Encounters between aphids and their predators: the relative frequencies of disturbance and consumption

Ecologists may wish to evaluate the potential for predators to suppress prey populations through the costs of induced defensive behaviors as well as through consumption. In this paper, we measure the ratio of non-consumptive, defense-inducing encounters relative to consumptive encounters (henceforth the 'disturbed : consumed ratio') for two species of aphids and propose that these disturbed : consumed ratios can help evaluate the potential for behaviorally mediated prey suppression. For the pea aphid, Acyrthosiphon pisum (Harris) (Homoptera: Aphididae), the ratio of induced disturbances to consumption events was high, 30 : 1. For the cotton aphid, Aphis gossypii (Glover) (Homoptera: Aphididae), the ratio of induced disturbances to consumption events was low, approximately 1 : 14. These results indicate that the potential for predators to suppress pea aphid populations through induction of defensive behaviors is high, whereas the potential for predators to suppress cotton aphid populations through induced behaviors is low. In measuring the disturbed : consumed ratios of two prey species, this paper makes two novel points: it highlights the variability of the disturbed : consumed ratio, and it offers a simple statistic to help ecologists draw connections between predator-prey behaviors and predator-prey population dynamics.