Light suppresses the acoustically prompted predator-avoidance behaviour in moths

Urbanization exposes wild animals to increased levels of light, affecting particularly nocturnal animals. Artificial light at night might shift the balance of predator-prey interactions, for example of nocturnal echolocating bats and eared moths. Moths exposed to light show less last-ditch manoeuvres in response to attacking close-by bats. In contrast, the extent to which negative phonotaxis, moths first line of defence against distant bats, is affected by light is unclear. Here, we aimed to quantify the overall effect of light on both types of sound-evoked anti-predator flight, last-ditch manoeuvres and negative phonotaxis. We caught moths at two light traps, which were alternately equipped with loudspeakers that presented ultrasonic playbacks to simulate hunting bats. The light field was omnidirectional to attract moths equally from all directions. In contrast, the sound field was directional and thus, depending on the moths approach direction, elicited either only negative phonotaxis, or negative phonotaxis and last-ditch manoeuvres. We did not observe an effect of sound playback on the number of caught moths, suggesting that light might suppress both types of anti-predator flight, as either type would have caused a decline in the number of caught moths. As control, we confirmed that our playback was able to elicit evasive flight in moths in a dark flight room. Showing no effect of a treatment, however, is difficult. We discuss potential alternative explanations for our results, and call for further studies to investigate how light interferes with animal behaviour.