Hyperparasitoids as new targets in biological control in a global change context

Keywords: Pest-control, intraguild interactions, food-web, climate-change, parasitoid, conservation, ecosystem services

Agricultural and forest insect pests generate important yield losses worldwide. Globalchange is expected to increase pest outbreaks and their impact on human-managed ecosystems. Pest control performed by natural enemies is also likely to be influenced by global-change. Yet, pests and their natural enemies are part of complex food-webs and interact with other species, notably at upper trophic levels (e.g. hyperparasitoids, predators). These interactions have to be considered as important facets of food-web structure, functioning, and pest control efficiency. Relatively recent evidence suggests that global-change may translate to modifications in uppertrophic level abundance, phenology, and geographic range. The combination of these shifts at different trophic-levels may ultimately threaten ecosystem services such as biological pest control, yet these shifts have largely been overlooked. Little information is available on hyperparasitoid ecology and therefore little is known about the potential impact of climate change on these species. Improving our knowledge on this topic is important if we aim at adopting biological control programs in the near future. In this overview, we first emphasize that hyperparasitoids may have huge potential to disrupt biological control in natural and agricultural settings. We then stress that this disruption may increase in frequency and magnitude in the near future due to global-change. We finally propose that hyperparasitoids may become new targets in biological control and recommend different methods to control them, or limit their impact.

Introduction

Agricultural and forest productivity will likely increase to meet intensifying human demand for food, manufactured goods and houses [START_REF] Foley | Global Consequences of Land Use[END_REF]. In the meantime, the Earth is experiencing rapid changes in both climatic conditions and land-use [START_REF] Tilman | Global environmental impacts of agricultural expansion: the need for sustainable and efficient practices[END_REF][START_REF] Karl | Modern Global Climate Change[END_REF]. These changes have led to a drastic decrease in biodiversity and to important consequences on species ecology, including distribution ranges, phenology and physiology, ultimately threatening the stability of ecosystems [START_REF] Hautier | Anthropogenic environmental changes affect ecosystem stability via biodiversity[END_REF][START_REF] Walther | Ecological responses to recent climate change[END_REF] and degrading ecosystem services [START_REF] Tscharntke | Landscape perspectives on agricultural intensification and biodiversity-ecosystem service management[END_REF][START_REF] Montoya | Climate change, biotic interactions and ecosystem services[END_REF]. One of the most important ecosystem services is biological pest control, which helps to maintain pest damage below tolerable thresholds in human-managed ecosystems, such as agrosystems or forests, and can help to sustain the increasing production demand [START_REF] Van Lenteren | The state of commercial augmentative biological control: plenty of natural enemies, but a frustrating lack of uptake[END_REF][START_REF] Van Lenteren | Biological control using invertebrates and microorganisms: plenty of new opportunities[END_REF]. Whether global-change will negatively or positively impact the efficiency of biological-control service remains unclear, but every type -either natural, classical, conservation or augmentative biological control -is likely to be impacted [START_REF] Aguilar-Fenollosa | Can we forecast the effects of climate change on entomophagous biological control agents?[END_REF][START_REF] Andrew | Effect of Climate Change on Insect Pest Management[END_REF]Björkman and Niemelä, 2015).

Global change is expected to increase damage caused by agricultural pests by increasing their population growth, generations per year, and dispersal, by inducing earlier pest outbreaks, decreasing natural enemies' efficiency, and decreasing plant resistance (Björkman and Niemelä, 2015;[START_REF] Klapwijk | Assessing the impact of climate change on outbreak potential[END_REF][START_REF] Pincebourde | Plant-Insect Interactions in a Changing World[END_REF]. Environmental changes might also be beneficial for biological control agents if, for example, they are able to find more hosts/prey for a longer time window [START_REF] Hance | Impact of Extreme Temperatures on Parasitoids in a Climate Change Perspective[END_REF][START_REF] Thomson | Predicting the effects of climate change on natural enemies of agricultural pests[END_REF] since early suppression of pests is crucial for efficient biological control (Gómez-Marco et al., 2016a;[START_REF] Langer | Seasonal activity and distribution of cereal aphid parasitoids in Belgium[END_REF][START_REF] Neuville | The delay in arrival of the parasitoid Diaeretiella rapae influences the efficiency of cabbage aphid biological control[END_REF]. Parasitoids are among the main natural enemies, and have been widely used in biological control against insect pests [START_REF] Jervis | Insects as natural enemies: a practical perspective[END_REF][START_REF] Van Lenteren | Biological control using invertebrates and microorganisms: plenty of new opportunities[END_REF]. Globalchange will either (i) enhance biological control by parasitoids if they adapt better to new conditions than their hosts, (ii) maintain unchanged biological control by parasitoids if they are able to keep pace with their hosts' distribution or phenology shifts, or (iii) disrupt biological control by parasitoids if they adapt less well to new environmental conditions than their hosts.

However, parasitoids and their hosts are not alone in their environment and they interact with other species in complex food-webs. Following global-change, novel interactions will appear through shifts in geographic ranges ( [START_REF] Van Der Putten | Climate Change, Aboveground-Belowground Interactions, and Species' Range Shifts[END_REF] or in phenology [START_REF] Forrest | Complex responses of insect phenology to climate change[END_REF][START_REF] Visser | Phenology: Interactions of climate change and species[END_REF], altering the abundance, distribution, and functions of species in a food-web [START_REF] Facey | Upsetting the order: how climate and atmospheric change affects herbivore-enemy interactions[END_REF][START_REF] Gilman | A framework for community interactions under climate change[END_REF][START_REF] Walther | Ecological responses to recent climate change[END_REF], potentially leading to increased antagonism among natural enemies (predation, parasitism or competition). These changes in food-web interactions are an additional challenge that biological control practitioners will have to face in the upcoming years [START_REF] Andrew | Effect of Climate Change on Insect Pest Management[END_REF][START_REF] Hance | Impact of Extreme Temperatures on Parasitoids in a Climate Change Perspective[END_REF]. Thus, if we aim at predicting the effect of global-change on biological control, there is a need to consider such effects not only at the pest-parasitoid scale, but at the whole community scale (e.g. shifts in interactions involving more than two species) [START_REF] Frago | Interactions between parasitoids and higher order natural enemies: intraguild predation and hyperparasitoids[END_REF][START_REF] Thackeray | Phenological sensitivity to climate across taxa and trophic levels[END_REF][START_REF] Thompson | Means and extremes: building variability into community-level climate change experiments[END_REF][START_REF] Tylianakis | Global change and species interactions in terrestrial ecosystems[END_REF][START_REF] Van Der Putten | Predicting species distribution and abundance responses to climate change: why it is essential to include biotic interactions across trophic levels[END_REF][START_REF] Visser | Phenology: Interactions of climate change and species[END_REF].

Intraguild interactions among parasitoid species (i.e. species sharing the same host) are good examples of the complexity underlying food-web functioning and of the difficulty to develop successful biological control programs [START_REF] Gómez-Marco | Untangling the aphid-parasitoid food web in citrus: Can hyperparasitoids disrupt biological control?[END_REF][START_REF] Traugott | Endoparasitism in cereal aphids: molecular analysis of a whole parasitoid community[END_REF]. Specifically, the effects of intraguild competition (i.e. direct competition, multiparasitism) (reviewed in [START_REF] Boivin | Intra-and Interspecific Interactions among Parasitoids: Mechanisms, Outcomes and Biological Control[END_REF][START_REF] Harvey | Intrinsic Inter-and Intraspecific Competition in Parasitoid Wasps[END_REF] and intraguild predation (i.e. when at least two species that share the same prey or host also predate or parasitize each other) (reviewed in [START_REF] Rosenheim | Intraguild predation among biological-control agents: theory and evidence[END_REF] on biological control have been well explored. Surprisingly, and despite their importance in biological control and their ubiquity in food-webs, the effects of upper trophic levels such as hyperparasitoids on pest population regulation have been neglected relative to interactions mentioned above. Hyperparasitoids are secondary parasites of immature stages of primary parasitoids and belong to the fourth (or upper) trophic levels [START_REF] Sullivan | Insect hyperparasitism[END_REF] and are the only specific natural enemies of parasitoids [START_REF] Boivin | Intra-and Interspecific Interactions among Parasitoids: Mechanisms, Outcomes and Biological Control[END_REF]. Hyperparasitoids have a wide host range and harbor complex life-cycles and host-foraging behaviour since they can be either facultative or obligate [START_REF] Brodeur | Host specificity and trophic relationships of hyperparasitoids[END_REF][START_REF] Hunter | Evolution and behavioral ecology of heteronomous aphelinid parasitoids[END_REF]. They come from a few taxa but are present in a wide variety of ecosystems, including forests, crops, orchards and greenhouses [START_REF] Brodeur | Host specificity and trophic relationships of hyperparasitoids[END_REF][START_REF] Frago | Interactions between parasitoids and higher order natural enemies: intraguild predation and hyperparasitoids[END_REF][START_REF] Prado | Ecological Interactions Affecting the Efficacy of Aphidius colemani in Greenhouse Crops[END_REF]. As high trophic level species, they are more susceptible to perturbations because of bottom-up effects in the food-web, as predicted by the trophic rank hypothesis [START_REF] Holt | Trophic rank and the species-area relationship[END_REF][START_REF] Gilman | A framework for community interactions under climate change[END_REF]). Yet, hyperparasitoids did not gain the attention that they deserve in most biocontrol studies. We first review the effect of hyperparasitoids on biological control provided by parasitoids; we then analyze the potential effect of climate change on hyperparasitoids and biological control; and, finally, we propose several methods to overcome the effect of hyperparasitoids on biological control.

Hyperparasitoids and biological control disruption

When any type of biological control fails, it is often because scientists overlooked natural enemies as part of a food web [START_REF] Goldson | If and when successful classical biological control fails[END_REF]. Studies of natural host-parasitoid communities reveal that hyperparasitoids are common in many networks [START_REF] Gómez-Marco | Untangling the aphid-parasitoid food web in citrus: Can hyperparasitoids disrupt biological control?[END_REF][START_REF] Müller | Indirect interactions in aphid-parasitoid communities[END_REF][START_REF] Tena | Parasitoid complex of black scale Saissetia oleae on citrus and olives: parasitoid species composition and seasonal trend[END_REF][START_REF] Traugott | Endoparasitism in cereal aphids: molecular analysis of a whole parasitoid community[END_REF]. In fact, numerous field studies have reported hyperparasitism levels up to 90% (e.g. [START_REF] Holler | The Relationship between Primary Parasitoids and Hyperparasitoids of Cereal Aphids: An Analysis of Field Data[END_REF]. [START_REF] Prado | Ecological Interactions Affecting the Efficacy of Aphidius colemani in Greenhouse Crops[END_REF] also state that high hyperparasitism rates may be reached in greenhouses due to temperature advantage. Naturally occurring hyperparasitism is traditionally thought to disrupt biological control by primary parasitoids on target pests because hyperparasitoids develop at the expense of parasitoids and are, thus, likely to limit the control of herbivorous hosts by the following parasitoid generation [START_REF] Rosenheim | Higher-order predators and the regulation of insect herbivore populations[END_REF][START_REF] Sullivan | Hyperparasitism: multitrophic ecology and behavior[END_REF]. Among the detrimental effects on primary parasitoids, hyperparasitoids may affect their establishment, occurrence and abundance not only through direct parasitism but also by acting as intraguild competitors (i.e., facultative hyperparasitoids that exploit herbivores and primary parasitoids) [START_REF] Boivin | Intra-and Interspecific Interactions among Parasitoids: Mechanisms, Outcomes and Biological Control[END_REF][START_REF] Snyder | Behavior influences whether intra-guild predation disrupts herbivore suppression by parasitoids. Behavioral ecology of insect parasitoids[END_REF]. For example, [START_REF] Pérez-Lachaud | Insect gladiators: competitive interactions between three species of bethylid wasps attacking the coffee berry borer, Hypothenemus hampei (Coleoptera: Scolytidae)[END_REF] showed that Cephalonomia hyalinipennis, a facultative bethylid hyperparasitoid of the coffee berry borer, affected the primary biocontrol agents through direct behavioural contests for hosts. In another example, primary parasitoids Metaphycus and the facultative hyperparasitoid Coccophagus lycimnia compete for hosts of the same size when they parasitize the citricola scale Coccus pseudomagnoliarum leading to reduced levels of biological control [START_REF] Bernal | Seasonal and Scale Size Relationships between Citricola Scale (Homoptera: Coccidae) and Its Parasitoid Complex (Hymenoptera: Chalcidoidea) on San Joaquin Valley Citrus[END_REF]. As a consequence, the potential negative impact of hyperparasitoids may be different between obligate and facultative hyperparasitoids. Obligate hyperparasitoids generally hinder the effect of primary parasitoids but may circumstantially play a role in stabilizing hostparasitoids interactions through density-dependent effects [START_REF] Beddington | On the Dynamics of Host-Parasite-Hyperparasite Interactions[END_REF][START_REF] Hassell | Host-Parasitoid Population Interactions[END_REF][START_REF] Holt | The Coexistence of Competing Parasites. Part II-Hyperparasitism and Food Chain Dynamics[END_REF], and may thus in some cases enhance biological control by primary parasitoids [START_REF] Rosenheim | Higher-order predators and the regulation of insect herbivore populations[END_REF][START_REF] Sullivan | Hyperparasitism: multitrophic ecology and behavior[END_REF]. The latter (facultative hyperparasitoids) may both outcompete and parasitize primary parasitoids, in addition to ovicide and larvicide effects [START_REF] Bernal | Seasonal and Scale Size Relationships between Citricola Scale (Homoptera: Coccidae) and Its Parasitoid Complex (Hymenoptera: Chalcidoidea) on San Joaquin Valley Citrus[END_REF][START_REF] Pérez-Lachaud | Wasp eat wasp: facultative hyperparasitism and intra-guild predation by bethylid wasps[END_REF], but may still be efficient at controlling pests since some facultative hyperparasitoids have been considered as candidate species for classical or augmentative biological control [START_REF] Boivin | Intra-and Interspecific Interactions among Parasitoids: Mechanisms, Outcomes and Biological Control[END_REF]. Thus, the debate about the extent to which hyperparasitoids can disrupt or even enhance biological control remains open, and would benefit from more theoretical and empirical studies [START_REF] Brodeur | Host specificity and trophic relationships of hyperparasitoids[END_REF][START_REF] Rosenheim | Intraguild predation among biological-control agents: theory and evidence[END_REF][START_REF] Schooler | The potential for hyperparasitism to compromise biological control: Why don't hyperparasitoids drive their primary parasitoid hosts extinct?[END_REF][START_REF] Snyder | Behavior influences whether intra-guild predation disrupts herbivore suppression by parasitoids. Behavioral ecology of insect parasitoids[END_REF].

In biological control, the actual impact of naturally occurring hyperparasitoids on the efficacy of primary parasitoids can be confused with other biotic and abiotic factors. For example, immature mortality of the primary host can be due not only to hyperparasitoids but also to host defenses or extreme climatic conditions [START_REF] Godfray | Parasitoids: behavioral and evolutionary ecology[END_REF]. Moreover, the effect of hyperparasitoids can go beyond the direct mortality of immature parasitoids. For example, they can deter primary parasitoids of foraging on host patches through volatiles [START_REF] Höller | Enemy-induced dispersal in a parasitic wasp[END_REF][START_REF] Petersen | Hyperparasitoid volatiles as possible foraging behaviour determinants in the aphid parasitoid Aphidius uzbekistanicus (Hymenoptera: Aphidiidae)[END_REF]; or be responsible of the apparent competition between primary parasitoids, leading to the extinction of the loser species [START_REF] Van Nouhuys | Apparent competition between parasitoids mediated by a shared hyperparasitoid[END_REF]. From a population dynamics point of view, hyperparasitoids are likely to act similarly as superpredators on the prey/pest trophic level (e.g. [START_REF] Mbava | Prey, predator and super-predator model with disease in the super-predator[END_REF], which can be observed directly by monitoring food-webs over time or recording behavioural activities. In addition to direct population monitoring, other indirect molecular tools such as barcoding or stable isotopes analyses can be used to assess and quantify the impact of hyperparasitoids on lower trophic levels [START_REF] Sanders | Trophic assimilation efficiency markedly increases at higher trophic levels in four-level host-parasitoid food chain[END_REF][START_REF] Traugott | Endoparasitism in cereal aphids: molecular analysis of a whole parasitoid community[END_REF].

Hyperparasitoids can act on the efficiency of natural (discussed above), classical, augmentative or conservation biological control.

Hyperparasitoids in classical biological control

In classical biological control, native hyperparasitoids can already be present at the location of introduction and can therefore reduce its own potential, although complex density-dependent interactions can appear in some other host-parasitoid-hyperparasitoid systems [START_REF] Hougardy | Comparative life history and parasitism of a new colour morph of the walnut aphid in California[END_REF]. For example, the native hyperparasitoid Conura albifrons was shown to attack the primary parasitoid Diadromus pulchellus, an exotic biological control agent released in North America to control the leek moth [START_REF] Miall | Potential impact of the native hyperparasitoid Conura albifrons (Hymenoptera: Chalcididae) on the exotic biological control agent Diadromus pulchellus (Hymenoptera: Ichneumonidae)[END_REF]. Hyperparasitoids can also be fortuitously released with the new primary parasitoid, in both cases impairing the long-term establishment of the primary parasitoid [START_REF] Goldson | If and when successful classical biological control fails[END_REF]. Although great care is made to exclude hyperparasitoids when importing insects, several cases of fortuitous introduction of hyperparasitoids -such as in Southern California at the beginning of the 20 th centuryreinforced the importance of quarantine procedures before the introduction [START_REF] Sawyer | To make a spotless orange: Biological control in California[END_REF]. Moreover, the important steps taken to reduce the risks associated with biological control introductions over the past 20 years have led to greatly improved safety of introductions [START_REF] Heimpel | Biological control: ecology and applications[END_REF][START_REF] Van Lenteren | Environmental risk assessment of exotic natural enemies used in inundative biological control[END_REF]. Therefore, it seems unlikely that the fortuitous introduction of hyperparasitoids will occur in the coming years.

Hyperparasitoids in augmentative biological control

Hyperparasitoids can also disrupt augmentative biological control. The best known example is the augmentative releases of Aphidius colemani (Hymenoptera: Braconidae) in greenhouses to control aphids [START_REF] Prado | Ecological Interactions Affecting the Efficacy of Aphidius colemani in Greenhouse Crops[END_REF]. There are several species of hyperparasitoids that attack and hinder the effect of A. colemani [START_REF] Acheampong | Survey of parasitoids and hyperparasitoids (Hymenoptera) of the green peach aphid, Myzus persicae and the foxglove aphid, Aulacorthum solani (Hemiptera: Aphididae) in British Columbia[END_REF][START_REF] Bloemhard | Seasonal abundance of aphid hyperparasitoids in organic greenhouse crops in The Netherlands[END_REF][START_REF] Van Steenis | Evaluation of four aphidiine parasitoids for biological control of Aphis gossypii[END_REF]. These hyperparasitoids are more abundant and active in summer [START_REF] Van Steenis | Evaluation of four aphidiine parasitoids for biological control of Aphis gossypii[END_REF][START_REF] Bloemhard | Seasonal abundance of aphid hyperparasitoids in organic greenhouse crops in The Netherlands[END_REF]. For this reason, augmentative releases of A. colemani are recommended in spring, when hyperparasitism does not interfere with aphid control [START_REF] Prado | Ecological Interactions Affecting the Efficacy of Aphidius colemani in Greenhouse Crops[END_REF]. The use of banker plants can also hinder the efficacy of A. colemani releases if hyperparasitoids can find hosts and overwinter there [START_REF] Nagasaka | Impact of secondary parasitism on Aphidius colemani in the banker plant system on aphid control in commercial greenhouses in Kochi, Japan[END_REF].

Naturally present primary parasitoid and hyperparasitoid populations around agricultural fields can interact with released parasitoids. This should be taken into account for the development of pest management strategies in the future. However, as mentioned before, little is known about what hyperparasitoid species are present around cultivated fields, and we know even less about their behaviour and ecology. Several questions remain to be answered if our aim is to include hyperparasitoids as an important factor when making management decisions. For example; how do they interact with other species, especially with released and naturally present primary parasitoids, and with the abiotic environment?

Hyperparasitoids in conservation biological control

In conservation biological control, hyperparasitoids can take advantage of the conservation procedures aimed to improve the fitness of primary parasitoids [START_REF] Müller | Indirect interactions in aphid-parasitoid communities[END_REF]. In conservation biological control, there may be a trade-off between improving primary parasitoid efficiency and attracting hyperparasitoids or improving their fitness. For instance, nectar sources added around the fields to improve efficiency of Aphidius ervi, a primary parasitoid of aphids, could also benefit hyperparasitoids such as Dendrocerus aphidum [START_REF] Araj | Floral nectar affects longevity of the aphid parasitoid Aphidius ervi and its hyperparasitoid Dendrocerus aphidum[END_REF][START_REF] Araj | Adding floral nectar resources to improve biological control: Potential pitfalls of the fourth trophic level[END_REF]. Several elements in the landscape such as the composition of field margins may also provide hyperparasitoid alternative host species or suitable over-wintering sites [START_REF] Plećaš | Landscape composition and configuration influence cereal aphidparasitoid-hyperparasitoid interactions and biological control differentially across years[END_REF].

The effects of global-change on hyperparasitoids

As global-changes are altering trophic interactions [START_REF] Tylianakis | Global change and species interactions in terrestrial ecosystems[END_REF], and the scheduling of species activities in a food-web [START_REF] Forrest | Complex responses of insect phenology to climate change[END_REF], the effect of hyperparasitoids on biological control could be modified. Predicting hyperparasitoids' range or phenology shifts following global-change is problematic because their life-cycles are often understudied and strongly linked to those of lower trophic levels (primary parasitoids, hosts and host-plants). Land-use change and climate change are the main drivers of global environmental changes and are expected to have both additive and synergistic effects on organisms ecology [START_REF] Oliver | Interactions between climate change and land use change on biodiversity: attribution problems, risks, and opportunities: Interactions between climate change and land use change[END_REF]. Among potential interactions between both global-change drivers, one could expect that due to climate warming, the build-up of hyperparasitoid populations would be faster in simplified landscapes with more host resources (e.g. aphids) than in more complex landscapes, especially for facultative hyperparasitoids and generalist species. Predictions may vary between generalist and specialist hyperparasitoids species because the latter are not expected to shift their range or phenology beyond that of their hosts, while generalist hyperparasitoids are more likely to find an alternative host and spread to other systems in the global-change context.

Landscape complexity tends to increase species diversity and the action of primary parasitoids as well as hyperparasitoids [START_REF] Ortiz-Martínez | The effect of landscape context on the biological control of Sitobion avenae: temporal partitioning response of natural enemy guilds[END_REF][START_REF] Plećaš | Landscape composition and configuration influence cereal aphidparasitoid-hyperparasitoid interactions and biological control differentially across years[END_REF], especially for generalist species such as parasitoid communities associated with aphids [START_REF] Brodeur | Host specificity and trophic relationships of hyperparasitoids[END_REF][START_REF] Heimpel | Biological control: ecology and applications[END_REF]. By analysing food-webs of cereal aphids along an agricultural landscape gradient of structural complexity, [START_REF] Gagic | Food web structure and biocontrol in a four-trophic level system across a landscape complexity gradient[END_REF] have shown that both parasitism and hyperparasitism levels are higher in complex landscapes with little percentage of arable lands. Yet, [START_REF] Rand | Landscape complexity differentially benefits generalized fourth, over specialized third, trophic level natural enemies[END_REF] found that generalist hyperparasitoids benefit landscape complexity more than specialist primary parasitoids. Therefore, the question remains as to the extent to which landscape simplification may act on the services/disservices balance (i.e. will landscape simplification decrease more hyperparasitism than it decreases primary parasitism and favor pests). More studies are necessary to understand how agricultural intensification and landscape simplification can act indirectly on hyperparasitoids by modifying trophic interactions between pests and primary parasitoids, which can in turn trigger effects on the upper trophic level through bottom-up effects [START_REF] Gagic | Agricultural intensification and cereal aphid-parasitoid-hyperparasitoid food webs: network complexity, temporal variability and parasitism rates[END_REF][START_REF] Lohaus | Farming practices change food web structures in cereal aphidparasitoid-hyperparasitoid communities[END_REF][START_REF] Zhao | Effects of inter-annual landscape change on interactions between cereal aphids and their natural enemies[END_REF].

Concerning the effects of climate-change, higher temperatures will likely lead to a decrease in longevity in insects from all trophic levels through increasing metabolic rates [START_REF] Van Baaren | Consequences of Climate Change for Aphid-Based Multi-trophic Systems[END_REF]. In the hyperparasitoid Asaphes vulgaris (Hymenoptera: Pteromalidae), the lifespan is four months at 15°C while it decreases to only 46 days at 25°C [START_REF] Brodeur | Life history of the aphid hyperparasitoid Asaphes vulgaris Walker (Pteromalidae): possible consequences on the efficacy of the primary parasitoid Aphidius nigripes Ashmead (Aphidiidae)[END_REF]. However, hyperparasitoids may benefit from higher temperatures for several reasons. First, climate-change may allow hyperparasitoids to terminate diapause earlier than reported in past decades, thus increasing their damage on early-season parasitoids [START_REF] Gómez-Marco | Untangling the aphid-parasitoid food web in citrus: Can hyperparasitoids disrupt biological control?[END_REF][START_REF] Tougeron | Are aphid parasitoids from mild winter climates losing their winter diapause?[END_REF]. Hyperparasitoids can be active in winter only if primary parasitoids are available, which in turn requires the presence of hosts. Such a scenario is now present in western and southern Europe where winters are mild enough to allow insect activity throughout the year, including by hyperparasitoids [START_REF] Andrade | Impact of change in winter strategy of one parasitoid species on the diversity and function of a guild of parasitoids[END_REF]Gómez-Marco et al., 2016a;[START_REF] Lumbierres | Seasonal parasitism of cereal aphids in a Mediterranean arable crop system[END_REF]. Hyperparasitoids were only observed from spring to fall a few decades ago [START_REF] Krespi | Variability in the development of cereal aphid parasitoids and hyperparasitoids in oceanic regions as a response to climate and abundance of hosts[END_REF][START_REF] Rabasse | Overwintering of primary parasites and hyperparasites of cereal aphids in western France[END_REF] but, as in many insect species, their phenology seems to be changing following both softening of climatic conditions and modifications in their host's phenology [START_REF] Tougeron | Are aphid parasitoids from mild winter climates losing their winter diapause?[END_REF]. Secondly, although no long term analysis on hyperparasitoid voltinism (i.e. the production of supplementary generation each year) are available to our knowledge, we expect an increase in voltinism in hyperparasitoids as occurs in many insects under warmer climates [START_REF] Sgrò | What Can Plasticity Contribute to Insect Responses to Climate Change[END_REF]. Finally, basal thermal tolerance and thermal optima can differ between trophic levels which may condition the way hyperparasitoids interact with lower trophic levels in a warmer climate [START_REF] Furlong | Climate change and biological control: the consequences of increasing temperatures on host-parasitoid interactions[END_REF][START_REF] Gilman | A framework for community interactions under climate change[END_REF], and therefore change the way pest populations are controlled. [START_REF] Rice | Temperature and developmental interactions in a multitrophic parasitoid guild[END_REF] showed that the developmental threshold from larva to adult was 3°C higher for the hyperparasitoid Mesochorus sp. (Hymenoptera: Mesochorinae) than for three primary parasitoids species it attacks. Differences in plastic response and in adaptive potential to rapid temperature warming among trophic levels are thus critical to consider in this context and can be viewed as a "thermal arming race" between interacting species [START_REF] Ferris | The physiological arms race: Exploring thermal acclimation among interacting species[END_REF][START_REF] Somero | The physiology of climate change: how potentials for acclimatization and genetic adaptation will determine "winners" and "losers[END_REF]. Nevertheless, the extent to which higher temperatures would benefit hyperparasitoids more than it would primary parasitoids or pest insects still remains to be clarified in most host-parasitoid-hyperparasitoid systems.

If temperatures remain in an optimal or quasi-optimal range for hyperparasitoids, we expect an overall positive effect of climate-change on hyperparasitoid abundances and species richness at a given location, likely due to increase in foraging capacities, and increasing voltinism and seasonal activity levels. However, whether these responses will negatively or positively affect biological pest control is a question that remains to be answered, especially because many cropspecific and species-specific situations will likely appear. Numerous gaps exist concerning our understanding of hyperparasitoid ecology and biology. However, we can speculate that both evolutionary and plastic adjustments, including maternal effects and bet-hedging strategies, will be involved in their adaptation to environmental changes. The response of hyperparasitoids to ongoing global-changes is likely to be the same as the response of primary parasitoids to changing environments (Figure 1, [START_REF] Tougeron | Diapause variability in aphid parasitoids in the context of climate changes; implications for biological control[END_REF]. More insights on plant-herbivore interactions in a global-change context are proposed in [START_REF] Pincebourde | Plant-Insect Interactions in a Changing World[END_REF] and in Björkman and Niemelä (2015) regarding the impact on biological control. These processes are described in the main text of the manuscript. In brief, both climate and land-use changes will modify local environmental conditions. Hyperparasitoids may adapt to new conditions through environmental plasticity or natural selection. These adjustments may not only affect different components of hyperparasitoid ecology, but also of species with which they interact. These changes in the biotic compartment ultimately act on the efficiency of biological pest control, although both positive and negative outcomes could be predicted. This figure is not exhaustive and other mechanisms (e.g., pesticides, artificial light at night) may be involved.

Hyperparasitoids as new targets in biological control?

In light of the above, it would be interesting to consider hyperparasitoids as potential targets in biological control and investigate methods to buffer their negative impacts on pest suppression. As species at high trophic levels, hyperparasitoids undergo many ecological and physiological constraints on which we could act to reduce their fitness or their establishment and persistence in the target environment. Some methods have already proved efficient in classical biological control such as quarantine procedures to avoid the introduction of hyperparasitoids. However, controlling hyperparasitoids in conservation and augmentative biological control without also impairing primary parasitoids' fitness is thorny because hyperparasitoids often are ecologically, behaviourally and physiologically very close to their hosts due to their common evolutionary origin [START_REF] Buitenhuis | Comparing and contrasting life history variation in four aphid hyperparasitoids[END_REF].

Use of chemical volatiles

A first promising method is to unravel hyperparasitoid host-foraging behaviour and determine whether there are volatile chemicals that could lure hyperparasitoids away from primary parasitoids without disrupting the latter. If this occurs, the use of attractants or repellents could be a promising approach to keep hyperparasitoids away from target crops. Limited information on hyperparasitoid foraging behaviour indicates that they exploit host and plant volatile chemicals to find their parasitoid hosts [START_REF] Buitenhuis | Foraging behaviour at the fourth trophic level: a comparative study of host location in aphid hyperparasitoids[END_REF][START_REF] Poelman | Hyperparasitoids Use Herbivore-Induced Plant Volatiles to Locate Their Parasitoid Host[END_REF][START_REF] Sullivan | Hyperparasitism: multitrophic ecology and behavior[END_REF]. Host-related kairomones also elicit behavioural responses in the hyperparasitoid. For example, hyperparasitoid females of Alloxysta victrix (Hymenoptera: Figitidae) show arrested development and antennal examination behaviour when exposed to aphid cornicle secretions, honeydew and solvent extracts of parasitized aphids by the primary parasitoid [START_REF] Grasswitz | Contact Kairomones Mediating the Foraging Behavior of the Aphid Hyperparasitoid Alloxysta victrix (Westwood) (Hymenoptera: Charipidae)[END_REF]. Exploiting plant volatiles and/or insect-induced plant volatiles to manipulate target species such as pests or their natural enemies is a commonly used strategy (e.g., push-pull methods in biological control) [START_REF] Cook | The Use of Push-Pull Strategies in Integrated Pest Management[END_REF]. However, as pointed out by [START_REF] Poelman | Complexity of Plant Volatile-Mediated Interactions Beyond the Third Trophic Level[END_REF], there is a risk that both primary and secondary hyperparasitoids are attracted by the same volatile chemicals. Efforts may thus concentrate in finding chemical lures -or concentrations or mixes of these chemicals -that are specific to hyperparasitoids. If these volatile chemicals are attractive only to hyperparasitoids and are affordable, they could be used to control hyperparasitoids without interfering with the activity of primary parasitoids. To the same extent, chemical volatiles used to attract primary parasitoids to crops should ideally not be attractors to hyperparasitoids. An interesting but yet untested approach arises from the fact that primary and secondary parasitoids may be attracted in a different way by plant and host cues (discussed in [START_REF] Poelman | Complexity of Plant Volatile-Mediated Interactions Beyond the Third Trophic Level[END_REF].

Environmental management

In a conservation biological control approach, it would be interesting to evaluate whether specific flower mixes and/or specific sugars (for sugar spraying) could benefit primary parasitoid more than hyperparasitoids. In this sense, it has been recently demonstrated that two Aphidius parasitoids increase their longevity much more than their hyperparasitoid Dendrocerus aphidum when they feed on melibiose [START_REF] Goelen | Gustatory response and longevity in Aphidius parasitoids and their hyperparasitoid Dendrocerus aphidum[END_REF], a common sugar of honeydew [START_REF] Wäckers | A comparison of nectar-and honeydew sugars with respect to their utilization by the hymenopteran parasitoid Cotesia glomerata[END_REF]. This knowledge can be exploited in tailoring food sources to selectively support Aphidius parasitoids, enhancing the biological control of aphids [START_REF] Damien | Flowering crops in winter increases pest control but not trophic link diversity[END_REF]. As trophic levels can have different thermal requirements or plastic adjustment capacities to microclimatic conditions [START_REF] Thackeray | Phenological sensitivity to climate across taxa and trophic levels[END_REF] appropriate landscape management to manipulate pests', parasitoids' and hyperparasitoids' thermal niches could also be considered [START_REF] Alford | The effect of landscape complexity and microclimate on the thermal tolerance of a pest insect[END_REF][START_REF] Tougeron | Comparing thermal tolerance across contrasting landscapes: first steps towards understanding how landscape management could modify ectotherm thermal tolerance[END_REF].

Another potential solution in conservation biological control would be to enhance predator presence at the beginning of the season, when hosts are at low densities. For example, in citrus, the aphid Aphis spiraecola (Hemiptera: Aphididae) has a large and diverse complex of hyperparasitoids which may make unfeasible its biological control with parasitoids in the Mediterranean Basin [START_REF] Gómez-Marco | Untangling the aphid-parasitoid food web in citrus: Can hyperparasitoids disrupt biological control?[END_REF]. There, the biological control services of these predators can be improved by means of cover crops based on Poaceae plants that increase their presence at the beginning of the season (Gómez-Marco et al., 2016a, 2016b).

Use of natural enemies of hyperparasitoids

Another method is to use the specific natural enemies of hyperparasitoids. For this, we first need to identify them and then determine the extent to which hyperparasitoids' negative effects on primary parasitoids can be limited by their predators, pathogens and parasites in the foodweb. Focus should be made on finding hyperparasitoid or predator species that are able to attack a target hyperparasitoid without interfering with the lower trophic levels (i.e., primary parasitoids). At least two groups of insects have been identified as potential natural enemies of hyperparasitoids. Among specialists, some hyperparasitoids are in the fifth trophic level (i.e. tertiary parasitoids) and parasitize hyperparasitoids. or interfere with their development through larval or adult competition [START_REF] Harvey | Interactions to the fifth trophic level: secondary and tertiary parasitoid wasps show extraordinary efficiency in utilizing host resources[END_REF]. For example, the use of molecular techniques have allowed researchers to untangle the role of hyperparasitoids in food-webs that include the aphid A. spiraecola in citrus, and some hyperparasitoids appeared to be in the fifth trophic level [START_REF] Gómez-Marco | Untangling the aphid-parasitoid food web in citrus: Can hyperparasitoids disrupt biological control?[END_REF]. Ants generally protect aphids from primary parasitoids. However, ants are also the best known generalist natural enemy of hyperparasitoids and can provide an "enemy-free space" for the primary parasitoid. For example, the primary parasitoid of aphids Lysiphlebus cardui (Hymenoptera: Braconidae) can avoid ants attending aphids, whereas hyperparasitoids are detected and attacked by ants [START_REF] Völkl | Aphids or their parasitoids: who actually benefits from ant-attendance?[END_REF]. This interaction is, however, quite specific as some hyperparasitoids can handle ant attendance [START_REF] Hübner | Behavioral strategies of aphid hyperparasitoids to escape aggression by honeydew-collecting ants[END_REF]. The presence of tertiary hyperparasitoids and/or ants could be promoted in future conservation biological control programs. To rear tertiary parasitoids for their release in augmentative biological control seems unfeasible, however, because of their cost [START_REF] Van Lenteren | Biological control using invertebrates and microorganisms: plenty of new opportunities[END_REF]. Development of specific strains of microbial biological control agents targeting hyperparasitoids could also be interesting solutions in the future.

Recommendations for augmentative biological control

In augmentative biological control, we would recommend, when possible, to select and release primary parasitoids that can avoid/escape hyperparasitism, either behaviorally or immunologically, [START_REF] Brodeur | Seasonal microhabitat selection by an endoparasitoid through adaptive modification of host behaviour[END_REF][START_REF] Höller | Movement away from the feeding site in parasitized aphids: host suicide or an attempt by the parasitoid to escape hyperparasitism[END_REF], even if they are less effective. In case species with different suitability for hyperparasitoids and efficacy as biological control agents can be released, we would recommend releasing first the "tolerant to hyperparasitism but less effective species", and then release the "intolerant but effective" at the end of the season once hyperparasitoids cannot built up their populations. In terms of augmentative releases, it is also important to determine the correct timing to control a pest while avoiding the presence of hyperparasitoids. For this, it is crucial to include diapause and changes in diapause expected in insects of all trophic levels under climatic change in population dynamic models and phenology models intended to biological control practitioners [START_REF] Bale | Insect overwintering in a changing climate[END_REF][START_REF] Lalonde | Some dynamical consequences of parasitoid diapause[END_REF]. Moreover, as hyperparasitoid population dynamics seem to depend on both pest hosts and primary parasitoids [START_REF] Gagic | Food web structure and biocontrol in a four-trophic level system across a landscape complexity gradient[END_REF], future research needs to focus on how manipulating either or both of these lower-trophic-level populations could make hyperparasitoid populations crash (e.g., by establishing complex multitrophic-level population dynamics models to identify the levers on which acting to eliminate hyperparasitoids from a system).

Potential of endosymbionts

Endosymbionts are important players in many food-webs. They are known to be involved in host-parasitoid interactions [START_REF] Dion | Symbiont infection affects aphid defensive behaviours[END_REF]Oliver et al., 2014), but have only recently been studied regarding higher trophic levels. Protective symbionts in pests may have effects that extend to higher trophic levels, including hyperparasitoids [START_REF] Oliver | How resident microbes modulate ecologically-important traits of insects[END_REF][START_REF] Ye | Facultative bacterial endosymbionts shape parasitoid food webs in natural host populations: A correlative analysis[END_REF]. For example, in populations of the pea aphid Acyrthosiphon pisum (Hemiptera: Aphididae) that are protected by the bacterial endosymbiont Hamiltonella defensa against primary parasitism, there is a reduction in hyperparasitoid emergence not by a direct effect of the endosymbiont on the hyperparasitoid, but through indirect cascading effect resulting from the reduction of specific hosts available for primary parasitism [START_REF] Mclean | Cascading effects of herbivore protective symbionts on hyperparasitoids: Symbionts and hyperparasitoids[END_REF]. In this sense, protection by endosymbionts is an additional type of bottom-up effect that influences the composition of a parasitoid-hyperparasitoid community [START_REF] Rothacher | Bacterial endosymbionts protect aphids in the field and alter parasitoid community composition[END_REF][START_REF] Van Nouhuys | Wolbachia increases the susceptibility of a parasitoid wasp to hyperparasitism[END_REF][START_REF] Ye | Facultative bacterial endosymbionts shape parasitoid food webs in natural host populations: A correlative analysis[END_REF]. There are, however, no empirical analyses available on how protective symbionts in the host pest or in the primary parasitoid might directly impact hyperparasitoids, nor on how we could control hyperparasitoid fitness through manipulation of endosymbiosis within food-webs [START_REF] Mclean | Insect symbionts in food webs[END_REF]. Moreover, endosymbionts within hyperparasitoids remain, to our knowledge, poorly studied and their potential ecological and evolutionary implications are yet unknown, especially concerning species interactions within food-webs. Endosymbionts are also involved in insect resistance to stress such as thermal stress [START_REF] Dunbar | Aphid thermal tolerance is governed by a point mutation in bacterial symbionts[END_REF]. They are themselves sensitive to thermal stress [START_REF] Tada | Obligate association with gut bacterial symbiont in Japanese populations of the southern green stinkbug Nezara viridula (Heteroptera: Pentatomidae)[END_REF] and their interaction with the host and the parasitoid differs depending on temperature [START_REF] Cayetano | Effects of Heat Shock on Resistance to Parasitoids and on Life History Traits in an Aphid/Endosymbiont System[END_REF][START_REF] Guay | Impact of environmental stress on aphid clonal resistance to parasitoids: Role of Hamiltonella defensa bacterial symbiosis in association with a new facultative symbiont of the pea aphid[END_REF][START_REF] Thomas | Thermal biology in insect-parasite interactions[END_REF]. Through changes in temperatures in greenhouses, it could thus be possible to manipulate the efficiency of endosymbiotic microorganisms to change the outcome of plant-pest-parasitoid-hyperparasitoid interactions. This option would however impact plant production and development of other pests such as fungi.

Conclusion

It is now clear that the study of binary pest-parasitoid interactions is not sufficient to assess the impacts of global-change on biological pest control, and that multitrophic interactions from the host-plant to the hyperparasitoid [START_REF] Dicke | The evolutionary context for herbivore-induced plant volatiles: beyond the 'cry for help[END_REF][START_REF] Duffy | The functional role of biodiversity in ecosystems: incorporating trophic complexity[END_REF][START_REF] Harvey | Conserving host-parasitoid interactions in a warming world[END_REF][START_REF] Pincebourde | Plant-Insect Interactions in a Changing World[END_REF], as well as endosymbiotic interactions [START_REF] Mclean | Insect symbionts in food webs[END_REF][START_REF] Ye | Facultative bacterial endosymbionts shape parasitoid food webs in natural host populations: A correlative analysis[END_REF] should be considered. Predicting interaction shifts within food-webs and their ecological consequences in a changing world is currently one of the greatest challenges in community ecology [START_REF] Chevin | Adaptation, Plasticity, and Extinction in a Changing Environment: Towards a Predictive Theory[END_REF][START_REF] Gilman | A framework for community interactions under climate change[END_REF]. Hyperparasitoids may positively respond to global-change through increasing population growth and activity rates, but whether this will negatively or positively affect biological pest control is still unknown. While their potential detrimental effects on primary parasitoids must be evaluated and limited as part of biological control programs, we also need to account for their potential positive influence on food-web stabilization through top-down effects (stabilization of host-parasitoids interactions).

Realistically, hyperparasitoids cannot be removed from food-webs, aside from food-webs maintained in greenhouses and other highly controlled systems. Some of the control methods exposed in the last section of our manuscript are promising, but remain limited because their application will depend on both the crop system (greenhouses, fields, orchards, forests, etc.) and local biotic and abiotic constraints. In order to reduce the negative effect of hyperparasitoids in the future and to better adopt pest management decisions, more empirical studies are required on the ecology of hyperparasitoids in different environmental contexts. In particular, research priorities should be given to: (i) improving knowledge on hyperparasitoid ecophysiology, including determining their current thermal tolerance ranges and seasonal strategies under diverse environmental conditions; (ii) determining the extent to which hyperparasitoids' physiology and behaviour are liable to plastic or evolutionary adjustments in the face of climatechange; and (iii) focusing on plant-host-parasitoid-hyperparasitoid community assembly rules, species interactions and food-web functioning in various systems.

Figure 1 :

 1 Figure 1: Schematic summarizing the factors involved in modulating the efficiency of biological pest control when hyperparasitoids are present in the ecosystem, and in the context of global changes.These processes are described in the main text of the manuscript. In brief, both climate and land-use changes will modify local environmental conditions. Hyperparasitoids may adapt to new conditions through environmental plasticity or natural selection. These adjustments may not only affect different components of hyperparasitoid ecology, but also of species with which they interact. These changes in the biotic compartment ultimately act on the efficiency of biological pest control, although both positive and negative outcomes could be predicted. This figure is not exhaustive and other mechanisms (e.g., pesticides, artificial light at night) may be involved.

Acknowledgments

The authors thank Dr. Gagic for sharing her recommendations, Dr. Klein for English revisions and two anonymous reviewers who helped improve an earlier version of this manuscript. KT was supported by the Fondation Fyssen during the writing process of this manuscript. The authors have no competing interests to declare.


Adrien Savoyant

O Pilone

S Bertaina

Fabian Delorme

Fabien Giovannelli

Electronic and nuclear magnetic anisotropy of cobalt-doped ZnO single-crystalline microwires

Keywords: numbers: 61.72.uj, 75.30.Gw, 75.50.Pp, 76.30.Fc

I. INTRODUCTION

ZnO micro-and nanocrystals, doped or not with magnetic impurities, are very interesting materials for a wide range of applications [1][2][3][4][START_REF] Ozgur | Proc. IEEE[END_REF] , particularly because of their inverse and direct piezoelectric (PZ) properties combined with their semiconducting n-type character [START_REF] Wang | [END_REF][7][8][9][10] . For example, if the contact between a ZnO micro-crystal and a metal results in a Schottky barrier, a micro-scaled mechanical-to-electrical convertor is realized. These PZ properties arise on the one hand from the ionic and binary character of ZnO and, on the other hand, from the absence of inversion symmetry at each site of the ZnO wurtzite structure.

Different crystal-grain orientations within a given sample results in a destructive PZ effect, so that this latter is positively correlated with the crystal quality of the considered sample. The obtention of very high crystal quality ZnO micro-and nano-structures is thus a key point to maximize the direct and inverse PZ effects to be used in concrete applications.

Besides, inserting magnetic ions in such a PZ semiconductor could result in interesting magneto-electrical, magneto-optical or magnetomechanical couplings [11][12][13][14][15][16][17] .

Apart from these hoped-for magnetic properties, cobalt-doping of ZnO nanostructures is also realized for many other reasons: to reduce some intrinsic defects 18 , to enhance the PZ response and dielectric constant 8,9 , to improve crystal quality 19 , etc., these different effects being certainly linked together. Aside of these cobalt-induced properties, this doping is also performed for probing crystallinity of single crystals 20 or orientation's coherence of a set of nanorods 21 , subjects on which we will focus thereafter. In particular, the use of Electron Paramagnetic Resonance (EPR) techniques allows for detecting very low impurities contamination or weak crystal-grain disorientation, provided that the resonant centers are sufficiently diluted and display a notable magnetic anisotropy. This is the case of ZnO:Co diluted magnetic semiconductor in which, at low temperature, the localized electronic spin of Co 2+ shows a preferred orientation within the hexagonal plane of the wurtzite structure. The effective g-factor in a direction perpendicular to the c-axis is about twice that in the parallel direction. As a consequence, each local environment around a Co 2+ ion gives EPR lines at a specific position, depending on its environment's orientation. Then, the total EPR spectrum contains information about the orientation's distribution, which can be extracted from suitable simulation. However, when dealing with very few numbers of paramagnetic impurities (as often in a nanostructure context), high-power microwave is required in order to detect something. This often leads to a saturated regime beyond the current-simulation possibilities. Thus, experimental reference spectra of single crystals for any microwave power are desirable data for understanding the saturated regime of nanostructures.

In this paper, we present the EPR results obtained on ZnO:Co single crystalline microwire (MW), grown by the optical furnace method. In this material, the nuclear hyperfine interaction of the 59 Co 2+ ions displays a quite unusual large anisotropy (A ∥ /A ⊥ = 5.4). Due to its low value in the perpendicular direction (A ⊥ = 2.98 × 10 -4 cm -1 ≡ 0.3 mT ), the full hyperfine tensor can only be resolved in very high crystal-quality samples, for which the EPR line width is significantly lower than 0.3 mT. We show that such a crystal quality can be reached in the studied material, which allows for the electronic and nuclear magnetic anisotropy characterization, through the determination of the full g-factor and hyperfine tensors. Three different static-and microwavefield configurations are used, for which the temperature and microwave-power dependence are studied. The fits of the temperature-dependent intensities confirm the correctness of the spin 3/2 model for each configuration, while the power-dependent intensities curves reveal the different dynamics of each. Qualitative analysis of these data shows that the spin-lattice relaxation time T 1 has axial anisotropy, while the spin-spin one T 2 is almost isotropic. Then, in the wider context of nanostructures, we explain how the power-study of the intensity ratio I ∥ /I ⊥ allows for the separation between signals arising from ordered and disordered phases when working in an EPR saturated regime.

II. EXPERIMENTAL

The wurtzite-crystallized ZnO:Co MW under study has been grown by the optical furnace method, described in previous papers 20,22 . The SEM image in Fig. 1 shows a typical MW, with its hexagonal cross section. In all cases, the c-axis of the wurtzite structure is parallel to the long dimension of the MW, which is also taken to be the z-axis of the laboratory frame (see Fig. 1). In order to obtain the smallest EPR line width necessary to resolve the perpendicular hyperfine coupling (A ⊥ ≡ 0.3 mT), the least Co-doped MW was chosen, among the completely transparent ones (green coloration increases with Co-doping 20 ). Thus, dipolar broadening is at its minimum and only excited-level lifetime and structural disorder, respectively, contribute to homogeneous and inhomogeneous EPR line broadening. For the intensity measurements in the three configurations, another MW slightly more doped has been used: in this one, all intensities are increased but also the line width so that the perpendicular hyperfine coupling A ⊥ is not resolved.

The X-band (ν = 9.621 GHz) EPR spectra were recorded using a Bruker EMX spectrometer equipped with a standard TE 102 cavity. The low temperature (5 K) equilibrium was obtained by using a continuous helium-flow cryostat stabilized with an Intelligent Temperature Controller (Oxford, ITC 503). Regarding the orientation of the static (B) and of the microwave (B 1 ) fields with respect to the c-axis of the wurtzite MW, three configurations are possible, depicted in Fig. 1 and denoted by C ∥ , C ⊥ and C ′ ⊥ . Each of them gives a particular EPR intensity, with its own power-and temperature-dependence. Sensitivity improvement was achieved by the use of a lock-in amplifier modulating the static B-field at a 100 kHz frequency. The amplitude of this mod- ulation was set to always avoid over-modulation phenomena (while keeping sufficiently intense signal) down to 0.05 mT. Regarding the saturation recovery measurements, the incident microwave power P was controlled by an attenuation A = 10 log(P 0 /P ), with P 0 = 200 mW. ZnO:Co MW images were performed in a FEI Quanta 200 scanning electron microscope (SEM).

III. RESULTS

A. Experimental and simulated spectra

The experimental spectra of the ZnO:Co MWs recorded at T = 5 K in the C ∥ and C ⊥ field configurations are shown in Fig. 2 for high and low microwave power. The C ′ ⊥ configuration spectrum is not shown because of the too low signal/noise ratio (see Sec. III C). No other signal than the Co-related one is detected, indicating the absence of contamination with paramagnetic impurities. In particular, none of the intrinsic defects observed in ZnO nanostructures are detected. The anisotropy of the S = 3/2 electronic spin of the Co 2+ ion is clearly seen by the different spectra position for the two orientations, while that of the hyperfine coupling is represented by the different spacing between two adjacent lines of the octet spectrum. Such spectra can be simulated by the following spin Hamiltonian, acting in the 32-dimensional space produced by the electronic and nuclear spins, S = 3/2 and I = 7/2 respectively:

H = DS 2 z + µ B -→ S .g. - → B + - → S . Ã. - → I (1)
This Hamiltonian includes, respectively, the uniaxial anisotropy (D > 0, easy plane), the Zeeman interaction (g) and the hyperfine coupling ( Ã). The principal components of the g and à tensors can be directly read on these spectra, without any simulation, and independently of the microwave power. These are in accordance with the usual ZnO:Co values, g ∥ = g ∥ ef f = 2.256 and g ⊥ = g ⊥ ef f /2 = 2.283, A ∥ = 16.07 × 10 -4 cm -1 and A ⊥ = 2.98 × 10 -4 cm -1 . We remark that the measurement of the A ⊥ value necessitates a very low line width (< 0.3 mT), only reachable in very high quality single crystals. Only three previous works have reported such a measurement on ZnO:Co, Estle & De Wit (bulk material, 1967) 23 , Hausmann (bulk material, 1969) 24 and Jedrecy et al. (thin film, 2004) 25 . Here, the crystal quality is such that the apparent EPR line width is as low as 0.08 mT, allowing for a precise determination of A ⊥ . This value can be used for simulation of low-resolved spectra. The determination of the axial anisotropy parameter D can not be directly performed with such X-band EPR spectra, but a relation linking it to the difference ∆g = g ⊥ -g ∥ = 0.027 has been derived in a previous work 21 , which gives an identical result here (D = 2.57 cm -1 ). While these spin Hamiltonian parameters are intrinsically power-independent, we observe that the EPR line width and intensities vary with the incident microwave power (Fig. 2). This is because these are characteristics of the EPR resonance phenomena described by the time-dependent Hamiltonian H 1 (t) = µ B S.g.B 1 (t), with B 1 ∝ √ P , and not of the material itself. Line width reaches its minimum as the incident power goes to zero, when the resonance is not saturated. The intensities (double integration of spectra) obtained in C ∥ and C ⊥ configurations appear to depend on the incident microwave power, as well as their ratio I ∥ /I ⊥ . Within the experimental power range, this ratio varies between 1 and 2; this cannot be accounted for by spectra simulation. Indeed, spectra obtained by simulation softwares as Easyspin 26 do not include power dependence, which relies on complex relaxation phenomena, but do describe a non-saturating limit, when P tends to zero and intensity is proportional to √ P . By trying to simulate the twoorientations spectra of figure 2, only that with the lowest microwave power (P = 0.4 mW) gives a satisfactory result (Fig. 3) indicating a non-saturating regime. No parameter adjustment allows for a reasonable fit of the high power (P = 50.2 mW) measurement, which is then supposed to be saturated, out of current-simulation possibilities. Sim.

Magnetic Field (mT)

Exp.

Sim. The best obtained simulation with Easyspin 26 shown in figure 3 includes a slight g-strain within the hexagonal cross section, that is a normalized narrow gaussian distribution of g ⊥ values of δg ⊥ = 0.002 width. This g-strain is likely to arise from minute stress within the hexagonal MW section. We remark that the line shape is Lorentzian, indicating a predominance of homogeneous broadening. The global agreement between simulated and off-saturation experimental spectra in both configurations C ∥ and C ⊥ is satisfactory, so that we conclude that the S = 3/2 and I = 7/2 model coupled to the Hamiltonian (1) well describe substitutional Co 2+ ions in ZnO.

B. Temperature dependence

In order to test the validity of the model in the three configurations C ∥ , C ⊥ , and C ′ ⊥ , we examine the temperature dependence of their intensity obtained by the double-integration method. These measurements have been performed on another MW, a bit more cobalt-doped in order to increase the C ′ ⊥ EPR intensity, but with the consequence that the line width is increased so that the perpendicular hyperfine coupling is no longer resolved. However, this increase in the line width does not affect at all the intensities, which might be fitted by EPR intensity formulae as well. In offsaturating conditions, the EPR intensities in the three configurations are described by formula ( 2), ( 3) and ( 4). These take into account the transition probability and the Boltzmann population difference between the two resonating levels, but also the important frequency-to-field conversion factor 27,28 and the T ∥ 2 and T ⊥ 2 transverse relaxation times [START_REF]T2 times characterizes the relaxation time along the microwave field B1 direction, with a priori two possible values in an axial-anisotropy case[END_REF] . These latter can be passed in the K constant when fitting only one line, just as in Ref. 21, but not in the present case.

I ∥ (T ) = K g 2 ⊥ T ⊥ 2 √ P g ∥ T (1 + e -2D/kT ) (2)
I ⊥ (T ) = K g ⊥ T ⊥ 2 √ P 2T (1 + e -2D/kT ) ( 3
)
I ′ ⊥ (T ) = K g 2 ∥ T ∥ 2 √ P g ⊥ 8T (1 + e -2D/kT ) (4)
The K-factor takes account for all other experimental parameters (static-field modulation, etc.) identical in the three configurations. Although the same in the three configurations, the √ P dependence is explicitly written for completeness of the involved physical parameters. In order to reduce the number of parameters, we have fitted the experimental intensities by taking T ⊥ 2 = T ∥ 2 , so that only the common factor K has been adjusted to obtain the fit of figure 5. Considering the uncertainty on the intensity measurement (from about 5% at low temperature to 10% at high temperature), the fitting agreement can be considered as fully satisfactory. Taking into account different transverse relaxation times (T

∥ 2 ̸ = T ⊥
2 ) results in a very poor improvement of the fits, so that these two relaxation times may reasonably assumed to be almost identical.

As the temperature tends to zero, the theoretical ratio I ∥ /I ⊥ (in non-saturating condition, P = 0.4 mW) tends to 2g ⊥ /g ∥ , that is 2.05, to be compared with the experimental ratio of 1.94 at T = 5 K. The temperature dependence of this ratio is slight and not of great importance because 2), ( 3) and ( 4).

the EPR intensity in the three configurations rapidly goes to zero (for relaxation reasons), as the temperature exceed 15 K. On the contrary, this I ∥ /I ⊥ ratio highly depends on incident microwave power P , as discussed now.

C. Power dependence

Aside of the precise determination of the spin Hamiltonian parameters from EPR spectra simulations and the temperature dependence study, it is interesting to study the spin dynamics in the three configurations. Such information can be partially extracted from the study of the EPR intensity-saturation curves (Fig. 5). Offsaturation regime is characterized by an intensity proportional to √ P (see Eqs. ( 2), ( 3), ( 4)), thus appearing as straight lines at low power in figure 5. Saturation occurs when the rate of absorption become greater than the rate of relaxation, so that the Co 2+ ions can no longer absorb all the incident photons. Indeed, for the absorption phenomenon to reach a stationary state, the excited state lifetime must be much smaller than the mean time between two photon incidences, this latter depending on microwave power. In other words, the faster a set of microwave-excited Co 2+ ions go back to their ground state by evacuating quantized energy toward the lattice, the more it can absorb microwave power.

Qualitatively, the power saturation phenomenon arises naturally from the classical Bloch equations. In the simple case of isotropic magnetic moment and without any inhomogeneous broadenning, the following factor can multiply the intensities (2), ( 3) and ( 4) in order to give account for the saturated regime 29 :

(1 + C.T 1 .T 2 .P ) -1 (5) where C is a constant assumed to be identical in the three configurations, T 2 is the isotropic transverse (along B 1 field) relaxation time discussed before, and T 1 the longitudinal (along B field), or spin-lattice, relaxation time. T 1 is equal to its parallel (T ∥ 1 ) or perpendicular (T ⊥ 1 ) value, according to the B-field orientation in the considered configuration (see Fig. 1). Then, qualitatively, the EPR phenomenon will enter a saturated regime even more rapidly as the T 1 T 2 product gets larger, which will be visible in the saturation curves by departure from the straight dashed-line. As saturation occurs at lower power in the C ∥ configuration (see Fig. 5:

P ∥ < P ⊥ ), we deduce that T ∥ 1 > T ⊥ 1 .
We can then conclude that there is some anisotropy in the spin-lattice coupling, which is stronger in the hexagonal plane (shorter relaxation) than along the c-axis of the wurtzite structure (longer relaxation). It must be stressed out that the considered relaxation times are phenomenological parameters arising from the classical isotropic Bloch equations, so that they can in fact include some other parameters such as the component of the effective g-tensor. Access to the real relaxation times can be quantitatively reached by the combination of pulsed EPR methods and fitting of the saturation curves including inhomogeneous phenomena, thus giving greater insight into the spin dynamics.

Another interesting information to be extracted from the saturation curves in figure 5 is the maximum power to which the resonance, in both configuration C ∥ and C ⊥ , is off-saturation and can then be properly simulated by Easyspin software. Beyond this power, no adjustment of any parameter can give a satisfactory fitting. It is to be noted that this power can slightly depend on various experimental conditions, such as cavity quality-factor, sample volume, filling factor, etc, and must then be regarded as an approximate value. Figure 5 shows that the first apparition of saturated regime occurs in the C ∥ configuration at a power defined by √ P ∥ /P 0 = 0.05, that is P ∥ = 0.5 mW.

D. Intensities anisotropy

We now turn to the study of the I ∥ /I ⊥ ratio. As pointed out before, this ratio only slightly depends on temperature in the range 5-10 K, but notably does on the incident microwave power (see Fig. 6). Starting from a non-saturating regime in which I ∥ /I ⊥ ≃ 2 at low temperature (around 5 K), each intensity increases as the incident microwave power is augmented, as shown in Fig. 5. However, due to different spin-lattice relaxation times in C ∥ and C ⊥ configurations, the increase has a different rate and maximum value in each case, so that the intensity ratio changes with power.

The strongest interest for the knowledge of this I ∥ /I ⊥ ratio power-dependence in single a microcrystal is that this latter is equivalent to an ideally orientated set of nanorods or nanowires. In this context of ZnO:Co nanostructures, the number of spins is weak so that high incident microwave power is often required in order to obtain a detectable signal, leading to a saturated regime. Thus, a simple EPR simulation is not relevant and, as shown in a previous work 18 , the experimental I ∥ /I ⊥ ratio is systematically lower than 1, that is out of the figure 6 curve. The simplest interpretation for this deviation is that some other signal positively contributes to the I ⊥ intensity, thus leading to a reduction of the considered ratio. Indeed, this contribution is likely to arise from a disordered phase of the ZnO:Co nano-objects which, in the case of totally random orientations (powder), give the spectrum of Fig. 7-c, that is a single assymmetric line located at the same B-field as for the C ⊥ configuration (∼ 150 mT). This hypothesis is strongly supported by the shape of the I ⊥ intensity observed in Ref. 18, which shows a pronounced asymmetry just as in the powder spectrum signal (Fig. 7). In this scenario, the ZnO:Co nanostructures EPR spectra would be the superposition of ordered-and disordered-phase signals.

In a non-saturating resonance regime, the total signal can be simulated, which directly gives access to the proportion of ordered and disordered phases. However, as seen before, the maximum power for such a non-saturating case is very low (0.5 mW) so that, in many cases, the incident microwave power will be greater than that, which correspond to an abscissa greater than 0.05 in figure 6. Then, at such a given microwave power, the knowledge of the single-crystal I ∥ /I ⊥ ratio would permit, if it exists, to determine the amount of non-ordered signal intensity at 150 mT. Appropriate simulation of this remaining intensity would thus allow for the approximate quantification of Co 2+ ions in ordered and disordered phase. Refinement of this quantification could be reached by considering orientated powder, or other particular distribution of spin orientations, rather than a totally disordered phase. Finally, the only difficulty would be to correctly simulate the (oriented-) powder spectra for any microwave power, including the saturated regime.

IV. CONCLUSION

Electronic and nuclear magnetic properties of 59 Co 2+ ions embedded in a tetrahedral local symmetry, as arising in ZnO:Co, show interesting and sizeable orientation dependence. In this paper, we have reported the use of these magnetic anisotropies for the crystal-quality characterization of low-doped ZnO:Co microcrystals. The use of three EPR configurations and the study of each resulting intensity temperature-and powerdependence have allowed for the validation of (S = 3/2, I = 7/2) model with easy-plane uniaxial electronic spin anisotropy. By the way, these studies have shown that the phenomenological spin-spin relaxation time must be isotropic, while the spinlattice relaxation time appears to be anisotropic, with

T ∥ 1 > T ⊥ 1 .
Finally, in the context of orientated nanostructures as nanorods or nanowires, we have shown how the power dependence of the I ∥ /I ⊥ ratio can reveal the proportion of ordered and disordered phases.

Figure 1 :

 1 Figure 1: (color online) Top and bottom-right: Schematic of the three field-configurations, C ∥ , C ⊥ and C ′ ⊥ , with respect to the c-axis (∥ z) of the wurtzite MW. Bottom-left: SEM image of individual ZnO:Co microcrystal showing the hexagonal section.

Figure 2 :

 2 Figure 2: (color online) EPR spectra of ZnO:Co MW in C ⊥ (left, red) and C ∥ (black, right) configurations, recorded in saturating (a) and off-saturating (b) conditions, at T = 5 K. All intensities are normalized.

Figure 3 :

 3 Figure 3: (color online) Experimental EPR spectra of the ZnO:Co MW in C ∥ (up, black circles) and C ⊥ (down, red circles) configurations, recorded at T = 5 K and P = 0.4 mW (same data as Fig 2-b). Blue lines show simulated spectra from Eq. 1 with parameters discussed in the text.

Figure 4 :

 4 Figure 4: (color online) Open circles, squares and triangles: experimental EPR spectra intensities in C ∥ , C ⊥ and C ′ ⊥ field configurations, recorded in non-saturating condition (P = 0.4 mW). Solid lines: corresponding calculated intensities from Eq. (2), (3) and (4).

Figure 5 :

 5 Figure 5: (color online) Saturation curves of EPR intensities recorded at T = 5 K and 0.5 G modulation for the three magnetic-field configurations. Dashed lines illustrate the off-saturating regime while lines indicate saturation occurences.

Figure 6 :

 6 Figure 6: Evolution of the I ∥ /I ⊥ intensities ratio as a function of the microwave power (P0 = 200 mW), recorded at T = 5 K.

Figure 7 :

 7 Figure 7: (color online) EPR spectra simulations of ideally orientated [(a) and (b)] and totally disordered (c) sets of ZnO:Co nanorods.

V. ACKNOWLEDGEMENT

This work was supported by the NATO project Science for Peace (SfP) 984735, Novel nanostructures. A. Savoyant acknowledges Prof. André Ghorayeb for fruitful discussions and corrections.