First in, last out: asymmetric competition influences patch exploitation of a parasitoid.

FR2116International audienceParasitoid females exploiting a patchy environment may encounter conspecifics on the host patches they visit or arrive in patches where other females have already parasitized hosts. When 2 or more foragers with differential arrivals exploit a resource patch simultaneously, the solution for the evolutionary stable patch residence times is the outcome of an asymmetric war of attrition. A theoretical prediction is that the forager that arrives first should stay longer than those arriving later, as a result of a resource value asymmetry. This study aims to examine how the arrival order on a host patch affects patch time in the solitary aphid parasitoid Aphidius ervi. For this purpose, 3 situations of competition were tested: single individuals foraging on unexploited patches (no competition), individuals foraging on previously exploited patches, and individuals exploiting patches in the presence of a competitor. Our data confirm the theoretical prediction: first-arriving females stay longer on a patch of hosts than second-arriving females. Neither host rejections nor host attacks affect patch-leaving decisions of females, but foraging with a competitor and previous encounters with a competitor increase the patch residence time of first-arriving females. This experiment is the first to test the effect of arrival order on patch exploitation strategies in nonfighting species. Key words: differential arrival, exploitative competition, foraging behavior, interference, parasitic wasp, war of attrition

Data from: Multi-scale and antagonist selection on life-history traits in parasitoids: a community ecology perspective

1) Life-history traits within ecological communities can be influenced by regional environmental conditions (external filters) and community-wide density-dependent processes (internal filters). While traits in a regional context may converge to a narrow range of values because of environmental filtering, species belonging to a guild may present contrasting traits as a means of niche differentiation, allowing coexistence whilst exploiting the same resources. 2) To disentangle the role of external and internal filters on phenotypic diversity within ecological communities, we examined the patterns of life-history trait variation within a guild of insect parasitoids during two successive years across three contrasted regions in relation to several ecological factors. 3) By combining a mean-field approach and an analysis of phenotypic variance across organizational levels (from individual to guild), we showed that the patterns of life-history trait variation across regions are consistent with local adaptation or adaptive phenotypic plasticity while the patterns of phenotypic variation within regions suggested how coexistence modulates life-history traits expression through niche differentiation. 4) Within a given region, phenotypic pattern of parasitoid life-history traits may also arise from bottom-up effects of trophic webs: insect host species could also control parasitoid life-history traits in nature. Our results also showed that parasitoid life-history traits presented contrasting variation patterns according to the sampling year, suggesting temporal variations in evolutionary and ecological dynamics of parasitoid species. 5) The application of such trait-based studies to insect parasitoids has the potential to provide further insight on how agricultural environments contribute to differential diversification among natural enemies guilds, highlighting the main role of agricultural landscape management for organisms' responses

Playing the hare or the tortoise in parasitoids: could different oviposition strategies have an influence in host partitioning in two Aphidius species?

In this paper, we compare the host selection behaviours of two parasitoids, Aphidius rhopalosiphi and Aphidius picipes, in order to analyse whether behavioural adaptations to the defensive behaviour of their host (the grain aphid Sitobion avenae) could, in part, be responsible for the simultaneous presence of both species in cereal fields. The oviposition behaviour of A. picipes differed from that of A. rhopalosiphi by,including a number of 'fluttering wings' sequences followed by immobility It resulted in a 44 times longer host-handling time for A. picipes than for A. rhopalosiphi. Hosts attacked by A. picipes exhibited fewer defensive behaviours than hosts attacked by A. rhopalosiphi.. A. picipes and A. rhopalosiphi rejected respectively 0% and 53% of unparasitized hosts presenting cornicle secretions, one of the defensive means of aphids. Furthermore, A. picipes females rejected 100% of the hosts that were already parasitized, whereas A. rhopalosiphi was previously described to reject only 20 to 40% of such hosts. Such differences could be explained by the way the two species deal with the aphid defensive behaviour. Field analyses, showed that A. rhopalosiphi was already present in wheat fields in early April whereas A. picipes appeared later and only achieved a low level of parasitism. However, when both species were present simultaneously, they shared the same resource

Seasonal and annual genotypic variation and the effect of climate on population genetic structure of the cereal aphid Sitobion avenae in northern France

Changes in the genetic structure and genotypic variation of the aphid Sitobion avenae collected from cereal crops in northern France were examined by analysing variation at five microsatellite loci across several years and seasons. Little regional and temporal differentiation was detected, as shown by very low F(ST) among populations. Repeated genotypes, significant heterozygote deficits, positive F(IS) values and frequent linkage disequilibria were found in nearly all samples, suggesting an overall pattern of reproductive mode variation in S. avenae populations. In addition, samples from Brittany (Bretagne) showed greater signs of asexual reproduction than those from the north of France, indicating a trend toward increasing sexuality northward. These patterns of reproductive variation in S. avenae are consistent with theoretical models of selection of aphid reproductive modes by climate. Contrasting with little changes in allelic frequencies, genotypic composition varied substantially in time and, to a lesser extent, in space. All important part of changes in genotypic arrays was due to the variation in frequency distribution of common genotypes, i.e. those that were found at several instances in the samples. Genotypic composition was also shown to vary according to climate, as genotypic diversity in spring was significantly correlated with the severity of the previous winter and autumn. We propose that the genetic homogeneity among S. avenae populations shown here across large temporal and spatial scales is the result of two forces: (i) migration conferred by high dispersal capabilities, and (ii) selection over millions of hectares of cereals (mostly wheat) bred from a narrow genetic base