The non-consumptive effects of a parasitoid wasp on pea aphids interfere with suppression by natural enemies

Background/Question/Methods

Predator/prey and parasitoid/host dynamics are traditionally described by consumptive effects, or who eats whom. However, natural enemies can also impact prey populations through non-consumptive effects. Non-consumptive effects include defensive behaviors of prey, such as escaping to a refuge habitat in response to a threat of predation. These defensive behaviors help prey avoid being eaten, but they can be physiologically costly.  For example, pea aphids, Acyrthosiphon pisum, drop from their host plant when threatened, which stops a feeding bout. This behavior will eventually lead to decreases in fecundity and overall population size. Previous work demonstrated that the parasitoid wasp Aphidius colemani, which does not consume pea aphids, can reduce pea aphid abundance by inducing the same defensive dropping behavior as the natural enemy Aphidius ervi. Here we examined whether this non-consumptive effect of A. colemani on pea aphids influences the suppression of pea aphids by their actual enemy, A. 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 pea aphid abundance and consumption by parasitoids.

Results/Conclusions

As expected, A. colemani failed to consume pea aphids, yet still reduced pea aphid population size relative to the no-parasitoid control. A. ervi suppressed pea aphids to a much greater degree than A. colemani, but through a combination of both consumptive and non-consumptive effects. Aphid population size when A. ervi was alone was not different than when both A. ervi and A. colemani were present. Despite no difference in the final pea aphid population size, A. ervi consumed four times as many pea aphids when alone as opposed to when A. colemani was also present. Since each consumed aphid results in one new parasitoid adult, the reduction in aphid consumption by parasitoids in the presence of A. colemani would translate into a four-fold reduction in the size of the parasitoid population in the next generation. Therefore, we found that a parasitoid that does not directly consume pea aphids, A. colemani, indirectly interferes with the effectiveness of the pea aphid natural enemy, A. ervi, by likely altering pea aphid behavior.