Dispersal-dependent oviposition and the aggregation of parasitism

The prediction that parasitoid foraging effort should increase with distance traversed to reach or to locate hosts has had little experimental attention. Consistent with a number of models of foraging behavior, we found that the per capita number of ovipositions by the minute fairyfly-egg parasitoid Anagrus sophiae increased significantly with dispersal distance to planthopper hosts in the field in experimental patches of many host eggs. In large continuous stands of cordgrass host plants, after dispersal of decimeters or less, female wasps laid approximately 18% of their average of 18.6 eggs. After dispersal to plants isolated 10 m from other cordgrass, they laid approximately 84%, and they laid virtually all of their eggs after dispersal of 250 m to experimental floating islands of cordgrass. The increased oviposition following dispersal tripled the CV2 index of aggregation of parasitism to a level theoretically sufficient to promote locally stable parasitoid-host dynamics in isolated patches. At the same time, the change in wasp behavior did not affect the relationship between parasitism and host density, which was consistently density independent. Our results suggest that increased foraging effort with distance traversed can counter Allee effects in colonization and increase spatial spread of populations of natural enemies.