Can Defensive Symbionts Change the Composition of Natural Enemies Attacking the Pea Aphid?

Inherited facultative symbionts commonly infect herbivorous insects and can mediate their interactions with food plants and natural enemies. A well-studied example of defensive symbiosis is found in the pea aphid, Acyrthosiphon pisum, where the bacterial symbiont Hamiltonella defensa confers high levels of protection against the aphid’s most prevalent parasitoid Aphidius ervi.  Recent work, however, shows that H. defensa confers no protection against a related, but less common parasitoid Praon pequodorum.  A. ervi is a dominant external competitor and following its introduction to the USA to control A. pisum has completely displaced all other parasitoids except the native P. pequodorum. Here, we conducted population cage experiments to determine if defensive symbionts (H. defensa) can influence the composition of enemies attacking the pea aphid. Replicated cages were established with susceptible or H. defensa-protected aphids (same aphid genotype) and the proportion of successful parasitism for each wasp species was estimated over time.  In the cages with susceptible aphids, A. ervi outcompeted P.pequodorum, but in cages with symbiont-protected aphids, the native wasp P. pequodorum outperformed A. ervi. This finding shows that protective symbionts can create a reservoir of hosts susceptible only to particular enemies and thereby influence the composition of enemies using this host.