An increased risk of parasitism mediated by the facultative symbiont Regiella insecticola

Previous data showed that the parasitoid wasp Aphelinus asychis was more successful on Sitobion avenae clones artificially infected with different strains of Regiella insecticola compared to the same clones lacking facultative symbionts. To test whether this resulted from a specific interaction between the aphid and symbiont genotype, we used new clonal lines from a different genotype of S. avenae (Linyi) infected with the same R. insecticola strains. The parasitism rate of A. asychis was higher on the two Linyi lines infected with R. insecticola than on the uninfected control line, while the emergence rate was lower due to higher mortality during development. However, a different wasp species, Aphidius gifuensis, showed no difference in the parasitism rate and emergence rate between the three Linyi lines, suggesting that the increase in parasitism rate could be wasp species-dependent. The mortality of A. asychis during development in the presence of R. insecticola may be linked to (i) a direct toxic or metabolic effect since the weight of the emerged wasps was also reduced and (ii) a general effect on the aphid fitness since the survival and fecundity of infected aphids having survived A. asychis attack were clearly reduced compared to the control, while those surviving A. gifuensis attack had only a slight increase in their survival rate. Our data therefore enriched the panel of phenotypic effects that R. insecticola could contribute to the aphid and emphasized the potential implications of symbionts on biological pest control.