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

We investigated the role of the foraging behavior of the parasitoid wasp Ap hidius ervi in producing nonrandom spatial patterns of parasitism among pea aphids, Aeyrthosiphon pisum. We measured spatial variability in percent pa rasitism by determining the number of aphids and percent parasitism in 10 s ampling plots (0.65-m(2) circles) located within a homogeneous alfalfa fiel d. In one replicate of this experiment, mean parasitism of aphids was 18.7% , and percent parasitism showed density-independent aggregation (i.e., grea ter 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 p articular, we asked whether the presence or absence of density-independent aggregation at different mean percent parasitism can be explained even if p arasitoids forage randomly, without changing their behavior in response to encounters with aphids. Observations show that parasitoids tend to move sho rt distances between nearby alfalfa stems (mean = 10.8 cm), and the turning angle between successively visited stems was uniformly distributed. We inc orporated this behavior into both simulation and analytical models of paras itoid foraging. The models show the same pattern as that observed in the fi eld: parasitism is aggregated in a density-independent fashion when mean pe rcent parasitism is low but not when mean percent parasitism is high. There fore, density-independent aggregation in percent parasitism does not necess arily imply behavioral responses of parasitoids to host encounters and prev iously parasitized hosts.