Multi-scale approach to biodiversity proxies of biological control service in European farmlands.

Intensive agriculture has profoundly altered biodiversity and trophic relationships in agricultural landscapes, leading to the deterioration of many ecosystem services such as pollination or biological control. Information on which spatio-temporal factors are simultaneously affecting crop pests and their natural enemies is required to improve conservation biological control practices. We conducted a study in 80 winter wheat crop fields distributed in three regions of North-western Europe (Brittany, Hauts-de-France and Wallonia), along intra-regional gradients of landscape complexity. Five taxa of major crop pests (aphids and slugs) and natural enemies (spiders, carabids, and parasitoids) were sampled three times a year, for two consecutive years. We analysed the influence of regional (meteorology), landscape (structure in both the years n and n-1) and local factors (hedge or grass strip field boundaries, and distance to boundary) on the abundance and species richness of crop-dwelling organisms, as proxies of the service/disservice they provide. Firstly, there was higher biocontrol potential in areas with mild winter climatic conditions. Secondly, natural enemy communities were less diverse and had lower abundances in landscapes with high crop and wooded continuities (sum of interconnected crop or wood surfaces), contrary to slugs and aphids. Finally, field boundaries with grass strips were more favourable to spiders and carabids than boundaries formed by hedges, while the opposite was found for crop pests, with the latter being less abundant towards the centre of the fields. We also revealed temporal modulation-and sometimes reversion-of the impact of local elements on crop biodiversity. To some extent, these results cause controversy because they show that hedgerows and woodlots should not be the unique cornerstones of agro-ecological landscape design strategies. We point out that combining woody and grassy habitats to take full advantage of the features and ecosystem services they both provide (biological pest control, windbreak effect, soil stabilization) may promote sustainable agricultural ecosystems. It may be possible to both reduce pest pressure and promote natural enemies by accounting for taxa-specific antagonistic responses to multi-scale environmental characteristics.

Effects of surfactins, Bacillus lipopeptides, on the behavior of an aphid and host selection by its parasitoid.

BACKGROUND: Lipopeptides extracted from the Bacillus genus are emerging biopesticides, especially in protecting crops against phytopathogens. Among the three main families of lipopeptides, surfactins have been identified as having insecticidal properties against several insect orders. However, the sublethal effects of these promising biopesticides on insect pests and their natural enemies remain largely unknown. The aim of this study was to evaluate the effects of surfactins topically applied on black bean aphid Aphis fabae mortality. First, the effects of surfactins on aphid mortality were determined by delivering increasing concentrations to adults and nymphs. Second, the sublethal effects of surfactins on locomotor activity and feeding behavior of surviving aphids were evaluated using the electropenetrography method. Finally, the effect of host exposure to surfactins on host selection behavior by Aphidius matricariae parasitoid females was analyzed. RESULTS: Four surfactins concentrations were studied (0.5, 1, 2.5 and 5 g L-1 ). There was concentration-dependent mortality in response to surfactins at 24 h after treatment. Surfactins impacted aphid behavior when delivered at 1 g L-1 by inducing a greater locomotor activity and a reduction in feeding activity. By contrast, at the third trophic level, exposure of aphid hosts to surfactins did not affect behaviors leading to host recognition and acceptance by parasitoid females. CONCLUSION: This study highlighted the consequences of aphid exposure to surfactins in the context of bottom-up regulation. Although surfactins could directly impact aphid behavior, they had no apparent consequences on the host selection behavior exhibited by parasitoid wasps.

Antagonist effects of the leek Allium porrum as a companion plant on aphid host plant colonization.

Combining a non-host plant (companion plant or CP) with a target cultivated plant is considered as a promising strategy to reduce pest pressure. Among the companion plants (CP) commonly used in integrated systems, those belonging to the Amaryllidaceae family (chives, garlic, onion, leek) exhibit characteristics related to certain volatile organic compounds (VOCs) with promising repellent potentialities. The aim of this work was to investigate the potential disruption of sweet pepper (host plant) colonization by the green peach aphid (Myzus persicae) when exposed to leek (Allium porrum) as a CP. Retention/dispersion, EPG and clip-cage/Petri dish laboratory experiments were thus performed to study the effect of leek VOCs on aphid settlement/migration, feeding behavior and life history traits parameters, respectively. This work revealed that leek as a CP had a negative effect on aphid feeding behavior, by disturbing the balance between phloem and xylem sap ingestion, but had no influence concerning aphid settlement. Surprisingly, leek as a CP triggered some unexpected probiotic effects on certain life history traits such as aphid survival, biomass, and fecundity, suggesting a possible hormetic effect of leek VOCs on aphid physiology. The possibility of experience-induced preference of aphids for leek VOCs was also discussed.

Plant infection by two different viruses induce contrasting changes of vectors fitness and behavior.

Insect-vectored plant viruses can induce changes in plant phenotypes, thus influencing plant-vector interactions in a way that may promote their dispersal according to their mode of transmission (i.e., circulative vs. noncirculative). This indirect vector manipulation requires host-virus-vector coevolution and would thus be effective solely in very specific plant-virus-vector species associations. Some studies suggest this manipulation may depend on multiple factors relative to various intrinsic characteristics of vectors such as transmission efficiency. In anintegrative study, we tested the effects of infection of the Brassicaceae Camelina sativa with the noncirculative Cauliflower mosaic virus (CaMV) or the circulative Turnip yellows virus (TuYV) on the host-plant colonization of two aphid species differing in their virus transmission efficiency: the polyphagous Myzus persicae, efficient vector of both viruses, and the Brassicaceae specialist Brevicoryne brassicae, poor vector of TuYV and efficient vector of CaMV. Results confirmed the important role of virus mode of transmission as plant-mediated effects of CaMV on the two aphid species induced negative alterations of feeding behavior (i.e., decreased phloem sap ingestion) and performance that were both conducive for virus fitness by promoting dispersion after a rapid acquisition. In addition, virus transmission efficiency may also play a role in vector manipulation by viruses as only the responses of the efficient vector to plant-mediated effects of TuYV, that is, enhanced feeding behavior and performances, were favorable to their acquisition and further dispersal. Altogether, this work demonstrated that vector transmission efficiency also has to be considered when studying the mechanisms underlying vector manipulation by viruses. Our results also reinforce the idea that vector manipulation requires coevolution between plant, virus and vector.

Response of life-history traits to artificial and natural selection for virulence and nonvirulence in a Drosophila parastitoid, Asobara tabida.

Co-evolution of host-parasitoid interactions is determined by the costs of host resistance, which received empirical evidence, and the costs of parasitoid virulence, which have been mostly hypothesized. Asobara tabida is a parasitoid, which mainly parasitizes Drosophila melanogaster and D. subobscura, the first species being able to resist to the parasitoid development while the second species is not. To parasitize resistant hosts, including D. melanogaster, A. tabida develops sticky eggs, which prevent encapsulation, but this virulence mechanism may be costly. Interindividual and interpopulation variation in the proportion of sticky eggs respectively allowed us to (i) artificially select and compare life-history traits of a virulent and a nonvirulent laboratory strain, and (ii) compare a virulent and a nonvirulent field strain, to investigate the hypothetical costs of virulence. We observed strong differences between the 2 laboratory strains. The nonvirulent strain invested fewer resources in reproduction and walked less than the virulent one but lived longer. Concerning the field strains, we observed that the nonvirulent strain had larger wings while the virulent one walked more and faster. All together, our results suggest that virulence may not always be costly, but rather that different life histories associated with different levels of virulence may coexist at both intra- and interpopulation levels.

Cascading effects of N input on tritrophic (plant-aphid-parasitoid) interactions.

Because N is frequently the most limiting mineral macronutrient for plants in terrestrial ecosystems, modulating N input may have ecological consequences through trophic levels. Thus, in agro-ecosystems, the success of natural enemies may depend not only from their herbivorous hosts but also from the host plant whose qualities may be modulated by N input. We manipulated foliar N concentrations by providing to Camelina sativa plants three different nitrogen rates (control, optimal, and excessive). We examined how the altered host-plant nutritional quality influenced the performances of two aphid species, the generalist green peach aphid, Myzus persicae, and the specialist cabbage aphid, Brevicoryne brassicae, and their common parasitoid Diaeretiella rapae. Both N inputs led to increased N concentrations in the plants but induced contrasted concentrations within aphid bodies depending on the species. Compared to the control, plant biomass increased when receiving the optimal N treatment but decreased under the excessive treatment. Performances of M. persicae improved under the optimal treatment compared to the control and excessive treatments whereas B. brassicae parameters declined following the excessive N treatment. In no-choice trials, emergence rates of D. rapae developing in M. persicae were higher on both optimum and excessive N treatments, whereas they remained stable whatever the treatment when developing in B. brassicae. Size of emerging D. rapae females was positively affected by the treatment only when it developed in M. persicae on the excessive N treatment. This work showed that contrary to an optimal N treatment, when N was delivered in excess, plant suitability was reduced and consequently affected negatively aphid parameters. Surprisingly, these negative effects resulted in no or positive consequences on parasitoid parameters, suggesting a buffered effect at the third trophic level. Host N content, host suitability, and dietary specialization appear to be major factors explaining the functioning of our studied system.

Aphid Parasitoid Mothers Don't Always Know Best through the Whole Host Selection Process.

Parasitoid host selection behaviour has been extensively studied in experimentally simplified tritrophic systems formed by one single food chain (one plant, one herbivore and one parasitoid species). The "Mother knows best" hypothesis predicts that the preference for a plant-host complex should be positively correlated with plant quality for offspring performance. We studied the host selection behaviour of the generalist endoparasitoid Aphidius matricariae towards the black bean aphid Aphis fabae in the intercrop system including Vicia faba as a focal plant and its companion plant Camelina sativa. Dual-choice laboratory bioassays revealed that parasitoid females preferred to orientate towards (1) the plant-aphid complex over the non-infested plant whatever the complex (2) the C. sativa-A. fabae complex over the V. faba-A. fabae complex. In dual choice attack rate bioassays, parasitoid females showed more interest towards the aphids on C. sativa but paradoxically chose to oviposit more in aphids on V. faba. Ultimately, parasitoids that had developed on the V. faba-A. fabae complex exhibited better fitness parameters. By demonstrating that parasitoid females were able to discriminate the aphid host that offered the highest fitness to their offspring but selected beforehand the least suitable plant-aphid complex, we provide key insight into the disruption in their host selection behaviour potentially triggered by diverse habitats. This suggests that the "Mother knows best" hypothesis could be thwarted by increasing the complexity of the studied systems.

Trans-generational effects of mild heat stress on the life history traits of an aphid parasitoid.

Temperature changes are common in nature and insects are particularly exposed and sensitive to such variations which can be potential stresses, ultimately affecting life history traits and overall fitness. Braconids have been widely used to study the effects of temperature on host-parasitoid interactions and the present work focused on the solitary endoparasitoid Aphidius ervi Haliday (Hymenoptera: Braconidae Aphidiidae), an efficient biological control agent commercially used against aphids such as the potato aphid Macrosiphum euphorbiae Thomas (Sternorrhyncha: Aphididae). Contrary to previous studies using heat shocks at extreme temperatures, we evaluated the effects of mild heat stresses by transferring young parasitoid adults from the constant temperature of 20°C to either a warm (25°C) or hot (28°C) temperature, for either 1 h or 48 h. Such treatments are consistent with situations commonly experienced by parasitoids when moved from their rearing conditions to greenhouses or field conditions. The effects were evaluated both on the heat stressed A. ervi adults (G0) (immediate effects) and on their first generation (G1) progeny (trans-generational effects). G0 wasps' mortality was significantly affected by the temperature in interaction with the duration of the stress. Longevity of G0 wasps surviving the heat stress was negatively affected by the temperature and females lived longer than males. Heat stress applied to A. ervi parents also had consequences on their G1 progeny whose developmental time, rates of mummification and percentage of parasitoid completing total development were negatively affected. Surprisingly, the egg load at emergence of the G1 female progeny was increased when their mothers had been submitted to a mild heat stress of 25°C or 28°C. These results clearly demonstrate trans-generational phenotypic plasticity, showing that adaptation to thermal stresses may be achieved via maternal effects. This study also sheds light on the complexity of insect responses and underlying mechanisms to fluctuating conditions in their natural environment.

Dynamic use of fruit odours to locate host larvae: individual learning, physiological state and genetic variability as adaptive mechanisms.

This chapter presents a series of behavioral studies designed to document how Leptopilina spp. learn fruit odours in order to find and explore host-infested fruits. Experimental analyses of conditioned responses explored individual learning, physiological changes and genetic variability as adaptive mechanisms of the host searching behavior. Both oriented walking and substrate probing can be easily observed and quantified in laboratory devices. We studied walking in a four-arm olfactometer and probing in an agar substrate in response to olfactory stimulation by fruit odours. We analyzed the odour learning process and the dynamics of the memory. We next investigated how odour memory is influenced by motivation factors such as mating or egg-load, and how much variation is due to inheritance, using isofemale lines. Next, we addressed the adaptive significance of innate and conditioned responses to fruit odour by comparing and crossing populations originating from areas with contrasted levels of host availability.