Aggregation in Aphis Varians: An Effective Strategy for Reducing Predation Risk

It has been suggested that by clumping together, even defenseless prey may reduce their chances of being consumed by a predator. However, the advantage of clumping to prey may be negated by a strong aggregative response of predators to prey clusters. We studied the balance of forces that determine the advantage or disadvantage of clumping to prey in one particular case: the interaction between the aphid Aphis varians (Homoptera: Aphididae) and its predator Hippodamia convergens (Coleoptera:Coccinellidae). Aphid movement was congregative, i.e., aphids tended to stay within clumps of conspecifics. Furthermore, by quantifying the per capita population growth rates for aphid colonies of differing sizes, we found that individual aphids benefited by being clumped together. However, a predator—exclusion experiment indicated that clumping was beneficial to aphids only in the presence of predators. To gain a mechanistic understanding of why aphid congregation reduced predation risk to individuals, we examined the behavioral response of predators to aphid density. Predators exhibited a strong aggregating response: large colonies of aphids were attacked by more predators than were small colonies. In addition, ladybird feeding rate increased with increasing colony size. The total number of prey taken from colonies by predators, however, could not keep pace with the increase in aphid colony size, and thus the risk of predation to individual prey was "diluted" by prey numbers.