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International audience; Generalist aphid parasitoids seem to behave as specialists at the local scale. The temporal pattern of host exploitation by parasitoids is key to understand this result. We investigated the temporal dynamics during twenty-one consecutive months of host exploitation by Diaeretiella rapae (M'Intosh) (Hymenoptera: Braconidae), one of the most generalist aphid parasitoid, in cultivated and uncultivated areas. We applied two complementary approaches: molecular detection of parasitism within living aphids and rearing of aphid mummies collected in Brassicaceae crops and adjacent areas. We built a Maximum Likelihood tree to determine the influence of host plant and aphid species on the genetic structure of D. rapae. We confirmed the existence of a cryptic species developing on Hayhurstia atriplicis (L.) (Hemiptera: Aphididae), but we found no other host-related genetic variation. D. rapae exploited almost exclusively Brevicoryne brassicae (L.) (Hemiptera: Aphididae), a pest of Brassiceae and rarely other hosts. When the crop is absent, D. rapae parasitized B. brassicae on wild Brassicaceae and cover crop instead of alternative aphid host on other plants. These results show the limited role of uncultivated habitats as a reservoir for D. rapae and the interest of wild Brassiceae and cover crops to enhance the control of B. brassicae.

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; The ecological role of bacterial symbionts associated with insects is increasingly recognized. Notably, aphids are known to harbour facultative bacterial symbionts that influence their phenotypes like conferring a protection against natural enemies. The most well-known Gamma-Proteobacteria Hamiltonella defensa can provide a defence against hymenopteran parasitoids to their aphid hosts. Aphidiine parasitoids are broadly used as biological control agents in inundative releases against aphids of various greenhouse crops so that defensive symbionts can potentially compromise efficiency of the released parasitoids. Here, we first measured the prevalence and spatiotemporal dynamics of bacterial symbionts infecting eight aphid species colonizing protected strawberry crops in France. Considering the three major aphid pest species of strawberry crops (Acyrthosiphon malvae (Mosley), Macrosiphum euphorbiae (Thomas) and Rhodobium porosum (Sanderson)), we then estimated their possible effects on protection against Aphidius ervi (Haliday), a parasitoid species currently used as a biological control agent. Both symbiotic composition and symbiont prevalences highly differed between aphid species. In the three dominant aphid species, many individuals were single infected or co-infected with H. defensa and Regiella insecticola. Prevalences of these bacterial symbionts highly varied across French regions. Parasitism experiments showed that infection with R. insecticola did not provide any protection against parasitoids while infection with H. defensa would be associated with a strong reduction of parasitism in A. malvae only. These results are valuable to understand the implications of defensive symbionts in the effectiveness of 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.

Session 17. Tools to design, manage and monitor diversified systems from the field to the landscape levels; International audience

International audience; For many crops, we have poor knowledge about the relationship between pest density and damage. However, investigating pest harmfulness is particularly relevant currently in the search for alternative crop protection strategies that are unlikely to totally suppress pest populations. Here, we assessed the harmfulness of Delia radicum (L.) (Diptera: Anthomyiidae) on broccoli (Brassica oleracea var. italica Plenck). We worked inside insect-proof cages set up in the field with additional pitfall traps to remove ground-dwelling predators. Plants were manually infested with 10 levels of pest density ranging 0–100 individuals per plant, following a natural infestation pattern. Surprisingly, no plants died but almost 100% of the pests introduced died over the course of the experiment. However, all broccoli development and growth traits were negatively correlated with pest density and broccoli head mass at harvest decreased linearly with pest density. The observation over time of development and growth traits showed evidence of plant compensation, suggesting that the head mass of individual plants may have reached similar values if allowed to fully mature. The relationship between pest density and damage, together with forecast models of pest population dynamics could be used to develop decision support tools assessing the relevance of preventative treatments.

International audience; Potencial regulador de parasitoides sobre poblaciones de Aphis gossypii Glover, 1877 (Hemiptera: Aphididae) en laboratorio, bajo condiciones semiáridas Este estudio tiene como objetivo evaluar el potencial regulador de Lysiphlebus testaceipes y Aphidius matricariae sobre Aphis gossy-pii en condiciones de laboratorio semiáridas. Nuestros resultados mostraron que la longevidad de las hembras de L. testaceipes fue significativamente menor que la de A. matricariae. El porcentaje diario de momificación fue más importante para L. testaceipes que para A. matricariae y disminuyó con la edad de los parasitoides. En general, las tasas de momificación y de emergencia fueron más altas en L. testaceipes. La descendencia de A. matricariae se des-arrolló en menos tiempo que la de L. testaceipes. La proporción sexual no varió respecto a las especies parasitoides. Con nuestros resultados, L. testaceipes fue más efectivo para el control biológico de A. gossypii en patata en agro-ecosistemas semiáridos. Abstract This study aims to evaluate the regulatory potential of Lysiphlebus testaceipes and Aphidius matricariae over Aphis gossypii in semi-arid laboratory conditions. Our results showed that the longevity of L. testaceipes females was significantly lower than A. matricariae ones. The daily mummification rate of aphid was more important for L. testaceipes than for A. matricariae and declined with parasitoids age. Overall, both the mummification rates and the emergence rates were the highest in L. testaceipes. A. matricariae offspring developed in a shorter time compared to L. testaceipes ones. The overall sex ratio did not vary according to parasitoid species. From our results L. testaceipes was more effective for the biological control of A. gossypii on potato in the semi-arid agro-ecosystems.

National audience; Ce chapitre synthétise les connaissances autour de la biologie et des traits de vie des différents groupes d’organismes acteurs des processus de régulations biologiques, en mettant à la fois l’accent sur les bioagresseurs (plantes adventices, champignons, bactéries, virus, insectes ravageurs) et sur leurs ennemis naturels. Il présente aussi les caractéristiques des différents groupes impliqués dans la pollinisation

Aphidiinae are mostly composed of specialist parasitoids and the few species described as generalist are suspected to be composed of cryptic specialists, almost indistinguishable based on morphological characteristics. The use of molecular markers has proven to be a useful tool for revealing cryptic species complexes and here we use seven mitochondrial and nuclear gene fragments to study possible genetic differentiation among seven Aphidiinae generalists. Maximum likelihood (ML) trees and Bayesian Poisson tree processes (bPTP) models were conducted on each gene separately and on the seven genes together. The standard cytochrome c oxidase I barcode region appeared to be the most polymorphic and probably the best marker to reveal putative cryptic species. However, we showed with ML trees and bPTP models that a complementary use of mitochondrial and nuclear genes was the most relevant approach to reliably identify cryptic genetic clades in the Aphidiinae. Overall, most of the analysed generalist morphospecies were shown to be composed of subgroups related to the aphid host, some of them revealed as cryptic species by the species delimitation analysis. Further studies are needed to reveal the generality of this result in Aphidiinae.

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

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.

Volatiles resulting from plant–herbivore interactions play an important role in the behavioral decisions of phytophagous, predatory, and parasitoid insects and could be used for managing pest insects. However, to date and after about 40 years of research, documented studies on applications in the field remain extremely scarce. Delia radicum L. (Diptera: Anthomyiidae), the cabbage root fly, is a major pest of brassicaceous crops for which classical control strategies are currently lacking. Our previous studies showed that dimethyl disulfide (DMDS), a compound emitted by roots heavily infested by D. radicum larvae, was attractive for the fly’s main natural enemies and could lead to a reduction of 60% in number of eggs laid on treated plants in the field. As a follow-up of this work, we conducted another field study to select additional volatiles that could be used in a push–pull approach. Several synthetic herbivore-induced plant volatiles, selected on the basis of their potential action on the behavior of both the fly and its natural enemies, were placed in odor dispensers in experimental broccoli plots and their influence on oviposition by D. radicum and egg predation by ground-dwelling predators was assessed. Our results confirmed the role of DMDS in reducing D. radicum egg numbers on broccoli plants and revealed that (Z)-3-hexenyl acetate, a green leaf volatile released by recently damaged plants, strongly stimulated fly oviposition. Also, two of the compounds tested slightly modified predation activity of ground-dwelling predators: acetophenone decreased the proportion of predated patches, whereas methyl salicylate increased it. This study is a first step in designing a push–pull strategy to control the cabbage root fly.

International audience; Many organisms, including entomopathogenous fungi, predators or parasites, use aphids as ressources. Parasites of aphids are mostly endoparasitoid insects, i.e. insects which lay eggs inside the body of an other insect which will die as a result of their development. In this article, we review the consequences of the numerous pecularities of aphid biology and ecology for their endoparasitoids, notably the Aphidiinae (Hymenoptera: Braconidae). We first examine the various mechanisms used by aphids for defence against these enemies. We then explore the strategies used by aphidiine parasitoids to exploit their aphid hosts. Finally, we consider the responses of both aphids and parasitoids to ecological constraints induced by seasonal cycles and to environmental variations linked to host plants and climate. The fundamental and applied interest of studying these organisms is discussed.

International audience; The efficiency of the biological controlstrategies based on parasitoids, either as a release or a conservative measure, depends on the timely arrival or release of the parasitoids after colonization of the field by the pest. In this study, the effects of the delay between the cabbage aphid Brevicoryne brassicae (Hemiptera: Aphididae) infestation and the release of its parasitoid Diaeretiella rapae (Hymenoptera: Braconidae) and of the number of parasitoid releases on aphid population dynamics and host plant growth were investigated. When the parasitoids were released at the start of the infestation, 89.6 % of the aphids were parasitized one month later, and the damage to cabbage was low. The identical number of parasitoids released two weeks after the start of the infestation partially controlled the aphid population, but the growth of the cabbage was altered. The earliest introduction of the parasitoid to the crop led to an efficient host regulation, and the release in three times rather than in one time did not improve the control efficiency at this release delay. The implications for biological control strategies that use parasitoids of aphids are discussed.

International audience; The recognition by female phytophagous insects of a plant as a ‘good’ or ‘bad’ host for egg laying is based on a variety of cues (either visual, physical or chemical). Specific cues are often looked for during stereotypic oviposition behaviors, composed of several phases having their own function(s). In this study the oviposition behavior of the pollen beetle Meligethes aeneus, a pest which lays eggs in flower buds of only some brassicaceous plants, was described in detail on five oilseed rape (Brassica napus) genotypes. In parallel, setae borne by the ovipositor were characterized by scanning electron microscopy. Observations showed that the stereotypic oviposition sequence is functionally divided into three independent phases: external inspection, internal inspection and egg laying. The ovipositor plays a role in all phases by gaining information about external and internal bud parts. This role appears to be only physical since all the setae it bears are mechanoreceptors. Despite the fact that the pollen beetle is a specialist for oviposition, important variations in secondary metabolites that are typical of its host plant family (i.e., glucosinolates) on the bud did not influence clutch size. The crucial phase in the oviposition sequence seems to be the external inspection, during which poor and high-quality host plants are probably discriminated. Chemical information on bud surface is likely to be determinant in this process.

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

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.

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

International audience; Molecular methods are increasingly used to detect and identify parasites in their hosts. However, existing methods are generally not appropriate for studying complex host-parasite interactions because they require prior knowledge of species composition. DNA barcoding is a molecular method that allows identifying species using DNA sequences as an identification key. We used DNA amplification with primers common to aphid parasitoids and sequencing of the amplified fragment to detect and identify parasitoids in their hosts, without prior knowledge on the species potentially present. To implement this approach, we developed a method based on 16S rRNA mitochondrial gene and LWRh nuclear gene. First, we designed two primer pairs specific to Aphidiinae (Hymenoptera), the main group of aphid parasitoids. Second, we tested whether the amplified regions could correctly identify Aphidiinae species and found that 61 species were accurately identified of 75 tested. We then determined the ability of each primer pair to detect immature parasitoids inside their aphid host. Detection was earlier for 16S than for LWRh, with parasitoids detected, respectively, 24 and 48 h after egg injection. Finally, we applied this method to assess parasitism rate in field populations of several aphid species. The interest of this tool for analysing aphid-parasitoid food webs is discussed.

Correspondence: Stephane Derocles, Fax: +33 2 23 48 51 70; E-mail: stephane.derocles@agrocampus-ouest.fr; International audience; Reliable identification of Aphidiinae species (Braconidae) is a prerequisite for conducting studies on aphidparasitoid interactions at the community level. However, morphological identification of Aphidiinae species remains problematic even for specialists and is almost impossible with larval stages. Here, we compared the efficiency of two molecular markers [mitochondrial cytochrome c oxydase I (COI) and nuclear long wavelength rhodopsin (LWRh)] that could be used to accurately identify about 50 species of Aphidiinae that commonly occur in aphidparasitoid networks in northwestern Europe. We first identified species on a morphological basis and then assessed the consistency of genetic and morphological data. Probably because of mitochondrial introgression, Aphidius ervi and A.microlophii were indistinguishable on the basis of their COI sequences, whereas LWRh sequences discriminated these species. Conversely, because of its lower variability, LWRh failed to discriminate two pairs of species (Aphidius aquilus, Aphidius salicis, Lysiphlebus confusus and Lysiphlebus fabarum). Our study showed that no unique locus but a combination of two genes should be used to accurately identify members of Aphidiinae.

Structure du paysage en parcelles de Brassicacées : impact sur les insectes phytophages et leurs ennemis naturels.

Already parasitized hosts are often of poorer quality than healthy hosts. It is therefore usually advantageous for parasitoid females to recognize and reject them. Parasitized hosts can be identified on the basis of various physical or chemical marks present on the surface or inside the hosts or their surroundings in the case of concealed host. Here we studied host discrimination behaviors of females of a certain population of Pachycrepoideus vindemmiae, a solitary ectoparasitoid, which are known to reject large-sized parasitized hosts after an abdominal examination of their surface. We first investigated females' recognition behaviors of host parasitism status when confronted to small-sized hosts (Drosophila melanogaster pupae) as host size may influence the use of different cues for host selection. We showed that, in such a situation, females also discriminate parasitized hosts after an external host exploration with the tip of their ovipositor sheath (third valvulae). We then described the sense organs present on the different parts of the ovipositor by means of scanning and transmission electron microscopy analysis. As the extremity of the third valvulae bears only one type of sensilla that appears to be chemoreceptors, we considered these sensilla as highly likely to be involved in host discrimination in P. vindemmiae. To our knowledge, this is the first time that receptors located on the ovipositor sheath are described as implicated in host discrimination in parasitoid wasps. We discuss potential chemical markers that might be detected by these receptors.

Although a national strategy for the coexistence of GM and conventional/organic crops in Ireland has been published (McGill et al 2005) measures pertaining to the coexistence of GM and non-GM oilseed rape (OSR) crops was omitted pending the completion of

Structure du paysage et colonisation des parcelles de Brassicacées légumières par les insectes phytophages et leurs ennemis naturels

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.

International audience; In insect parasitoids, fitness is dependent on the host finding and recognition abilities of the female. Host recognition cues have been described for various host-parasitoid systems, but are still under investigation in aphid parasitoids. Our study aimed to clarify the respective role of physical and chemical cues in recognition of the aphid cuticle. Shed aphid exuviae were used as an elicitor in order to avoid any influence of color, movement, or volatiles present in a living aphid. We assessed the effect of chemical and heat treatments on the texture of the cuticle by using scanning electron microscopy and tested the recognition of treated cuticles by the parasitoid. We showed that recognition cues of the cuticle can be removed chemically (using combined treatments with n-hexane and methanol). Moreover, heat treatment destroyed the physical texture of the cuticle without significantly reducing parasitoid recognition. In a second step, we showed that epicuticular extracts deposited on pieces of paper triggered female attack behavior. First results concerning the chemical composition of the active extract are presented. This study shows that chemical compounds extractable by organic solvents mediate cuticle recognition by aphid parasitoids

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.

Blackcurrant crop, especially Noir de Bourgogne variety, is emblematic of Burgundy, with the well-known liquor “Crème de cassis”. Face to the sector’s difficulties to maintain a crop yield, a European project was built in association with scientific and technical partners. Two research topics of this project were developed here. The first topic consisted in looking for traits for the plant breeding program of Noir de Bourgogne variety. The aim was to adapt the Noir de Bourgogne to climate change and increasing pest pressure. The first topic comprised several tests: phenological observations, flowering and fruit initiation follow-ups, measurements of anther stigma distance and the development of a resistance test to white peach scale (WPS) Pseudaulacaspis pentagona on cuttings. The second topic based on the restoration of the blackcurrant pollination service. It focused on the installation of bumblebee and osmia hives in blackcurrant plots in order to measure the effectiveness of pollination. At the end of the internship, a link between anther stigma distance and fruit initiation rate was made, varieties were identified for their precocity or lateness. The main obstacles to the development of the resistance test have beenidentified, and the first infestations of WPS according to the susceptibility of the cutting variety seem to have worked. The addition of hives of certain pollinators was significant compared to "natural" pollination.; La culture du cassis, notamment de la variété Noir de Bourgogne, est emblématique de la Bourgogne avec la fameuse liqueur de crème de cassis. Face aux difficultés de la filière pour maintenir le rendement, un projet d'innovation européen a été élaboré en association avec des partenaires scientifiques et techniques. Deux axes de ce projet sont développés ici. Le premier axe a consisté à rechercher des caractéristiques d’intérêt pour l’amélioration génétique du Noir de Bourgogne, afin de s’adapter au changement climatique et à une pression croissante de ravageurs. Il s’est décliné en plusieurs tests: observations phénologiques, suivi des floraisons et nouaisons, mesures de distance anthère stigmate et développement d’un test de résistance à la cochenille du mûrier Pseudaulacaspis pentagona sur des boutures. Le deuxième axe du projet est basé sur la restauration du service de pollinisation du cassis. Celui-ci a porté sur l’installation de ruches de bourdons et osmies dans les parcelles de cassis afin de mesurer l’efficacité de la pollinisation. A l’issue du stage, un lien entre distance anthère stigmate et taux de nouaison a été fait, des variétés ont été identifiées pour leur précocité ou tardiveté. Les principaux freins au développement du test de résistance ont été identifiés, et les premières infestations de cochenille en fonction de la sensibilité de la variété semblent avoir fonctionné. L’ajout de ruches de certains pollinisateurs est significatif par rapport à la pollinisation « naturelle ».

Developping farming systems that use less pesticides is partly based on the intensification of ecosystem services. The main aim of this thesis was to quantify the insect pest regulation service provided by their natural enemies, both in experimental plots and in farmers fields. We worked on brassicaceous vegetables that are confronted to pest attacks on above- and belowground organs (aphids and the cabbage root fly). We assessed their harmfulness, in order to quantify the injuries and damage these pests can inflict to plants. Our results confirm that aphids are less harmful than the cabbage root fly. On flower vegetables, the latter not only leads to plant mortality but can also slow plant development.We also showed that ground dwelling predators significantly reduce the populations of both pests, leading, for the cabbage root fly, to a strong decrease in plant mortality rate. Finally, simplifying soil tillage in the spring did not favor ground dwelling predators overwintering inside the field and neither did it increase natural regulation service in the short term. However, early ground dwelling predators are fundamental and must be favored by tailored practices both inside and around the fields. The knowledge gained in the present thesis show the potential of conservation biological control for the agroecological management of insect pest populations.; Le développement d’une agriculture moins dépendante des pesticides repose en partie sur la maximisation des services écosystémiques. L’objectif principal de cette thèse était de quantifier le service de régulation des insectes ravageurs par leurs ennemis naturels, en conditions expérimentales et en situation de production. Le travail a porté sur les brassicacées légumières soumises aux attaques de ravageurs aériens et souterrains (pucerons et mouche du chou). Nous avons évalué leur nuisibilité pour quantifier les dégâts et dommages qu’ils peuvent générer. Nos résultats confirment la moindre nuisibilité des pucerons par rapport à celle de la mouche du chou. Pour celle-ci, la nuisibilité sur légume-fleur s’exprime non seulement par la mortalité des plants mais aussi par un effet sur leur développement.Nous avons également montré que les prédateurs épigés réduisaient significativement les populations des deux ravageurs, ce qui, pour la mouche du chou, conduit à une baisse conséquente de la mortalité des plants. Enfin, simplifier le travail du sol au printemps n’a pas permis de favoriser les prédateurs hivernant dans la parcelle, ni d’augmenter le service de régulation à court terme. Cependant les prédateurs précoces sont déterminants et doivent être favorisés par des pratiques intra- et péri-parcellaires ciblées. Les connaissances acquises dans ce travail démontrent le potentiel de la lutte biologique par conservation pour une gestion agro-écologique efficace des insectes ravageurs.