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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; Contexte: Il existe une diversité de stratégies de biocontrôle faisant appel aux macro-organismes, qu'elles soient pratiquées depuis longtemps en France, plus récemment implantées, voire encore prospectives. enjeux: Il est possible de définir un cadre de réflexion commun à l'ensemble de ces stratégies autour d'enjeux de recherche et développement : réglementation, zootechnie, amélioration génétique, efficacité au champ et risques éventuels. Quatre dynamiques sont susceptibles d'apporter un nouvel élan au biocontrôle à l'aide de macroorganismes : – la convergence entre santé des plantes et santé humaine autour de stratégies communes ; – la convergence entre biocontrôle et production d'insectes comestibles autour d'enjeux communs ; – la complémentarité entre les stratégies interventionnistes utilisant les macro-organismes et la lutte biologique par conservation ; – la nécessaire réflexion concernant de nouvelles organisations économiques et sociales.

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; Contexte : Malgré des besoins toujours plus pressants de trouver des alternatives aux produits phytopharmaceutiques, force est de constater que le secteur des macro-organismes de biocontrôle peine à se généraliser dans la pratique et, avant tout, à s'imposer dans l'esprit des parties intéressées : les utilisateurs finaux (les agriculteurs), les conseillers techniques, mais également les politiques et les financeurs. Stratégies de lutte : La régulation des bioagresseurs peut se réaliser en favorisant les individus déjà présents dans ou aux alentours des parcelles cultivées : c'est ce que l'on appelle la lutte biologique par conservation. Elle peut également se faire en introduisant certains auxiliaires délibérément dans les parcelles cultivées (acclimatation, augmentation, lutte autocide). Cet article présente les stratégies de biocontrôle à l'aide de macro-organismes (essentiellement des insectes, acariens ou nématodes entomopathogènes), qu'elles soient pratiquées depuis longtemps en France, plus récemment implantées, voire encore prospectives.

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.

Contexte : Malgré des be- soins toujours plus pressants de trouver des alternatives aux produits phytopharmaceutiques, force est de constater que le secteur des macro-organismes de biocontrôle peine à se généraliser dans la pratique et, avant tout, à s'imposer dans l'esprit des parties intéressées : les utilisateurs finaux (les agriculteurs), les conseillers techniques, mais également les politiques et les financeurs. Stratégies de lutte : La régulation des bioagresseurs peut se réaliser en favorisant les individus déjà présents dans ou aux alentours des parcelles cultivées : c'est ce que l'on appelle la lutte biologique par conservation. Elle peut également se faire en introduisant certains auxiliaires délibérément dans les parcelles cultivées (acclimatation, augmentation, lutte autocide). Cet article présente les stratégies de biocontrôle à l'aide de macro-organismes (essentiellement des insectes, acariens ou nématodes entomopathogènes), qu'elles soient pratiquées depuis longtemps en France, plus récemment implantées, voire encore prospectives.

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; 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

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