Patch assessment in the parasitoid Diaeretiella rapae MacIntosh (Hymenoptera: Aphidiidac)

Foraging parasitoids that use kairomones for patch assessment may be expected to leave a patch according to a count-down exploitation mechanism. Such a mechanism makes two basic predictions: (1) a kairomone should have a positive effect on search time, and (2) attacks of unattacked hosts may have a negative effect on search time. The parasitoid Diaeretiella rapae uses honeydew emitted from its principal host, the cabbage aphid Brevicoryne brassicae, to locate the host colony. It has been suggested that D. rapae uses a count-down exploitation mechanism to determine patch-residence time. I examined the relationship between the number of aphids attacked by D. rapae and aphid colony size over four different honeydew amounts. For each honeydew amount, the number of aphids attacked increased to a certain colony size, and then leveled off in spite of the increase in the number of aphids. The results suggest that D. rapae leaves the colony based on the predictions of a count-down exploitation mechanism, which maximizes the benefit-to-cost ratio.