The defensive potential of the bacterial endosymbiont Arsenophonus in the soybean aphid

Aphids commonly harbor bacterial facultative symbionts in addition to their primary symbiont, Buchnera aphidcola. Facultative symbionts have a variety of effects upon their aphid hosts, including defense against Hymenopteran parasitism. The soybean aphid, Aphis glycines, is infected with the symbiont Arsenophonus, which has unknown effects on its aphid host. A previously established correlation between Arsenophonus infection and rates of parasitism in another taxa suggests a defensive role for the symbiont. To address this defensive hypothesis, we cured soybean aphid of its natural Arsenophonus infection through ampicillin microinjection, resulting in infected and uninfected isolines within the same genetic background. These isolines were subjected to parasitoid assays using a recently introduced biological control agent, Binodoxys communis [Braconidae], and a naturally recruited parasitoid, Aphelinus certus [Aphelinidae]. We performed whole-plant assays in which individual parasitoids were given 24h access to 50 aphids. We also performed observational assays of parasitism. We did not find a difference in successful parasitism for either parasitoid species using either assay. Our tentative conclusion is that Arsenophonus does not provide parasitism defense to soybean aphid, but further studies with other parasitoid species, different symbiont accessions, and different ambient conditions are ongoing. We will also examine other potential benefits of infection to better understand how this symbiont affects the ecology and population dynamics of this important pest.