Working together for a common goal: how complementary consumptive and non-consumptive effects of parasitoid wasps suppress pea aphid populations

Predator/prey dynamics describe changes in prey population size from predator consumption. However, predators also impact prey populations through non-consumptive effects. For example, pea aphids (Acyrthosiphon pisum) drop from their host plant to escape predation, leading to decreases in fecundity and population size due to lost feeding time. Because non-consumptive effects do not involve consumption, they can occur between any organisms in a community, whether or not they consume one another. Our goal is to understand how a non-enemy wasp Aphidius colemani affects pea aphid suppression by the lethal enemy wasp Aphidius ervi. In a previous short-term greenhouse study, we found evidence of antagonism between the enemy and non-enemy wasps. Here we aim to determine how these interactions impact long-term pea aphid suppression in the field. In large cages, we manipulated the presence of the two wasps in a factorial design and monitored pea aphid abundance over four weeks, or two wasp generations. The non-enemy wasp alone suppressed pea aphid populations through non-consumptive effects such as inducing the defensive dropping behavior. However, the strongest and most consistent pea aphid suppression occurred when both wasps were present. This is likely due to temporally distinct adult emergences of the two parasitoid species, resulting in temporal niche partitioning of consumptive and non-consumptive effects on pea aphid populations. We found that a diverse community of wasps containing enemies and non-enemies most consistently suppresses herbivore populations. Therefore, it may be most effective to promote diverse predator communities of lethal enemies and non-enemies when controlling pest populations.