Can predators assess the quality of their prey’s resource?

Predator–prey interactions play a substantial role in shaping spatial distributions of organisms in biological communities. Recent theory has considered optimal habitat selection strategies for predators and prey that interact in real time. One consistent prediction across many theoretical studies is that it could be adaptive for predators to assess and respond to the quality of resources consumed by their prey (resources the predators do not consume). However, extremely few empirical studies have tested this prediction. We utilized a tritrophic system in which seven-spotted lady beetles (Coccinella septempunctata L., Coleoptera: Coccinellidae), pea aphids (Acyrthosiphon pisum (Harris), Hemiptera: Aphididae) and tic bean plants (Vicia faba L., Fabaceae) were, respectively, the predators, prey and prey’s resource. In microcosms, we created patches of varying quality (from the perspective of the prey) among which predators and prey were allowed to move freely. By utilizing treatments in which there were (1) prey but no predators, (2) predators but no prey and (3) predators and prey simultaneously, we determined which sources of information influenced movement and habitat selection. Predators were able to detect and respond to the quality per se of their prey’s resource, and they preferred the highest-quality patch regardless of the presence of prey. While theory has frequently predicted such behaviour, direct responses of predators to resource quality (rather than resource amounts, resource damage, or indirect cues of prey) have not previously been demonstrated. These results substantiate general predictions of ecological theory and have implications for predicting the behaviour of biological control agents.