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 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 by creating a reservoir of aphids susceptible only to P. pequodorum.  Replicated cages were established with susceptible or H. defensa-protected aphids and the proportion of successful parasitism for each wasp species was estimated over time.