Recognising one's enemies: a functional approach to risk assessment by prey

Little has been done to compare the relative importance of various mechanis ms through which prey assess the potential risk from natural enemies. We us ed predator-naive spider mites (Tetranychus urticae, Tetranychidae) to (1) compare the responses of prey to chemical cues from enemy and non-enemy spe cies and (2) investigate the source of these cues. In the laboratory, we ob served the distribution of T. urticae in response to cues from nine mite sp ecies, including (1) predators of spider mites. (2) predators/parasites of other animals, and (3) fungivores/pollen-feeders. When given a choice over 24 h, spider mites foraged and oviposited in fewer numbers on leaf discs th at were previously exposed to predatory or parasitic mites (including speci es incapable of attacking spider mites) than on clean leaf discs (unexposed to mites). Interestingly, previous exposure of leaf arenas to fungivores a nd pollen-feeders had no significant effect on spider mite distribution. We then observed the response of T. urticae to cues from two species of preda tor that had been reared on a diet of either spider mites or pollen, ir: ur ticae showed stronger avoidance of leaf discs that were previously exposed to spider-mite-fed predators than of discs exposed to pollen-fed predators. Nevertheless, for one predator species (Amblyseius andersoni), T. urticae still preferred to forage and oviposit on clean (unexposed) discs than on d iscs exposed to pollen-fed predators. Protein-derived metabolic wastes of p redatory or parasitic mites may provide a general cue about potential preda tion risk for T. urticae. However, T. urticae also avoided areas exposed to pollen-fed predators, suggesting there may be other sources of enemy recog nition by the spider mites. We discuss the ecological and evolutionary mech anisms that may influence the scope of information through which animals as sess predation risk.