Spatially aggregated parasitism on pea aphids, Acyrthosiphon pisum , caused by random foraging behavior of the parasitoid Aphidius ervi

Abstract

We investigated the role of the foraging,behavior of the parasitoid wasp Aphidius erui in producing nonrandom spatial patterns of parasitism among pea aphids, Acyrthosiphon pisum. We measured spatial variability in percent parasitism by determining the number of aphids and percent parasitism in 40 sampling plots (0.65-m 2 circles) located within a homogeneous alfalfa field. In one replicate of this experiment, mean parasitism of aphids was 18.7%, and percent parasitism showed density-independent aggregation (i.e., greater than random variability in percent parasitism among sampling plots). In the other replicate, mean parasitism was 56.3%, and percent parasitism was not aggregated among plots. We used a combination of field observations of parasitoid foraging and mathematical models to explore these results. In particular, we asked whether the presence or absence of density-independent aggregation at different mean percent parasitism can be explained even if parasitoids forage randomly, without changing their behavior in response to encounters with aphids. Observations show that parasitoids tend to move short distances between nearby alfalfa stems (mean = 10.8 cm), and the turning angle between successively visited stems was uniformly distributed. We incorporated this behavior into both simulation and analytical models of parasitoid foraging. The models show the same pattern as that observed in the field: parasitism is aggregated in a density-independent fashion when mean percent parasitism is low but not when mean percent parasitism is high. Therefore, density-independent aggregation in percent parasitism does not necessarily imply behavioral responses of parasitoids to host encounters and previously parasitized hosts.