Suppression of pea aphids by the biological control agent Aphidius ervi declines in the presence of the non-enemy Aphidius colemani

Predator/prey dynamics are traditionally studied from the perspective of consumptive effects, or the consumption of prey by predators. However, predators can also reduce prey abundance by causing prey to exhibit costly defensive behaviors, non-consumptive effects. For example, pea aphids, Acyrthosiphon pisum, drop from their host plant when threatened, leading to a decrease in fecundity and population size. Previous work demonstrated that the parasitoid wasp Aphidius colemani, which does not utilize pea aphids as a host, reduces pea aphid abundance by stimulating the dropping escape behavior. Here we examined whether this non-consumptive effect of A. colemani on pea aphids influences the effectiveness of the pea aphid biological control agent, Aphidius ervi. In greenhouse cages, we manipulated parasitoid presence in a factorial design (no-parasitoid control, A. colemani only, A. ervi only, or a mix of both species) and monitored aphid abundance and mummy formation. As expected, A. colemani failed to parasitize pea aphids, yet still reduced pea aphid population size relative to the no-parasitoid control. This non-consumptive effect of A. colemani on the pea aphid population led to an antagonistic interaction between the two parasitoid species with a more than five-fold drop in the rate of pea aphid parasitism by A. ervi when A. colemani was also present. Therefore, we found that a parasitoid that does not directly consume pea aphids, A. colemani, indirectly interferes with the effectiveness of the pea aphid biological control agent, A. ervi, by altering pea aphid behavior.