Ignoring the irrelevant: auditory tolerance of audible but innocuous sounds in the bat-detecting ears of moths
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Noctuid moths listen for the echolocation calls of hunting bats and respond to these predator cues with evasive flight. The African bollworm moth, Helicoverpa armigera, feeds at flowers near intensely singing cicadas, Platypleura capensis, yet does not avoid them. We determined that the moth can hear the cicada by observing that both of its auditory receptors (A1 and A2 cells) respond to the cicada’s song. The firing response of the A1 cell rapidly adapts to the song and develops spike periods in less than a second that are in excess of those reported to elicit avoidance flight to bats in earlier studies. The possibility also exists that for at least part of the day, sensory input in the form of olfaction or vision overrides the moth’s auditory responses. While auditory tolerance appears to allow H. armigera to exploit a food resource in close proximity to acoustic interference, it may render their hearing defence ineffective and make them vulnerable to predation by bats during the evening when cicadas continue to sing. Our study describes the first field observation of an eared insect ignoring audible but innocuous sounds.
KeywordsMoths Auditory ecology Bats Cicadas Discrimination
The authors wish to thank the management of De Hoop Nature Reserve for their hospitality, Michelle Venance for her help in the field and the anonymous reviewers whose suggestions greatly improved the paper. Peter Wall developed the MATLAB sound-generating and spike analysis applications. The study was funded by a Natural Sciences and Engineering Council of Canada Discovery Grant (JHF) and by the National Research Foundation (GUN 2053611) of South Africa (DSJ) and the University Research Committee of the University of Cape Town (DSJ). All experiments performed complied with the current laws of South Africa.
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