A fox in sheep's clothing: Furtive predators benefit from the communal defense of their prey

Many animals live in temporary or permanent groups. either as gregarious or social species, to reduce predation risk. The solitary midge, Aphidoletes aphidimyza, preys specifically on aphids and spends its entire larval devel opment within a prey colony where it is susceptible to intraguild predation . We hypothesized that midge larvae profit from a dilution effect produced by aphids which enhances their chances of survival. We examined the defensi ve behaviors of aphids in response to foraging midges, and investigated the effect of aphid density on the predation risk of A. aphidimyza by the lace wing Chrysoperla rufilabris. We found that a foraging midge displays furtiv e hunting behavior which triggers little defensive reaction by aphids, and does not stimulate significant disturbance of the gregarious prey. Within t he aphid colony the midge benefits from a dilution effect which reduces the incidence of predation by lacewing larvae. However, the effectiveness of s uch a mechanism is determined by the level of disturbance caused by foragin g intraguild predators. Large lacewing larvae tend to dislodge aphids from their feeding sites, thereby eliminating the dilution effect. We conclude t hat prey not only provide food to midge larvae, but also protection against natural enemies.