Complementary consumptive and non-consumptive effects of parasitoid wasps lead to consistent long-term suppression of pea aphid populations
Although predator/prey dynamics are traditionally described by consumptive effects, or who eats whom, natural enemies can also impact prey populations through non-consumptive effects. Non-consumptive effects result from defensive behaviors of prey, such as escaping to a refuge habitat to avoid the threat of predation. For example, pea aphids, Acyrthosiphon pisum, drop from their host plant when threatened. This defensive behavior helps pea aphids avoid predation, but is physiologically costly due to reduced feeding time. Although non-consumptive effects are commonly documented between predators and their prey, they can occur between any two organisms in a community, whether or not they consume one another. Therefore, non-enemy species can contribute to herbivore suppression. In a previous short-term study, we demonstrated that the non-enemy wasp Aphidius colemani, which does not consume pea aphids, reduces pea aphid populations by inducing the same defensive dropping response to the lethal wasp Aphidius ervi. Here we examined how interactions between the non-enemy A. colemani and lethal natural enemy A. ervi impact long-term pea aphid suppression. In large field cages, we manipulated parasitoid wasp presence in a factorial design and monitored pea aphid abundance and consumption by parasitoids over four weeks, or two parasitoid generations.
The non-enemy A. colemani suppressed pea aphid populations relative to the control that had no parasitoids present, confirming previous research that A. colemani suppresses pea aphid populations through non-consumptive effects. In a previous short-term study, we found that the non-enemy A. colemani interfered with pea aphid suppression by the natural enemy A. ervi. However, in our four week study, we saw more consistent suppression of pea aphids when both parasitoids were present, as compared to either the non-enemy A. colemani or natural enemy A. ervi alone. The consistent pea aphid suppression when both parasitoids are present is likely due to differences in development time of A. colemani and A. ervi, which result in temporally distinct adult emergences of each species. Therefore, the presence of both A. colemani and A. ervi together most successfully suppresses pea aphid populations through complementary consumptive and non-consumptive effects. From this study we determined that a more diverse community of parasitoids made up of enemies and non-enemies can more effectively and consistently suppress herbivore populations as compared to an enemy or non-enemy alone.