The outcome of competition between two parasitoid species is influenced by a facultative symbiont of their aphid host

1.Symbiotic bacteria can act to protect their host against natural enemies. Where this protection is asymmetric against different natural enemies protection conferred by symbionts has the potential to mediate interactions between natural enemies, as well as between enemies and the host. 2.In pea aphids (Acyrthosiphon pisum), resistance against parasitoid wasps can be conferred by facultative symbiotic bacteria. We investigated whether the outcome of competition between two parasitoid species can be influenced by the presence of a defensive symbiont in the host. 3.We exposed pea aphids from a single clonal line, with and without a strain of the protective endosymbiont Hamiltonella defensa, to multiparasitism by the parasitoid wasps Aphelinus abdominalis (Aphelinidae) and Aphidius ervi (Braconidae), and recorded the outcome. The symbiont strain is known to impact A. abdominalis more strongly than A. ervi. 4.We found that the presence of a strain of the protective endosymbiont Hamiltonella defensa can reverse the outcome of competition between the wasps. In the absence of the symbiont, A. ervi gains very little success when attacking an aphid previously parasitized by A. abdominalis. However, where the aphids possessed the symbiont, A. abdominalis did not develop successfully, and the success rate of A. ervi was significantly increased. 5.Our results show that defensive facultative symbionts are able to influence community interactions at the trophic level above their host