Do furtive predators benefit from a selfish herd effect by living within their prey colony?

Abstract

Protection from predators is a recognized benefit of group living for animals. The selfish herd effect implies that individuals in a central position within the group suffer lower predation than individuals at the edges of the group. The midge, Aphidoletes aphidimyza, is a furtive predator that lives within aphid colonies where it benefits from a dilution effect that reduces the incidence of intraguild predation. We hypothesized that the selfish herd effect also contributes to midge survival. In laboratory experiments, we examined the distribution of midge larvae within aphid (Macrosiphum euphorbiae) colonies and determined the susceptibility of central and peripheral individuals to lacewing, Chrysoperla rufilabris, and coccinellid, Coleomegilla maculata larvae, two common intraguild predators. The probability of observing a midge in the central zone of an aphid colony was higher than predicted after a 24-h period. During predation tests, midges and aphids in the peripheral zone were attacked first significantly more often than prey in the central zone of the colony. However, because foraging coccinellid and lacewing larvae rapidly disrupt the cohesion among aphids in the colony, midge mortality was subsequently similar in both zones. We conclude that A. aphidimyza preferentially choose the central position of their aphid prey colonies. Such a preference reduces the probability of becoming the first prey attacked by intraguild predators. The effectiveness of the selfish herd for slow-moving midge larvae would depend on the nature and foraging behavior of the predator, as well as on cohesion of the aphid colony.