Evaluation of predation risk by a caterpillar using substrate-borne vibrations

Assessment of predation risk and appropriate defensive responses are critical for most organisms. Prey individuals must distinguish between the cues produced by abiotic factors, competitors and predators, and respond appropriately. We determined whether larvae of Semiothisa aemulataria (Geometridae), which defend themselves from invertebrate predators by hanging on a silk thread, can detect predator threats and distinguish between different predators, other herbivores and abiotic factors. Furthermore, we determined the mechanism used by the caterpillars to detect predator threats and the degree to which predation risk was specified. The hanging behaviour of S. aemulataria caterpillars occurred in response to invertebrate predators (wasps and stink bugs) but not in response to birds, other herbivores or abiotic factors. Caterpillars distinguish these predators by perceiving their substrate-borne vibrations. These vibrations differed both qualitatively and quantitatively. Caterpillars did not hang in response to mechanically reproduced vibration stimuli representing herbivores, but they did show this defensive behaviour in response to vibration stimuli that duplicated invertebrate predator signals. Caterpillars responded differently to predatory wasps and stink bugs. The length of the silk thread produced by caterpillars in response to foraging wasps was significantly greater than that produced in response to stink bugs. These predator-specific responses led to increased survival. Our results show that invertebrate prey are able to distinguish among the cues produced by abiotic factors, competitors and predators and respond specifically, in accordance with the potential risk posed by a predator.