Parasitoid Response to Concentration of Herbivore Food Plants: Finding and Leaving Plants

We hypothesized that a parasitoid of an herbivore would be attracted to large rather than small patches of plants that provide food for the herbivore. We conducted experiments to determine whether such parasitoids would be more likely to find and/or less likely to leave large patches of their victims' host plants. We also examined whether prey density (independent of patch size) affects plant finding or leaving. These aspects of plant foraging behavior were examined for Diaeretiella rapae (Hymenoptera: Aphidiidae), a parasitoid wasp that attacks aphids primarily on cruciferous plants. Arrival rates of naturally occurring D. rapae in artificial patches of 5, 10, 20, and 40 potted collard plants were measured in a field experiment. There was no patch-size effect (plants in larger patches were not more likely to be found), and plants with aphids were not more likely to be found than aphid-free plants. Furthermore, there was no dosage response to water bowl traps baited with chemical plant attractant placed in the same field. Leaving rates were measured in a greenhouse experiment using dispersal cages containing 1, 4, or 16 plants and 0, 10, or 100 aphids. Leaving rate decreased in larger patches independently of aphid density, which also affected leaving rate. We conclude that plant leaving may be equally or more important than plant finding in determining abundance of D. rapae in patches of crucifers. The significance of this result is discussed.