Floral presence and flower identity alter cereal aphid endosymbiont communities on adjacent crops

Abstract

AbstractFloral plantings adjacent to crops fields can recruit populations of natural enemies by providing flower nectar and non-crop prey to increase natural pest regulation. Observed variation in success rates might be due to changes in the unseen community of protective endosymbionts hosted by many herbivorous insects, which can confer resistance to various specialist natural enemies, e.g. parasitoid wasps. Reduced insect control may occur if highly protective symbiont combinations increase in frequency via selection effects, and this is expected to be stronger in lower diversity systems.We used a large-scale field trial to analyse the bacterial endosymbiont communities hosted by cereal aphids (Sitobion avenae) collected along transects into strip plots of barley plants managed by either conventional or integrated (including floral field margins and reduced inputs) methods. In addition, we conducted an outdoor pot experiment to analyse endosymbionts inS. avenaeaphids collected on barley plants that were either grown alone or alongside one of three flowering plants, across three time points.In the field, aphids hosted up to four symbionts. The abundance of aphids and parasitoid wasps was reduced towards the middle of all fields while aphid symbiont species richness and diversity decreased into the field in conventional, but not integrated, field-strips. The proportion of aphids hosting different symbiont combinations varied across cropping systems, with distances into the fields, and were correlated with parasitoid wasp abundances.In the pot experiment, aphids hosted up to six symbionts. Flower presence increased natural enemy abundance and diversity, and decreased aphid abundance. The proportion of aphids hosting different symbiont combinations varied across the flower treatment and time, and were correlated with varying abundances of the different specialist parasitoid wasp species recruited by different flowers.Synthesis and applications. Floral plantings and flower identity can have community-wide impacts on the combinations of bacterial endosymbionts hosted by herbivorous insects. Our work highlights the potential of within-season selection for symbiont-mediated pest resistance to natural enemies with biological control impacts. This could be mitigated through increased recruitment of diverse natural enemies by incorporating functional diversity of floral resources into the environment.