-year-old orchards located in the Centre Region of Cameroon. The average orchard size was 0.3 ha. Every second row of trees in these orchards consisted of fruit trees (avocado: Persea americana, safou: Dacryodes edulis, and Citrus spp.) replacing cocoa every four plants. Original biometric methods were developed for this study as follows: the spatial distribution pattern was characterized through a semivariogram analysis, whereas the barrier effect of intercropped trees was analyzed with presence-absence data through joint count analysis and permutation tests [START_REF] Babin | Does cocoa-fruit intercropping impact infestation by the cocoa mirid bug Sahlbergella singularis (Hemiptera: Miridae)? In : COPAL. 17th Conférence Internationale sur la Recherche Cacaoyère[END_REF][START_REF] Mahob | Spatial distribution of Sahlbergella singularis Hagl. (Hemiptera: Miridae) populations and their damage in unshaded young cacao-based agroforestry systems[END_REF]. Mirid bug infestation was assessed at the population peak (from June/July to October/November) through counting of individuals and scoring of recent damage symptoms on cocoa pods and shoots (N' Guessan et al., 2008). Results showed that mirid bug populations were strongly aggregated at small distances, but damage symptoms tended to be more uniformly distributed across orchards, suggesting population movements during the study period. Since no barrier effect was detected, we concluded that intercropped fruit trees did not impact mirid bug movement and distribution in cocoa orchards [START_REF] Babin | Does cocoa-fruit intercropping impact infestation by the cocoa mirid bug Sahlbergella singularis (Hemiptera: Miridae)? In : COPAL. 17th Conférence Internationale sur la Recherche Cacaoyère[END_REF][START_REF] Mahob | Spatial distribution of Sahlbergella singularis Hagl. (Hemiptera: Miridae) populations and their damage in unshaded young cacao-based agroforestry systems[END_REF].

A similar survey of black pod rot occurrence was conducted in the same young unshaded cocoa orchards. Previous studies conducted in well-established orchards suggested that spatiotemporal dynamics of the disease was first determined by the presence of primary inoculum in the soil [START_REF] Deberdt | Impact of environmental factors, chemical fungicide and biological control on cacao pod production dynamics and black pod disease (Phytophthora megakarya) in Cameroon[END_REF]. Secondary inoculum would then appear and drive temporal infection at tree level, through different dispersion mechanisms, including rain splash and ant tents [START_REF] Ten Hoopen | Spatial and temporal analysis of a Phytophthora megakarya epidemic in a plantation in the Centre region of Cameroon[END_REF]. During the latter study however, infection by black pod rot was too low to reveal any impact of intercropping of fruit trees on disease dynamics. Results suggested an exogenous origin of the infection and dispersion through human activities, with microclimate and cocoa genetic origin as the main factors explaining the persistence of the initial infection in some parts of the orchards [START_REF] Ten Hoopen | Dispersal and distribution of Phytophthora megakarya in young cacao plantations[END_REF].

These results suggest that intercropping with fruit trees had no effect on cocoa infestation by mirid bugs, while such an effect (or lack of thereof) on cocoa infection by black pod rot was not evident due to too low infection levels at the time of the study.

2.6. Effects of farmscape fragmentation on orange rust epidemics and berry borer dynamics in coffee-based agroforestry systems in Costa Rica (case study 6, Table 1)

Coffee is affected by a number of diseases and insect pests, of which coffee leaf rust, caused by H.

vastatrix. and the coffee berry borer, H. hampei are among the major ones with a worldwide distribution [START_REF] Avelino | Multiple-disease system in coffee: From crop loss assessment to sustainable management[END_REF]. They are notably the main biotic stresses affecting coffee production in Costa Rica, where a recent study on landscape effects on pest dynamics may help to develop more sustainable management programs [START_REF] Avelino | Landscape context and scale differentially impact coffee leaf rust, coffee berry borer, and coffee root-knot nematodes: Appendix A[END_REF].

In a first study, coffee berry borer abundance in coffee tree plots was found to be positively correlated with the proportion of coffee area in the farmscape at a distance of 150 m around the plots. Negative correlations were obtained with other land uses, specifically forest, pasture and sugar cane. Since the coffee berry borer is a monophagous pest, large extensions of connected coffee tree areas probably increased the likelihood of flying individuals locating new coffee berries for colonization. The latter is especially important after coffee harvest, when coffee berries are rare. Finding new berries to infest and survive on during the post-harvest period supports and sustains pest populations and high infestation levels and damage in the subsequent season. In this study, fragmenting of the farmscape with forests or pastures that adversely affected the borer host location process was associated with a noted reduction in pest abundance.

However, fragmenting coffee farmscapes with pasture yielded higher incidences of coffee leaf rust (peak correlation at a distance of 200 m). We hypothesize that wind turbulence, produced by low-wind-resistance land uses such as pasture, favored removal of coffee leaf rust spore clusters from host surfaces, resulting in increased epidemics [START_REF] Avelino | Landscape context and scale differentially impact coffee leaf rust, coffee berry borer, and coffee root-knot nematodes: Appendix A[END_REF].

These results demonstrated that what is conceived as a barrier for one species may be conducive to the survival of another. It is therefore necessary to take into account the whole crop pest and pathogen complex to ensure efficient management. In this case, fragmenting coffee farmscapes with forest patches was suggested to limit coffee berry borer abundance, without favoring coffee leaf rust.

Another study was conducted to quantify shade tree effects on coffee leaf rust in Erythrina poeppigiana (Fabaceae)-based agroforestry systems. Shade tree effect on coffee leaf rust is an example of conflicting effects that certain environmental conditions may provide in terms of management of pests or diseases. On the one hand, shade helps to reduce leaf receptivity to the pathogen by preventing leaf exposure to radiation and high fruit loads, but, at the same time, shade may provide appropriate microclimate conditions for pathogen development. In order to quantify the individual effects of these antagonistic pathways, which are combined under natural conditions, these two factors were dissociated by manually homogenizing fruit loads under shade and in full sunlight conditions. Under each light regime, fruiting nodes were removed from coffee plants in order to obtain four fruit load levels (zero, 150, 250 and 500 fruiting nodes per coffee plant).

As expected, the intensity of the coffee leaf rust disease increased as fruit load per tree increased (28.9% increase in incidence and 129.2% increase in disease severity on plants with 500 fruiting nodes, as compared to plants with no fruits). With homogenous fruit loads, the intensity of the coffee leaf rust disease was greater under shady conditions, with a 21.5% increase in incidence and a 22.4% increase in severity. Two mechanisms were suggested. First, we found a dilution effect due to host growth, i.e. the continuous appearance of new healthy leaves in the system. The number of new leaves and new leaf area was 25.2% and 37.5% greater, respectively, in full sunlight conditions. Second, the microclimate was more conducive to coffee leaf rust under shade, due to lower intra-day temperature variations and higher leaf wetness. Shade, as expected, has antagonistic effects on coffee leaf rust: reduction of the disease due to reduced fruit load vs increased disease severity due to microclimatic conditions that favor the pathogen [START_REF] Avelino | Landscape context and scale differentially impact coffee leaf rust, coffee berry borer, and coffee root-knot nematodes: Appendix A[END_REF][START_REF] Lopez-Bravo | Shade is conducive to coffee rust as compared to full sun exposure under standardized fruit load conditions[END_REF].

These results suggest that fragmenting coffee farmscapes with forest patches could limit coffee berry borer abundance without favoring coffee leaf rust, while shade (e.g. in E. poeppigiana-based agroforestry systems) has antagonistic effects on coffee leaf rust, namely reduced disease intensity due to reduced fruit load vs increased disease severity due to favorable microclimatic conditions.

Major lessons learned in terms of PSD-based pest and disease regulation effects with emphasis on synergies and tradeoffs

Studies conducted under the Omega3 project yielded results of different natures demonstrating the complex nature of agroecological interventions for pest and disease control. One of the integrative objectives of the project was to document context specific pest and disease regulation processes, and verify whether generalizable principles could be elevated. Several of the case studies demonstrated that locally adapted pest and disease regulation impacts exist and that these can be effective (section 2). Synergies and tradeoffs highlighted in the project are discussed in the following sections, and some of them are summarized in Fig. 1. 3.1. Lack of beneficial effect of PSD deployment on some targeted pests and diseases Unsurprisingly, due to high variability across systems studied, we found no evidence of generic relationships between PSD and pest and disease suppression impacts, since there were instances where anticipated pest and disease reduction effects of PSD practices were not evident. For instance,

Genericity of mathematical models

Importance of withinspecies genetic variation in service plants

although we expected a barrier effect (physical obstruction) against whiteflies provided by trap crops in the Niger subcase study (See § 2.3), [START_REF] Ratnadass | Potential of pigeon pea as a trap crop for control of fruit worm infestation and damage to okra[END_REF] reported that late-maturing pigeon-pea and sorghum in particular did not have any effect on whitefly density. Similarly, trap crops did not affect leafhopper infestation, which turned out to be a benefit overall (See § 3.5), or at least not a disservice, since these less injurious early-season pests triggered a regulatory effect on a later occurring/highly damaging fruit pest (tomato fruitworm), via top-down pathways involving spiders.

Conversely, in the case of the assisted trap cropping for cucurbit fruit fly management in Réunion, predatory flies were attracted by GF-120 ® bait [START_REF] Duhautois | Structuration des communautés de Diptères sur le maïs, Zea mays, utilisé comme plante piège à La Réunion[END_REF], highlighting a potential indirect negative effect on natural biological control of other pest guilds, e.g. aphids and whiteflies.

In the case of cocoa-based agroforestry systems, no effect of intercropping with fruit trees was observed (particularly no barrier effect on mirid bugs) in our study. It should be also noted that conflicting effects were actually highlighted in similar studies, e.g. weaver ants and other ant species acting as both black pod rot disseminators and predators of plant bugs on the one hand, and shade both facilitating black pod rot infection and negatively affecting infestation by mirid bugs on the other hand [START_REF] Babin | Impact of shade on the spatial distribution of Sahlbergella singularis in traditional cocoa agroforests[END_REF][START_REF] Bagny-Beilhe | Identifying ant-mirid spatial interactions to improve biological control in cacao-based agroforestry system[END_REF][START_REF] Gidoin | Tree spatial structure, host composition and resource availability influence mirid density or black pod prevalence in cacao agroforests in Cameroon[END_REF][START_REF] Tadu | Ant diversity in different cocoa agroforest habitats in the Centre Region of Cameroon[END_REF]. However, ants could also help pollinating cocoa [START_REF] Toledo-Hernández | Neglected pollinators: Can enhanced pollination services improve cocoa yields? A review[END_REF].

In the case of coffee-based agroforestry systems, a positive relationship between shade and leaf rust due to favorable microclimatic conditions and absence of a dilution effect was highlighted at plot scale. Antagonistic effects were observed in the case of the coffee berry borer when the entomopathogenic fungus Beauveria bassiana was applied [START_REF] Sanchez | Shade has antagonistic effects on coffee berry borer[END_REF], possibly because shade trees favored the coffee berry borer as well as its natural enemy. The lack of effects of increased PSD or barrier crops in more complex and diverse environments such as the coffee production systems in this study could possibly be ascribed to the already high diversity in these agroforests. Such effects have been found at the landscape level, e.g. [START_REF] Batáry | Landscape-moderated biodiversity effects of agri-environment management -a meta-analysis[END_REF] reported that local agroecological management practices had strongest impacts in simple compared to already diverse, ecologically complex landscapes.

We did however observe that cropping systems were frequently robust vis-à-vis new invasive or emerging pests and pathogens. This is for example, the case of the emergent ecotype of R.

solanacearum (Phyl IIB/seq4NPB) [START_REF] Deberdt | Diversity of Ralstonia solanacearum in French Guiana expands knowledge of the "emerging ecotype[END_REF][START_REF] Wicker | Ralstonia solanacearum strains from Martinique (French West Indies) exhibiting a new pathogenic potential[END_REF] associated with certain rotation crops. This also holds true, as shown in other studies, for new fruit fly invaders such as Bactrocera dorsalis (Diptera: Tephritidae) on the Indian Ocean Islands [START_REF] De Villiers | The potential distribution of Bactrocera dorsalis: Considering phenology and irrigation patterns[END_REF], or of lepidopteran pests such as the fall armyworm Spodoptera frugiperda (Lepidoptera: Noctuidae) in Africa and adjacent islands [START_REF] Harrison | Agro-ecological options for fall armyworm (Spodoptera frugiperda JE Smith) management: Providing low-cost, smallholder friendly solutions to an invasive pest[END_REF][START_REF] Midega | A climate adapted pushpull system effectively controls Fall Armyworm, Spodoptera frugiperda (J E Smith), in maize in East Africa[END_REF].

Genericity/overarching nature of results obtained on GMCCPs

Some overarching results of the above-mentioned case studies considered the effects of several GMCCPs across soil and field scales, and stressed the importance of the genotypic aspect for GMCCPs, beyond deployment of main crop genetic diversity (Tooker and Franck, 2012), which was neither addressed in this project nor in an earlier review (Ratnadass et al., 2012a). The case study on trap cropping for control of Helicoverpa spp. highlighted the specificity of agricultural contexts in Martinique and Niger, e.g. sweet corn was ruled out under dry conditions in Niger [START_REF] Yabo | Etude de la distance d'influence de respectivement le maïs sucré et le pois d'angole en tant que plante-pièges pour la noctuelle Helicoverpa armigera sur respectivement la tomate et le gombo[END_REF].

Similarly, in Madagascar, plants adapted to the ecology where witchweed thrives are not the same as those species found in High Plateaus region of Madagascar where scarab beetles are the main problems. Table 3 shows some overarching results regarding some GMCCPs across case studies at the field scale.

The main aspects determining pros and cons of these GMCCPs are:

• their susceptibility to either abiotic stresses (e.g. case of susceptibility of corn to drought, of C. spectabilis to waterlogging causing stem rot) or biotic stresses (e.g. case of susceptibility of most rattlebox (Crotalaria) species to the mirid bug, Moissonia importunitas [START_REF] Ratnadass | First record of Moissonia importunitas as a pest of rattle box (Crotalaria spp.) in Réunion Island (Hemiptera, Miridae)[END_REF][START_REF] Ratnadass | Crop protection for agricultural intensification systems in sub-Saharan Africa[END_REF];

• the importance of the within-species genetic variation of GMCCPs (e.g. rattlebox species and fodder radish cultivars for bacterial wilt regulation on the one hand, and corn and pigeon-pea cultivars for tomato fruitworm regulation on the other hand); agriculture systems in Madagascar vis-à-vis scarab beetles and witchweed, while it is not the case for some GMCCPs in rotations vs bacterial wilt (although all species host R. solanacearum at least latently), and to a lesser extent with sweet corn on the tomato fruitworm (due to 'dead-end' regulation) or pigeon-pea (due to ease of manual management of the target pest on the same). This project also highlighted the complementary use of models with field trials as a tool for designing and developing PSD-based cropping systems that are resistant/tolerant to pest and disease attacks, particularly to optimize systems that can have antagonistic effects on pests and diseases. A specific IBM model based on the tomato/corn earworm biological model can in the future be extended to cucurbit fruit fly modeling, provided adequate parameterization. The latter model was also used to compare the efficiency of pest or disease regulatory services provided by trap crop patches, borders and strips, in terms of spatial design. This model can also be adapted to the Niger tomato fruitworm case study, by including the perimeter trap crop design (see [START_REF] Potting | Insect behavioural ecology and other factors affecting the control efficacy of agro-ecosystem diversification strategies[END_REF].

3.4. Synergies in terms of pest and disease complex management by PSD-deployment measures Crop rotation strategies were observed to provide valuable regulatory effects on pests and diseases in the relevant case studies of this project. Some plant species provided allelopathic effects against soilborne pests and diseases and nematicidal plants (e.g. some rattlebox and fodder radish species/cultivars: [START_REF] Aydinh | Biofumigation studies by using Raphanus sativus and Eruca sativa as a winter cycle crops to control root-knot nematodes[END_REF][START_REF] Wang | Crotalaria as a cover crop for nematode management: a review[END_REF] were found to have sanitizing effects against soil-borne bacteria (e.g. against tomato bacterial wilt: [START_REF] Deberdt | Crotalaria spectabilis and Raphanus sativus as previous crops show promise for the control of bacterial wilt of tomato without reducing bacterial populations[END_REF]2018). Some plant species also provided insecticidal effects (e.g. against scarab beetles [START_REF] Rafarasoa | Effect of cover crop residues on white grubs and adults of Heteronychus bituberculatus (Coleoptera: Dynastidae) and on the damage they cause to upland rice[END_REF].

Based on the experience from Martinique, Crotalaria spp. could also be evaluated for their potential to provide bacterial wilt control on potato in Madagascar [START_REF] Ravelomanantsoa | Molecular epidemiology of bacterial wilt in the Madagascar highlands caused by Andean (Phylotype IIB-1) and African (Phylotype III) brown rot strains of the Ralstonia solanacearum species complex[END_REF]. Conversely, based on the experience from Madagascar, Crotalaria spp. could also be evaluated for their potential as a management tool against the Taro black beetle (Tomarus ebenus) (Scarabaeidae: Dynastinae) [START_REF] Robin | Commercial dasheen (Colocasia esculenta (L.) Schott var. esculenta) production and post-harvest protocol for the OECS[END_REF] in Martinique.

Similarly, Crotalaria spp. could be used for bacterial wilt control in market gardening systems in Réunion. However, care should be taken to use the right genotype since rattlebox is also highly susceptible to the mirid bug, M. importunitas, which is an important pest in Réunion [START_REF] Ratnadass | First record of Moissonia importunitas as a pest of rattle box (Crotalaria spp.) in Réunion Island (Hemiptera, Miridae)[END_REF][START_REF] Ratnadass | Crop protection for agricultural intensification systems in sub-Saharan Africa[END_REF]. On the other hand, some Crotalaria spp., e.g. C. brevidens could also be evaluated as cover crops to control witchweed via induction of suicidal germination [START_REF] Gacheru | The potential of planted shrub fallows to combat Striga infestation on maize[END_REF][START_REF] Grubben | Plant Resources of Tropical Africa 2: Vegetables[END_REF] in cereal-based conservation agriculture systems in the Midwest region of Madagascar. Since the genus Crotalaria includes 702 species [START_REF] Le Roux | A global infrageneric classification system for the genus Crotalaria (Leguminosae) based on molecular and morphological evidence[END_REF], there are still many studies to be conducted to determine whether there are also other species that could provide regulatory effects but that are not susceptible to mirid bugs.

Sweet corn was found to be an effective trap crop for H. zea [START_REF] Rhino | Corn as trap crop to control Helicoverpa zea in tomato fields: importance of phenological synchronization and choice of cultivar[END_REF]2016), while field corn was found adequate for assisted push-pull against vegetable fruit flies in the cucurbit-based cropping systems of Réunion [START_REF] Deguine | Agroecological cucurbit-infesting fruit fly management. A review[END_REF]. Corn might therefore have potential for use in the management of Neoceratitis cyanescens (Diptera: Tephritidae) in irrigated tomato-based cropping systems in Réunion [START_REF] Brévault | Life-history strategy in an oligophagous tephritid: the tomato fruit fly, Neoceratitis cyanescens[END_REF]. The use of corn as a component of a management strategy for Bactrocera dorsalis in mango orchards should also be investigated further. In addition, in drier environments, sorghum and pigeon pea could also be used, as was done in okra-based systems for H. armigera in Niger [START_REF] Ratnadass | Potential of pigeon pea as a trap crop for control of fruit worm infestation and damage to okra[END_REF]. However, the effectiveness of sorghum and pigeon pea as a barrier crop against whitefly (B. tabaci) was not established. It is further suggested that genotypes other than those of forage sorghum be evaluated for their nematicidal properties [START_REF] Dover | Nematode management using sorghum and its relatives[END_REF].

Similarly, rather than extra-short duration pigeon-pea cultivars, longer-season and taller cultivars should be preferred, since they are more likely to act as physical barriers.

Corn was ruled out as a rotational crop in upland rice-based cropping systems [START_REF] Ratnadass | Impacts of some upland rice-based cropping systems on soil macrofauna abundance and diversity and black beetle damage to rice[END_REF] since it is an alternate host for both scarab beetles and witchweed. However, corn may provide possible indirect positive effects to control witchweed, along with other weeds, via the physical barrier effect of its residues in conservation agriculture systems [START_REF] Ranaivoson | Is mulching an efficient way to control weeds? Effects of type and amount of crop residue in rainfed rice based cropping systems in Madagascar[END_REF]. In this respect, further studies confirmed the potential of S. guianensis for witchweed and non-parasitic weed control in Malagasy conservation agriculture systems [START_REF] Ranaivoson | Is mulching an efficient way to control weeds? Effects of type and amount of crop residue in rainfed rice based cropping systems in Madagascar[END_REF][START_REF] Randrianjafizanaka | Combined effects of cover crops, mulch, zero-tillage and resistant varieties on Striga asiatica (L.) Kuntze in rice-maize rotation systems[END_REF][START_REF] Rafenomanjato | Weed management using a no-till system with Stylosanthes guianensis cover crop in upland rice-based cropping systems in the Mid-West of Madagascar[END_REF]. However, a recent study highlighted a clear tradeoff between corn yield and the biomass production of S. guianensis, highlighting the need for careful and timely management of this perennial legume to avoid interspecific competition with the cereal crop [START_REF] Rodenburg | Mixed outcomes from conservation practices on soils and Striga-affected yields of a low-input, rice-maize system in Madagascar[END_REF].

3.5. Synergies in terms of control of single pest or pathogen species via several pathways based on single PSD-deployment measures GMCCP biomass productions were observed to provide valuable regulatory effects of some soil-borne pests and pathogens in this study. Different pests and pathogens were observed to be regulated via different pathways. For example, increased percentage of ground cover was observed to provide weed suppression [START_REF] Ranaivoson | Is mulching an efficient way to control weeds? Effects of type and amount of crop residue in rainfed rice based cropping systems in Madagascar[END_REF], while the numbers of some scarab beetle species were suppressed because of GMCCP-related changes in macrofauna communities and the status of scarab beetle species [START_REF] Ratnadass | Interaction entre le système de culture et le statut (ravageur ou auxiliaire) des vers blancs (Coleoptera : Scarabaeoidea) sur le riz pluvial[END_REF]2017).

Bacterial wilt on tomato may be regulated both via biocidal effects [START_REF] Deberdt | Effect of Allium fistulosum extract on Ralstonia solanacearum populations and tomato bacterial wilt[END_REF] and via antagonistic microbial community mobilization due to GMCCP practices [START_REF] Diédhiou | Impact of biocontrol plants on bacterial wilt and non-targeted soil microbial communities on a naturally infested soil[END_REF].

Also, for some sanitizing species such as C. spectabilis, bacterial wilt on tomato may be regulated through a symbiotic effect caused by arbuscular mycorrhizal fungi (AMF) brought by the sanitizing plant. [START_REF] Chave | Rhizophagus irregularis MUCL 41833 transitorily reduces tomato bacterial wilt incidence caused by Ralstonia solanacearum under in vitro conditions[END_REF] similarly demonstrated that the association of AMF (Rhizophagus irregularis MUCL 41833) with tomato seedlings delayed the appearance of bacterial wilt disease symptoms caused by R. solanacearum phytotype IIB/4NPB. Conversely, R. solanacearum did not affect root colonization by the AMF within the 16 days of the experiment, suggesting that the AMF fungus was able to reduce bacterial wilt, probably by eliciting defense mechanisms in the plant. This strategy could possibly also be used to manage witchweed [START_REF] Lendzemo | Can arbuscular mycorrhizal fungi contribute to Striga management on cereals in Africa?[END_REF].

This project also highlighted the efficacy of certain intercropping/trap cropping strategies against some aerial pests. For example, biocidal/bottom-up effects (e.g. growth inhibition effect of corn silks on the tomato fruitworm) and top-down effects, e.g. predators like ladybugs (Coleoptera: Coccinellidae) on corn [START_REF] Rhino | Corn as trap crop to control Helicoverpa zea in tomato fields: importance of phenological synchronization and choice of cultivar[END_REF]2016) and pirate bugs (Hemiptera: Anthocoridae) and spiders on pigeon-pea [START_REF] Ratnadass | Potential of pigeon pea as a trap crop for control of fruit worm infestation and damage to okra[END_REF] were reported. Indirect effects (via food webs) such as the attraction of spiders into okra fields via perimeter pigeon-pea growing [START_REF] Ratnadass | Potential of pigeon pea as a trap crop for control of fruit worm infestation and damage to okra[END_REF] were highlighted.

The latter example highlighted the need to consider possible tradeoffs, between the prevention of infestation of the main vegetable crop by certain less important pests or the encouragement thereof.

This can be achieved, for example, by allowing early-season, less injurious sap-or mesophyll-feeding pests into crop fields, in anticipation of a regulatory effect on later occurring highly damaging fruit pests (e.g. tomato fruitworm), via top-down pathways involving generalist predators. This may be put in perspective with studies on tripartite trophic interactions between weaver ants, scale insects and fruit flies on different host tree species [START_REF] Olotu | Efficacy of the African weaver ant Oecophylla longinoda (Hymenoptera: Formicidae) in the control of Helopeltis spp. (Hemiptera: Miridae) and Pseudotheraptus wayi (Hemiptera: Coreidae) in cashew crop in Tanzania[END_REF][START_REF] Thurman | Weaver ants provide ecosystem services to tropical tree crops[END_REF][START_REF] Van Mele | Ant cues affect the oviposition behaviour of fruit flies (Diptera: Tephritidae) in Africa[END_REF], or mirid bugs on cocoa in West Africa [START_REF] Bagny-Beilhe | Identifying ant-mirid spatial interactions to improve biological control in cacao-based agroforestry system[END_REF][START_REF] Bisseleua | Ant-mediated ecosystem services and disservices on marketable yield in cocoa agroforestry systems[END_REF] and Australia [START_REF] Forbes | Increased pollinator habitat enhances cacao fruit set and predator conservation[END_REF].

3.6. Synergies or tradeoffs with respect to other ecosystem services or disservices (at the farm/cropping system level)

Soil improvement (via atmospheric Nitrogen fixation), is one of the objectives of the use of legume cover crops such as Crotalaria spp. Some species of the same genus can also be used for food, e.g. C.

brevidens [START_REF] Abukutsa-Onyango | Response of Slenderleaf (Crotalaria brevidens Benth) to inorganic nitrogen application[END_REF]. This is also true for pigeon-pea when used as trap crops, since pods can still be used even if partly damaged, unlike okra or tomato fruits.

Such a valorization of trap crop products occurs in Niger in the case of pigeon-pea seeds, while no such valorization is possible with sweet corn cobs in Martinique. On the other hand, it is precisely because sweet corn is not commonly grown that it can be proposed to farmers for cultivation as a "sacrificial crop", and harvested seed could still be used, e.g. as poultry feed [START_REF] Rhino | Corn as trap crop to control Helicoverpa zea in tomato fields: importance of phenological synchronization and choice of cultivar[END_REF]. [START_REF] Ranaivoson | Agroecological functions of crop residues under conservation agriculture[END_REF] have shown that much of the expected function of cover plant biomass depends on the amount of biomass left on the ground. However, with the notable exception of Crotalaria spp., due to its containing toxic pyrrolizidine alkaloids [START_REF] Williams | Occurrence, concentration, and toxicity of pyrrolizidine alkaloids in Crotalaria seeds[END_REF] almost all the plants used as cover crops, particularly in the Omega 3 project, are also valuable forage plants. In the context of family farming in sub-Saharan Africa, this biomass generally has a high economic value as livestock fodder [START_REF] Naudin | Trade-offs around the use of biomass for livestock feed and soil cover in dairy farms in the Alaotra lake region of Madagascar[END_REF]. While farmers may experience an almost certain gain in terms of livestock fodder, this may not always be the case in terms of pest control.

Furthermore, the seeding of cover crops requires a cost in time and money in an already-constrained agricultural context.

In addition to soil fertility enhancement, pigeon-pea borders may play a role in wind erosion alleviation. Similarly, legume trees used in agroforestry systems (e.g. Erythrina poepiggiana, but also other Fabaceae like Leucaena leucocephala or Albizzia lebeck) contribute both to soil fertility enhancement and reduced soil erosion.

In the reported studies, we have highlighted the importance of identification of tradeoffs and gains in GMCCP implementation on pest and disease management. Tradeoffs should be determined between allelopathic effects on witchweed of GMCCPs used in cereal-based conservation agriculture systems and competition for water with the main crop in mid-elevation areas (e.g. Arachis, Desmodium, which are more adapted to conservation agriculture systems in higher altitude/rainfall areas). In this regard, inspiration could be found in the "Climate-Smart Push-Pull Technology" approach [START_REF] Khan | Climate-smart Push-Pull: a Conservation Agriculture Technology for Food Security and Environmental Sustainability in Africa[END_REF], which was adapted from the earlier and highly successful "Push-Pull" technology [START_REF] Cook | The use of push-pull strategies in integrated pest management[END_REF] to address challenges provided by climate change. These adaptations implied that water-demanding trap crops and intercrops (e.g. respectively Napier grass and Silverleaf desmodium) were replaced by more drought-tolerant species (e.g. such as Brachiaria (cv Mulato) as trap crop and Greenleaf desmodium as intercrop) (Cheruiyot et al., 2018a), with the same pest-regulating effects (Cheruiyot et al., 2018b).

Environmental limitations to establishment and growth of trap crops are important and should be considered in planning of habitat management strategies such as increased PSD. For example, trap crops such as corn which has high water and soil fertility requirements can be grown wherever such resources are not limited (e.g. in Martinique). However, alternate trap crops which are drought tolerant and/or self-sufficient in nitrogen (e.g. sorghum, pigeon-pea) should be used wherever these resources are scarce. Trap crops such as pigeon-pea and corn (cf. Martinique, Niger, Réunion) furthermore contribute to biodiversity conservation, particularly of natural enemies of pests and may provide a natural pest regulation service to the whole farm [START_REF] Deguine | Agroecological cucurbit-infesting fruit fly management. A review[END_REF][START_REF] Ratnadass | Potential of pigeon pea as a trap crop for control of fruit worm infestation and damage to okra[END_REF][START_REF] Rhino | Corn as trap crop to control Helicoverpa zea in tomato fields: importance of phenological synchronization and choice of cultivar[END_REF].

Another example is the role that intercropped plants like shade trees may play in cocoa pollination ecology. In West African countries, cocoa yields hardly exceed a quarter of their potential. There are many reasons for this, one of which could be inadequate pollination. Cocoa flowers are thought to be almost exclusively pollinated by midges of the family Ceratopogonidae (Diptera), largely from the genus Forcipomyia [START_REF] Toledo-Hernández | Neglected pollinators: Can enhanced pollination services improve cocoa yields? A review[END_REF]. These insects prefer humid habitats where they breed in moist litter on the ground. Several studies have shown that adding organic matter to cocoa litter improves pollination and fruit set. By diversifying litter sources and increasing humidity in fullsun cocoa, plant diversification may thus improve pollination and consequently yield [START_REF] Adjaloo | Evaluation of breeding substrates for cocoa pollinator, Forcipomyia spp. and subsequent implications for yield in a tropical cocoa production system[END_REF][START_REF] Claus | Challenges in cocoa pollination: The Case of Côte d'Ivoire. Pollination in Plants[END_REF]. In addition, Toledo-Hernández et al. ( 2017) suggested that plant diversification, through habitat diversification, may enhance the role of other neglected groups of cocoa pollinators, such as ants, that could contribute to cocoa pollination.

3.7.

Tradeoffs that occur at other levels in the production system, landscape scale and/or crop value chain Possible tradeoffs include for example the acceptance of lower production levels vs premium prices due to certification (e.g. Organic Agriculture; Ethical Coffee labels: [START_REF] Perfecto | Biodiversity, yield, and shade coffee certification[END_REF][START_REF] Tscharntke | Conserving biodiversity through certification of tropical agroforestry crops at local and landscape scales[END_REF][START_REF] Waldron | Making biodiversity-friendly cocoa pay: combining yield, certification, and REDD for shade management[END_REF]. However, while there is a potential market for organic tomato in Martinique, no such market exists in Niger for okra [START_REF] Ratnadass | Crop protection for agricultural intensification systems in sub-Saharan Africa[END_REF]. Similarly, there are opportunities for agroecotourism [START_REF] Asare | Cocoa agroforestry for increasing forest connectivity in a fragmented landscape in Ghana[END_REF] in Martinique or in Réunion [START_REF] Deguine | Agroecological cucurbit-infesting fruit fly management. A review[END_REF] but less so in Africa. On the other hand, potential for organic or ethical cocoa or coffee production labels exists for West African or Costa Rican agroforestry systems [START_REF] Ayenor | Converging farmers' and scientists' perspectives on researchable constraints on organic cocoa production in Ghana: results of a diagnostic study[END_REF][START_REF] Babin | Pest management in organic cacao: Handbook of Pest Management in Organic Farming[END_REF].

As mentioned above, some GMCCPs can be valued for their market value per se, e.g. Allium fistulosum ("Onion péyi") in Martinique, which was reported to be a sanitizing plant used for controlling bacterial wilt on tomato caused by R. solanacearum [START_REF] Deberdt | Effect of Allium fistulosum extract on Ralstonia solanacearum populations and tomato bacterial wilt[END_REF]. Similar markets can be further developed for aromatic/medicinal companion plants in Martinique, as was suggested for the "push" or "pull" components of horticultural "push-pull" systems [START_REF] Cook | The use of push-pull strategies in integrated pest management[END_REF]. This was accomplished in Cuba with Plectranthus and Ocimum species, since the time of the US trade embargo which started at the beginning of the Revolution, and especially during the 'Special Period ' (1990-1991) after the collapse of imports of petroleum, agrochemicals and farm machinery from the Soviet bloc (Acosta de la [START_REF] Acosta De La Luz | Producción de plantas medicinales a pequeña escala: una necesidad de la comunidad[END_REF][START_REF] Altieri | The agroecological revolution in Latin America[END_REF].

In addition to adaptation to climate change, mitigation thereof may also be an incentive to deploy PSD-based pest and disease management strategies within agroecosystems. This might be of particular importance in relation to the carbon sequestration market (particularly for conservation agriculture, agroforestry and push-pull systems: [START_REF] Corbeels | The 4 per 1000 goal and soil carbon storage under agroforestry and conservation agriculture systems in sub-Saharan Africa[END_REF]. Actually, beyond the carbon sequestration aspect of PSD-based cropping systems, any crop protection measure per se contributes to climate change mitigation, as reduction of pest and disease-associated losses reduces the need for excessive input applications and unnecessary CO2 emissions linked with their production [START_REF] Lamichhane | Robust cropping systems to tackle pests under climate change[END_REF].

As for agroforestry systems, PSD which is deployed partly in view of pest or disease alleviation for the farmer's benefit, also contributes to watershed services [START_REF] Allinne | Delicate balance between pest and disease injuries, yield performance, and other ecosystem services in the complex coffee-based systems of Costa Rica[END_REF], for example improvement of water quality in dam reservoirs of Costa Rica (Gomez-Delgado et al., 2011). Benefits other than mere limitation of soil water erosion were reported by Ratnadass et al. (2012b), who indicated significant reduction in pollution levels of run-off water in conservation agriculture systems in Madagascar (Ratnadass et al., 2012b). Another example of the additional benefits of PSD in terms of limiting soil erosion and providing pest management services is that by [START_REF] Van Den Berg | Vetiver grass (Vetiveria zizanioides (L.) Nash) as trap plant for Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae) and Busseola fusca (Fuller) (Lepidoptera: Noctuidae)[END_REF] and [START_REF] Van Den Berg | Can vetiver grass be used to manage crop pests? In: Vetiver Grass -a World Technology and its Impact on Water[END_REF], who suggested the planting of Vetiver grass (Chrysopogon zizanioides) (Poaceae), which is commonly used as soil erosion management measure, for managing corn and rice stem borers.

Finally, other important aspects to keep in mind with regard to using PSD as a pest and disease management tool are the potential invasiveness of some GMCCPs, and their potential to host other pests and diseases that may become economically important. In this respect, some specific rules for their selection, as those proposed in the tomato bacterial wilt case study in Martinique [START_REF] Deberdt | The use of biocontrol plants to manage bacterial wilt of tomato in the tropics[END_REF] should be followed, which could be used in other locations, particularly in islands where there is a risk with invasive plants. The importance of several pests and diseases, as well as disease vectors that occur on plants that could be used as service plants in PSD systems have been reported by [START_REF] Van Den Berg | Developing habitat management systems for graminaceous stem borers in South Africa[END_REF], and should not be overlooked [START_REF] Ratnadass | First record of Moissonia importunitas as a pest of rattle box (Crotalaria spp.) in Réunion Island (Hemiptera, Miridae)[END_REF][START_REF] Ratnadass | Crop protection for agricultural intensification systems in sub-Saharan Africa[END_REF].

Conclusions

The Omega3 project presented in this review yielded results of different natures and each case study contributed to resolving local problems. For example, sanitizing crops for managing bacterial wilt on tomato in Martinique were identified and direct-seeding/mulch-based cropping systems were shown to reduce scarab beetle and witchweed numbers on upland rice in Madagascar. The potential of using trap cropping systems to control fruitworms on tomato in Martinique and on okra in Niger was reported and the suppressive effects of landscape fragmentation on coffee berry borer and orange leaf rust on coffee in Costa Rica, highlighted. However, this project also indicated that PSD deployment per se does not provide positive pest regulatory services; for example, no effects were observed on the cocoa mirid bug/brown pod rot case study in Cameroon.

A dynamic and spatially-explicit individual-based model was developed based on the fruitworm/tomato subcase study described above. This model will in the future be used as a generic tool to study pest and disease infestation patterns in response to plant characteristics as well as spatiotemporal deployment of commercial and trap crops, which will eventually lead to a better understanding of agroecosystem functioning.

Results also highlighted cross-cutting issues regarding plants used as green manure, cover crops or companion plants (GMCCP) in conservation agriculture and market gardening case studies (at the field scale), and stressed the importance of within-species genetic variation of these GMCCP species.

It is important to realize that tradeoffs exist when pest management tools such as PSD management is used to manage a complex of pests and diseases. A single pest or disease control tool may affect the pest/pathogen complex through several pathways and may also influence other ecosystem services or disservices at various scales.

Box 1 .

 1 An individual-based modeling approach to assess the efficacy of trap cropping in pest management: Helicoverpa zea management in

Figure 1 .

 1 Figure 1. Positioning of the reported case studies (cf. Table 1 for numbering and abbreviations), as a function of scale levels at which plant species diversity modalities and effects are deployed

Table 1 . Description of the six Omega3 project case studies and 11 biological models, as a function of scale levels at which plant species diversity modalities and effects translate, and a life history trait-based typology of pests and pathogens Case study Pest or disease Crop Cropping system 1 Spatial scale Geographical area Host specificity

 1

	Dispersal
	ability

11

Entwistle (1972); 12

Table 2 . References of publications reporting results of the Omega3 project case studies 120

 2 Sanitizing effects of GMCCPs planted as rotational crops vis-à-vis bacterial wilt on tomato in market-gardening systems of Martinique ("case study 1, Table

	Case	Crop	Geographical	Pest or disease	References
	study		area
		Tomato	Martinique	Bacterial wilt (Ralstonia solanacearum)	Deberdt et al. (2015); Deberdt et al. (2018); Diédhiou et al. (2012)
		Upland rice Madagascar	Scarab beetles (Heteronychus spp.)	Rabary et al. (2011); Rafarasoa et al. (2016)
		Upland rice Madagascar	Witchweed (Striga asiatica)	Michellon et al. (2011)
		Tomato	Martinique	Fruitworm (Corn earworm Helicoverpa zea)	Rhino et al. (2014); Grechi et al. (2012)
		Okra	Niger	Fruitworm (Tomato fruitworm Helicoverpa armigera) Ratnadass et al. (2014); Yabo (2010)
		Okra	Niger	Whitefly (Bemisia tabaci)	Ratnadass et al. (2014)
		Cucurbits	Réunion	Fruit flies (Diptera : Tephritidae)	Bonnet (2010); Duhautois (2010); Atiama-Nurbel et al. (2012);
					Deguine et al. (2015)
		Cocoa	Cameroon	Mirid bug (Sahlbergella singularis)	Babin et al. (2012); Mahob et al. (2015)
		Cocoa	Cameroon	Black pod rot (Phytophthora megakarya)	Ten Hoopen et al. (2012)
		Coffee	Costa Rica	Leaf rust (Hemileia vastatrix)	Avelino et al. (2012); Lopez-Bravo et al. (2012)
		Coffee	Costa Rica	Berry borer (Hypothenemus hampei)	Avelino et al. (2012)

Table 3 . Literature references on pros and cons of some GMCCPs* evaluated across some Omega3 project case studies

 3

	Case studies (Crops-	Upland rice
	Areas-Pests/
	Diseases)
	GMCCPs

[START_REF] Johanneson | Host pest relationship of the genus Hypothenemus (Scolytidae: Coleoptera) with special reference to the coffee berry borer (H. hampei)[END_REF];18 [START_REF] Decazy | Le scolyte du fruit du caféier, Hypothenemus hampei: considérations sur la lutte intégrée contre ce ravageur[END_REF]

Acknowledgments

The authors wish to thank, for assistance in the reported case studies, or the general implementation of the Omega3 project: Jessica Ambard, Alain-Paul Andrianaivo, Toulassi Atiama-Nurbel, Leila

Bagny Beilhe, Emilie Bonnet, François Bonnot, Raymond Bourgoing, Frédéric Chiroleu, Régine Coranson-Beaudu, Hector Cruz Cuellar, Clémentine D'Avout, Joël Daniel, Elisabeth Douraguia, Sophie Duhautois, Eric Gozé, Eva Groshens, Robert Habib, Adjara Liman Katiella, Harouna Idé Katouné, Jean-Baptiste Laurent, Marie-Ange Lebas, Donal Fernando Lopez-Bravo, Philippe Lucas, Abdoul-Aziz Maazou, Manana, Gaëlle Marliac, Félix Mathurin, Roger Michellon, Sonia Minatchi, Issa Mze Hassani, Amada Pastora Olivas, Alain Pelage, Aubéri Petite, Baptiste Quaranta, Serge Quilici, Meva Tahiry Randrianjafizanaka, Narcisse Randrianasolo, Richard Randriamanantsoa, Fiononamboahangy Ratahakarivony, Ali Romero-Gurdian, Philippe Ryckewaert, Halarou Salha, Esteban Sanchez, Jean-Louis Sarah, Saïdou Aoudou Siaka, Romuald Souga, Olivier Sounigo, Gerben Martijn Ten Hoopen, Catherine Thibaud, Philippe Tixier, Metty Trebeau, Aude Verchère, Sergio Vilchez, Elias de Melo Virginio-Filho, Sendrasoa Alphène Vololomanjaka, Oumarou Yabo, Yede, Ousmane Zakari-Moussa. The authors also thank all collaborative farmers at all study sites. In addition, they thank the three anonymous reviewers whose comments helped improve the manuscript.

Funding

This work was funded by Cirad's core budget (ATP Omega3 Project).

Declaration on conflict of interest

The authors declare that there is no conflict of interest

Author contribution

Conceptualization A.R., J.A., P.F., P.L., F.L. and J.V.D.B.; Methodology: J.A., R.B., P.D., J.P.D., K.N., B. Rhino; Investigation: A.R., H.A.K.K., J.A., F.D., R.B., R.M., P.D., P.F., J.P.D., K.N., L.S.R., B. Rabary, B. Rhino, I.G., P.L.; Writing -Original Draft: A.R.; Writing -Review & Editing: F.D., F.L., J.V.D.B., I.G., J.A., K.N., P.F., P.D., R.M., R.B., J.P.D., B. Rhino, H.A.K.K., P.L., L.S.R. and B.

Rabary; Funding Acquisition: A.R., J.A., P.F. and P.L.; Supervision: A.R.