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International audience; Plant viruses transmitted by vector pests are one of the most important worldwide threats to global food production and security. Biological control strategies to enhance natural enemies (parasitoids and predators) have mainly focused on their ability to reduce pest density. In contrast, few studies have examined how natural enemies affect the spread and the incidence of viruses in a crop, although those results could be used as levers for a more sustainable management of viral diseases. Vector-borne plant viruses can be classified in three categories based on their transmission mode: non-persistently transmit- ted viruses, semi-persistently transmitted viruses and persistently transmitted viruses, whereas vector density, fitness and movement were identified as main drivers of virus spread in a crop, their relative contributions to virus epidemiology may also depend on both the transmission mode and the presence of natural enemies. The first part of the review focuses on virus transmission dynamics in relation to vector activity and density. Because we identify different patterns for each type of plant viruses, control strategies that lead to changes in vector traits, should be adapted to the targeted virus. However, biological control of insect vectors has been rarely adapted to the mode of transmission of the target virus. Thus, the last part of the review explores the conditions required for natural enemies (parasitoids and predators) to prevent epidemics outbreaks of each type of plant viruses. Briefly, if combined with other practices, biological control of vectors to keep virus incidence below the economic threshold is a promising approach for persistently transmitted viruses but might be more difficult to achieve with non-persistently transmitted viruses and semi-persistently transmitted viruses.

International audience; Natural pest control is a significant service supporting agricultural production. However, the relative contribution of several functional groups of natural enemies to natural pest control is unknown for many crop-pest systems. Furthermore, the output of this regulation in terms of damage reduction is rarely assessed. In this study, based on three field experiments, we quantified the effect of ground dwelling predators and parasitoids on two functional groups of broccoli pests. Relying on physical exclusion, we showed that ground dwelling predators significantly lowered the abundance of both the cabbage root fly (Delia radicum) and aphids (Brevicoryne brassicae and Myzus persicae). Ground dwelling predators made a significant contribution to natural pest control, as they lowered pest populations by 37% on average. Parasitoids suppressed 22% of pest populations. Our results suggest that intraguild predation was not an issue since ground dwelling predators did not have a negative impact on the level of natural pest control by parasitoids. Finally, early predation by ground dwelling predators on the cabbage root fly, the most harmful pest in this study, reduced damage in a highly infested context, meaning that natural pest control can effectively support crop production. These results strongly suggest that agricultural practices limiting soil disturbances or even favoring ground dwelling predator overwintering or colonization could lead to fewer damage and losses for farmers. Keywords Natural pest control • Biological control • Intraguild predation • Damage • Ground dwelling predators • Parasitism Key message • We compared the efficiency of two groups of natural enemies on natural pest control. • Two contrasted types of pests, the cabbage root fly and aphids, were studied. • Early generalists and late specialists removed 37 and 22% of pest populations, respectively. • For the cabbage root fly, early ground dwelling predators were key to biological control. • Natural control of the cabbage root fly reduced damage to the crop by 26%.

International audience; Understanding how resource diversification affects ecological interactions, food web structure and ecosystem functioning is essential in both fundamental and applied ecology. While plant diversification strategies (either in-field or around-field) are often proposed in agricultural landscapes as practices to improve the biological control of herbivores by natural enemies, results remain variable and unsure. Here, we studied the effect of an in-field diversification practice (the intercropping of leguminous crops within cereal fields, an increasingly common practice but with inconsistent results on biological control) on cereal aphid control and the structure of a cereal aphid-parasitoid-hyperparasitoid food web for 2 years. We report that aphid control was not increased in mixed fields, nor was cereal parasitoid diversity and food web complexity. Nevertheless, the provision of alternative hosts in mixed fields led to a functional community composition shift, favouring generalist parasitoid species over specialist ones. Moreover, we observed a higher hyperparasitism rate in mixed fields, suggesting that secondary parasitoids were favoured by alternative resources, which may have disrupted aphid control by primary parasitoids. Synthesis and applications. This study demonstrates that parasitoid community composition shift and increased top-down control by the fourth trophic level can impact parasitoid efficiency to control herbivores. These results highlight the necessity to study fine-scale mechanisms within food webs to be able to set up efficient methods to support biodiversity and associated ecosystem services in agricultural landscapes.

International audience; Due to their ability to parasitize various insect species, generalist parasitoids are widely used as biological control agents. They can be mass-reared and released in agroecosystems to control several pest species in various crops. However, the existence of genetic differentiation among populations of generalist parasitoid species is increasingly recognized and this can be associated with an adaptation to local conditions or to a reduced range of host species. Moreover, constraints of mass-rearing conditions can alter genetic variation within parasitoid populations released. These features could be associated with a reduced efficiency of the control of targeted pest species. Here, we focused on strawberry greenhouses where the control of aphids with the generalist parasitoid Aphidius ervi appears to be inefficient. We investigated whether this inefficiency may have both genetic and ecological bases comparing wild and commercial populations of A . ervi . We used two complementary genetic approaches: one based on the mitochondrial marker COI and one based on microsatellite markers. COI analysis showed a genetic differentiation within the A . ervi species, but the structure was neither associated with the commercial/wild status nor with host species factors. On the other hand, using microsatellite markers, we showed a genetic differentiation between commercial and wild A . ervi populations associated with a loss of genetic diversity within the mass-reared populations. Our ecological genetics study may potentially explain the weak efficiency of biological control of aphids in protected strawberry crops and enable to provide some insights to improve biological control.

International audience; Ground dwelling predators provide regulation services of several insect pests. Enhancing these services may be a step toward integrated crop protection. Many studies have shown that soil tillage is deleterious to ground dwelling predators but pest regulation processes and services have rarely been measured. We performed an experiment to study whether simplifying soil tillage before the establishment of spring broccoli enhanced ground dwelling predator populations and the control they provide on Delia radicum. The direct effect of tillage on arthropods was assessed by comparing their emergence rates in plots differing in soil tillage management. The natural regulation service was assessed by comparing a control and an exclusion treatment in which predators were removed. The effect of soil tillage on carabids, spiders and staphylinids did not match the gradient of disturbance induced by tillage treatments. Tillage did not appear to affect the predators that likely contribute to D. radicum regulation. Consistently, the number of pests suppressed and the root injuries were unaffected by tillage treatments. The main deleterious effect of soil tillage was on the emergence of those carabid species that overwinter partly as larvae, suggesting that spring tillage could affect pest control in the following crops.

International audience; Greenhouse crops are thought to be simplified ecosystems because they often consist of mono-cultures that are relatively isolated from their environment. However, insect pests are still able to colonize these protected crops, which threaten their yields. Similarly, natural enemies of pests may be able to colonize greenhouses, providing a form of natural biological pest control. Protected strawberry crops are grown in several types of greenhouses that vary in their degree of openness. Crops often suffer from aphid outbreaks, which can be partly controlled by insect parasitoids immigrating from the surrounding environment. We investigated variations over space and time in both the aphid and parasitoid community diversity and species assemblages associated with protected strawberry crops. We sampled aphids and parasitoids in five regions of France in the spring and summer of two successive years. Despite the relative isolation of these protected crops, we identified a high aphid species richness in them, even at the greenhouse scale. Aphid community composition varied with spatial and temporal factors, but the species assemblages present were mostly determined by local factors. Parasitoid communities were mostly similar among the studied regions, but varied between seasons, with this temporal variation being related to changes in aphid species composition. The study of trophic interactions occurring between aphids and parasitoids allowed the most prevalent and efficient parasitoid species to be identified. The structures of food webs strongly varied in time and space, compromising any prediction of "natural" biological control. We also highlighted ecological factors that can disrupt aphid biological control, such as the occurrence of hyperparasitism or the possibility of apparent mutualism between aphid species. Finally, we showed that the degree of openness of greenhouses influenced both the aphid communities and the hyperparasitism rates in them. These results provide valuable information to improve aphid biological control in protected crops.

International audience; The degree of trophic specialization of interacting organisms impacts on the structure of ecological networks and has consequences for the regulation of crop pests. However, it remains difficult to assess in the case of parasitoids. Host ranges are often established by listing host records from various years and geographic areas in the literature. Here, we compared the actual hosts exploited at a local farm-scale by aphid parasitoids (Hymenoptera: Aphidiinae), to the available species listed as hosts for each parasitoid species. We sampled aphids and their parasitoids in cultivated and uncultivated areas in an experimental farm from April to November 2014 and thereafter used DNA-based data to determine whether a differentiation in sequences existed. Twenty-nine parasitoid species were found on 47 potential aphid hosts. Our results showed that the great majority of the parasitoid tested used fewer host species than expected according to data published in the literature and parasitized a limited number of hosts even when other potential hosts were available in the environment. Moreover, individuals of the most generalist species differed in their DNA sequences, according to the aphid species and/or the host plant species. At a local scale, only obligate or facultative specialist aphid parasitoids were detected. Local specialization has to be considered when implementing the use of such parasitoids in pest regulation within agroecosystems.

The idea that noncrop habitat enhances pest control and represents a win–win opportunity to conserve biodiversity and bolster yields has emerged as an agroecological paradigm. However, while noncrop habitat in landscapes surrounding farms sometimes benefits pest predators, natural enemy responses remain heterogeneous across studies and effects on pests are inconclusive. The observed heterogeneity in species responses to noncrop habitat may be biological in origin or could result from variation in how habitat and biocontrol are measured. Here, we use a pest-control database encompassing 132 studies and 6,759 sites worldwide to model natural enemy and pest abundances, predation rates, and crop damage as a function of landscape composition. Our results showed that although landscape composition explained significant variation within studies, pest and enemy abundances, predation rates, crop damage, and yields each exhibited different responses across studies, sometimes increasing and sometimes decreasing in landscapes with more noncrop habitat but overall showing no consistent trend. Thus, models that used landscape-composition variables to predict pest-control dynamics demonstrated little potential to explain variation across studies, though prediction did improve when comparing studies with similar crop and landscape features. Overall, our work shows that surrounding noncrop habitat does not consistently improve pest management, meaning habitat conservation may bolster production in some systems and depress yields in others. Future efforts to develop tools that inform farmers when habitat conservation truly represents a win–win would benefit from increased understanding of how landscape effects are modulated by local farm management and the biology of pests and their enemies., This work was supported through the National Socio-Environmental Synthesis Center (SESYNC) National Science Foundation Award DBI-1052875 for the project “Evidence and Decision-Support Tools for Controlling Agricultural Pests with Conservation Interventions” organized by D.S.K. and R.C.-K

Diaeretiella rapae is an aphid parasitoid with potential for use in biological control strategies. However, several recent genetic studies have challenged the long held view that it is a generalist parasitoid. We investigated its ecological specialization and ability to use resources in cultivated and uncultivated areas. Ecological specialization would reduce its ability to exploit the diversity of aphid species, particularly in uncultivated areas, and to control pest aphids. Four D. rapae strains were studied, three reared on pest aphids on Brassicaceae and one strain on a non-pest aphid on Chenopodiaceae. For each strain, we performed host-switching experiments, with a total of six aphid species, five of which D. rapae parasitizes in France. We tested cross-breeding ability between strains to detect potential reproductive isolation linked to aphid host species in D. rapae. The strain reared on non-pest aphids was able to develop on aphid species from both cultivated and uncultivated plants. The strains reared on pest aphids, however, exclusively parasitized aphid species on cultivated Brassicaceae. In addition, reproductive isolation was detected between strains from uncultivated and cultivated plants. Thus, the D. rapae populations examined here appear to be showing ecological specialization or they may even be composed of a complex of cryptic species related to the aphid hosts. The role of Chenopodium album as a reservoir for D. rapae, by providing a habitat for non-pest aphids on which it can feed, appears to be severely limited, and thus its efficiency to maintain local populations of D. rapae in the vicinity of crops is questionable.

International audience; The ecosystem service of insect pest regulation by natural enemies, such as primary parasitoids, may be enhanced by the presence of uncultivated, semi-natural habitats within agro-ecosystems, although quantifying such host–parasitoid interactions is difficult. Here, we use rRNA 16S gene sequencing to assess both the level of parasitism by Aphidiinae primary parasitoids and parasitoid identity on a large sample of aphids collected in cultivated and uncultivated agricultural habitats in Western France. We used these data to construct ecological networks to assess the level of compartmentalization between aphid and parasitoid food webs of cultivated and uncultivated habitats. We evaluated the extent to which uncultivated margins provided a resource for parasitoids shared between pest and nonpest aphids. We compared the observed quantitative ecological network described by our molecular approach to an empirical qualitative network based on aphid–parasitoid interactions from traditional rearing data found in the literature. We found that the molecular network was highly compartmentalized and that parasitoid sharing is relatively rare between aphids, especially between crop and noncrop compartments. Moreover, the few cases of putative shared generalist parasitoids were questionable and could be due to the lack of discrimination of cryptic species or from intraspecific host specialization. Our results suggest that apparent competition mediated by Aphidiinae parasitoids is probably rare in agricultural areas and that the contribution of field margins as a source of these biocontrol agents is much more limited than expected. Further large-scale (spatial and temporal) studies on other crops and noncrop habitats are needed to confirm this.

International audience; The high arctic Svalbard archipelago is characterized by a low insect species richness explained by both extreme living conditions and insularity. Here, we described two new species of Hymenopteran parasitoids found in Spitsbergen, the main island of Svalbard, using aphids as hosts. We named the first one Diaeretellus svalbardicum (Hymenoptera: Braconidae) because of morphological similarities with other European members of the Diaeretellus genus. D. svalbardicum parasitizes exclusively the aphid Acyrthosiphon svalbardicum, a species endemic to Svalbard. We also reported in D. svalbardicum a unique case of wing polymorphism with macropterous and micropterous forms in both genders. We named the second newly described parasitoid species Aphidius leclanti because of morphological similarities with other members of the Aphidius genus. Contrarily to D. svalbardicum, A. leclanti exploits the only two aphid species regularly found in Spitsbergen, A. svalbardicum and Sitobion calvulum. (C) 2012 Elsevier GmbH. All rights reserved.

International audience; Semi-natural structures of agricultural landscapes are known to provide important habitat functions for natural enemies of crop herbivores. Although several functions such as resource and shelter are related to plant species, the role of the floristic composition is not well understood. Using a correlative approach, we analysed interactions between plant functional groups of spontaneous field margin vegetation and the regulation of crop herbivores. We further compared the influence of the local plant community with that of landscape structure at different scales (50-500 m). We used Brassica oleracea crop plants as phytometers planted into field margins of 48 fields and observed correlations of explanatory variables (vegetation, landscape) and response variables (herbivore and natural enemy abundance, crop damage) for two consecutive years (24 fields x year(-1)). We observed significant positive and negative correlations between plant functional groups and variables related to crop herbivore regulation. In general, plant cover based-models provided a better fit than diversity-based models. Entomophilous flowering plants providing nectar showed a positive correlation to ladybird abundance and aphid parasitism and a negative relationship with major crop plant damage caused by leaf-chewing invertebrates. Cover and diversity of wild Brassicaceae species were positively correlated with flea beetle damage and abundance. Most correlations with plant functional groups remained significant after fitting variables of landscape structure. The density of semi natural habitats showed predominantly positive correlations to crop herbivore regulation, whereas the density of crops was negatively correlated. The number of significant relationships decreased from natural enemy to herbivore abundance and to plant damage. Our results demonstrate the relative importance of field margin vegetation for crop herbivore regulation. The differential responses to plant functional groups support the idea that designing specific plant mixtures may improve biological control. Additional effects of landscape variables suggest that a combination of local and landscape-scale management is the most promising strategy to optimise this ecosystem service. (C) 2016 Elsevier B.V. All rights reserved.

International audience; Resource distribution, through its effects on individual foraging and survival, drives population dynamics across the landscape. In an agricultural context, resource distribution is therefore a key information in assessing whether or not a pest population may invade and persist in a given environment. Addressing this issue by means of numerical exploration requires a population model with a sound dependence on the landscape. In this paper, we demonstrate that this dependence is effectively secured by a multi-scale description of the population. We derived a reaction-diffusion population model accounting for two individual-scale processes determining resource utilisation: (1) resource perception as a determinant of mobility and (2) energy supply management as a determinant of survival. In this model, the distribution of two competing resources (feeding and laying sites) affects the spatial population dynamics of a dipteran pest through a heterogeneous dispersion of the individuals and a metabolic currency. We conducted a global sensitivity analysis to evaluate the impact of both individual-scale processes on the population dynamics. This exploration demonstrated the biological relevance of the model according to field observations and theoretical expectations. Our key finding is that resource perception and energy supply management appear as significant as the demographic component regarding the resulting dynamics of the pest. Building on its acute multi-scale landscape dependence, this model may be particularly useful for investigating the putative relationships between agricultural landscape features and pest outbreaks

1. Host discrimination by Aphidius rhopalosiphi (De Stefani Perez) (Hymenoptera: Braconidae) was first studied on the grain aphid Sitobion avenae (Fabricius) (Homoptera: Aphididae). Females tended to avoid oviposition in hosts parasitised 3h earlier. No evidence of host discrimination ability on freshly parasitised hosts was suggested, however, and ovipositional experience had no effect on host discrimination. 2. The effects of host discrimination ability on the exploitation strategy of patches containing different proportions of unparasitised hosts and hosts parasitised for 3 h were studied. Females spent less time on patches with a higher proportion of parasitised hosts, reflecting the females' ability to perceive the potential profitability of the patch. This ability may be based on the nature of the hosts encountered (unparasitised or parasitised). 3. Incomplete exploitation of unparasitised hosts was also observed. It seems that this partial exploitation is related to the inability of A. rhopalosiphi to recognise freshly parasitised hosts. As a female may experience a risk of self-superparasitism during patch depletion, this could promote early departure from incompletely exploited patches. 4. The effect of previous experience on the patch exploitation strategy was also assessed. Females were tested twice on two patches of the same quality. Results suggested that the experience acquired during a previous visit led the females to leave the patch sooner and to lay fewer eggs in parasitised hosts. 5. Patch exploitation strategy may therefore be the result of different factors such as host discrimination and experience. The evolutionary consequences of the results are discussed.

International audience; Dispersal strategies and success of pests' natural enemies widely influence the efficiency of biological control. In this study, we compare two dispersal strategies among Aphidiinae parasitoids: eggs and larvae dispersal through winged aphid flight and active dispersal by adult parasitoids. Using a molecular method applied to a sample of >2,000 winged migratory aphids captured in a suction trap situated in Western France, we assessed the proportion of winged aphids carrying an aphidiine larva. In the six most abundant aphid species, we found an average parasitism rate of migrating aphids close to 1% and identified seven different, mainly generalist, parasitoid species. We also identified the species and the sex of adult Aphidiinae captured by the suction trap based on morphological criteria. We found that dispersing adult parasitoids were almost exclusively female. Parasitoid dispersal strategy seems to be species-dependant but this result needs to be confirmed by an exhaustive analysis of winged aphids captured. We discuss the possible impact of the low parasitism rate of winged aphids on parasitoid population dynamics and the importance of these results in the context of biological control and of the study of food webs between aphids and their natural enemies.

International audience; Agriculture intensification has deeply modified agroecosystems from field to landscape scales. To achieve successful pest control using natural enemies, understanding species interactions over all scales remains a challenge. Using the cabbage root fly as a model, we studied whether field and landscape characteristics influenced colonization and infestation of broccoli fields by the pest and its control by natural enemies. We also determined whether species of different trophic level or host specialization would respond to environmental characteristics at the same spatial extent. During a multiple-species and multiple-spatial extent study in northwestern France, we recorded pest colonization and infestation in 68 fields, collected associated natural enemies and assessed crop damages. In each field, we considered management practices and characterized the surrounding landscape in 50-500 m-wide buffers. Our main findings are that Delia radicum and its main natural enemies respond to both field and landscape characteristics. Semi-natural areas supported both crop colonization by pests and natural enemy action. The pest and its enemies differed in their responses to field or landscape variables. Landscape elements such as field banks favored the movement of the pest while impeding the movement of some natural enemies. Pest pressure did not increase with the neighboring density of Brassica crops. The presence of natural enemies did not reduce crop damage but reduced pest emerging rates. Finally, specialist parasitoids responded to the landscape at larger spatial extents than generalists. These results outline the complexity of improving pest control through landscape management.

Using transmission electron microscopy, egg content is studied in 10 parasitoid species, which either feed on host hemolymph or do not. As expected, hydropic eggs were found to contain little yolk which is only lipoid. In host-feeding species, the yolk contains a great number of typical protein bodies whereas in species that do not host-feed and which produce large eggs, the yolk contain very different bodies.

Prédation des pucerons par un cortège d’ennemis naturels sur Brassicacées légumières : quantification et facteurs de variation à l’échelle du paysage agricole

Les plantes sont sessiles et les flux génétiques entre populations sont donc limités. Cette faible mobilité renforce une sélection divergente dans des environnements contrastés. Par conséquent, on peut partir du principe que dans un milieu donné, les populations locales présentent une "fitness" supérieure à celle des populations non locales. Cependant, les mécanismes, les facteurs clés et les échelles de l'adaptation locale sont peu connus. Dans le cadre de deux projets, nous avons analysé les facteurs conduisant à l'adaptation locale et en particulier, le rôle des interactions biotiques. Le premier programme une transplantation réciproque entre 3 pays européens qui vise à comparer l'importance du sol et du climat, et à étudier l'influence de la communauté végétale du voisinage. Le deuxième projet est centré sur les interactions trophiques. Une manipulation des phytophages principaux dans des transplantations réciproques permet d'évaluer leur rôle dans l'adaptation locale. Ces transplantations sont accompagnées par des " common garden experiments " afin d'évaluer l'importance de l'origine des végétaux pour la restauration écologique. Dans les deux programmes de recherche, la théorie d'adaptation locale a été confirmée pour 3 espèces végétales sur 4. Nous avons trouvé une adaptation au site (climat) mais pas au sol d'origine. Une influence des facteurs biotiques a été mise en évidence mais dans des sens opposés, soit renforçant, soit affaiblissant l'adaptation. La co-évolution entre adaptation des plantes et des phytophages ne permet donc pas des conclusions générales. Néanmoins, nos études ont montré que chaque implantation de végétaux non-locaux change les interactions biotiques.

In this review, after giving some figures on the economic impact of aphids on agricultural production, we describe the different mechanisms leading to yield losses (direct damage due to sieve drain and plant reaction, indirect damage, often the most important, due to virus transmission). Then, after a history of chemical control and of its limits, the main control strategies (chemical control with decision rules, plant resistance, biological control, farming practices) are reviewed in the light of an integrated pest management approach. Several topics tackled in this article are exemplified for cereal aphids, which are among the most important in Europe as direct feeders and virus vectors.

Notice dans HAL : hal-00136285; International audience; The present study aimed to address how an insect parasitoid makes patch-departure decisions from various types of host patches and how previous patch experiences in the environment modify this decision-making process. Experiments were done with the parasitic wasp Aphidius rhopalosiphi attacking the grain aphid Sitobion avenae. In the experiments, wasps were observed in a laboratory environment containing several patches of various host densities, and behavioural records were analysed using a Coxrsquos proportional hazards model. Consideration of the effect of the within-patch experience gave a classic pattern of patch-leaving decision rules in parasitoids: A. rhopalosiphi used local information on host quality (i.e. numbers of ovipositions or rejections) and availability (i.e. patch density) to determine departure decision. However, consideration of previous patch experiences provided evidence that these departure rules are fundamentally dynamic, responding to the physiological state of the animal and the information it has about its environment. Results showed that A. rhopalosiphi decreased its tendency to leave the visited patch after an oviposition. However, when a female has already laid several other eggs in the environment, such an incremental mechanism gradually switched to a decremental one. Hence, A. rhopalosiphi responded to egg-load depletion by leaving the visited patches sooner and by depositing a smaller number of eggs in those patches, which probably led to a decreased level of superparasitism. Results also indicated that previous experiences enabled wasps to estimate spatial host distribution and then to adjust their behavioural decisions accordingly. Thus, A. rhopalosiphi was shown to adjust its patch residence time according to the quality and the number of the patches previously visited. These proximate mechanistic rules adopted by A. rhopalosiphi females are discussed in the context of general predictions from optimality models.

International audience; The distance from which hosts are perceived by foraging parasitoid females has been repeatedly considered to be an important behavioural trait that is supposed to be related to their efficiency as biological control agents. In the present study, such a ‘reactive distance’ was quantified using the automatic analysis of video-recorded females' walking paths. Inter-specific variation of this trait was analysed by comparison between Aphis gossypii Glover, Myzus persicae Sulzer, and Sitobion avenae Fabricius, and three of their parasitoid species known to present different levels of specificity to these hosts: Aphelinus abdominalis Dalman, Aphidius colemani Viereck, and Aphidius picipes Nees. Responses to two different host instars of different size were also compared in each combination. On average, parasitoid females were able to perceive aphids from a distance of about 3 to 4 mm. The only significant difference observed was between the Aphelinus and the two Aphidius parasitoid species. No difference was detected between the different host instars or species tested. Females seem to be unable to discriminate between host and non-host species from a distance. These results seem to confirm the hypothesis of the use of a cuticular contact kairomone for host recognition at the species level.

Experiments were conducted to understand how parasitoids and hyperparasitoids of cereal aphids in oceanic regions survive the periodic disappearance of their hosts. We recorded the number of parasitoids and hyperparasitoids emerging from mummies collected in summer on winter wheat and corn and in winter on cereal volunteers. In addition, we studied the duration of both summer and winter development of Aphidius rhopalosiphi De Stefani-Perez, the main parasitoid species of cereal aphids. In summer, interruption in development occurred in primary parasitoids which resulted in a longer development time (up to 4 m). These interruptions were fairly infrequent but they did ensure the survival of part of the population when there was a large decrease or disappearance of the aphid population. A longer duration of summer development also was recorded in a few hyperparasitoids. In winter, an important variability in the length of the development of primary parasitoids was noted which reflected their adaptive closeness to the aphid cycle. All hyperparasitoids studied here had long interruptions in their development and emerged only at the end of winter.