Do diverse endosymbiont compositions affect parasitism rates of different parasitoids?

Defensive symbionts confer various degrees of resistance against parasitism by different natural enemies. For example, pea aphids (Acyrthosiphon pisum) sometimes harbor endosymbiotic bacteria that confer resistance against the parasitoid wasp Aphidius ervi (Aphidius), but not a second parasitoid, Praon pequodorum (Praon).  However, it is unclear how these parasitoids respond to a mix of aphids with variable resistance. We investigated the responses of Aphidius, Praon, and both foraging simultaneously to 3 different populations of aphids: all infected with defensive symbionts, all uninfected, or a 50:50 mix. When both by itself and with the other species, we compared parasitism of each wasp observed in the aphid mix to the parasitism predicted by the average observed across the two uniform treatments. When foraging singly, parasitism by Aphidius in the mixed treatment did not differ from predicted parasitism. However, when foraging alongside Praon, parasitism by Aphidius for mixed aphids was significantly less than predicted from pure populations. Both singly and alongside Aphidius, aphid symbiont composition had no effect on parasitism by Praon. These results suggest that the presence of defended aphids and a competitor have synergistically negative effects on parasitism by Aphidius, whereas Praon is not affected by the endosymbiont composition of the aphid colony it attacks. The reduction in Aphidius’s parasitism of mixed aphids in the presence of Praon, but not in its absence, suggests thata competitor strengthens Aphidius’s response to resistant aphids among the host population.