Parasitoid Aggregation and the Stabilization of a Salt Marsh Host-Parasitoid System

We examine a salt marsh host-parasitoid system, consisting of the planthop- per Prokelisia marginate and its egg parasitoid Anagrus delicatus, for evidence of stabilizing parasitoid behavior. We first determine if there is sufficient parasitoid aggregation to po- tentially stabilize the Prokelisia-Anagrus interaction, using methods that infer parasitoid behavior from the spatial pattern of parasitism. We then test a basic assumption implicit in theory, that the degree of parasitoid aggregation is invariant with respect to changes in overall host and parasitoid density, and through time. Our results suggest that Anagrus behavior should, in theory, exert a consistent stabilizing effect on Prokelisia-Anagrus dy- namics. Using field experiments, and direct sampling of the spatial distribution of parasitoids, we then try to determine if parasitoid aggregation is in fact responsible for the heterogeneous pattern of parasitism observed in the field. The results of these studies indicate that there is insufficient parasitoid aggregation to account for this heterogeneity. Instead, factors such as the degree of tidal inundation, or the age of host eggs, may cause host patches to differ in their vulnerability to parasitoid attack.