Behavioral response of Spodoptera exigua under bat echolocation call stress

Abstract

Predation risk can shape prey behavior and limit population density. The ultrasonic sound emitted by bats creates a soundscape of fear which can affect moth trait expression and population dynamics via non-consumptive effects. Bat echolocation calls can be categorized as constant frequency (CF), frequency modulation (FM), or some combination of both (FM-CF-FM or FM-CF). Different sound characteristics may encode different levels of predation risk for moths and thus result in variable non-consumptive effects. To investigate the non-consumptive effects on moths of different types of bats echolocation calls, we played back three types of calls with distinct characteristics to an agricultural pest species, the beet armyworm, Spodoptera exigua, two CF calls from bats that prey on the moth, and one FM call from a bat that is sympatric with the moth and represents the echolocation sound typical of most insect-eating bats. Our results showed that the FM echolocation calls of the Oriental serotine bat, Eptesicus pachyomus, negatively affected the reproductive behavior of adult S. exigua, and altered larval energy metabolism and consumption. Calls of the big-eared horseshoe bat, Rhinolophus macrotis, with a dominant frequency of 58 kHz, only affected the reproductive behavior of adult S. exigua, whereas calls of the least horseshoe bat, Rhinolophus pusillus, with a dominant frequency of 110 kHz, had no measurable effects on S. exigua. The non-consumptive effects observed included reduced activity frequency, reduced food intake, and increased body weight of the moth in response to the soundscape of fear created by bats. Overall, FM calls with frequency within the moth's hearing range had greater effects on the moths. Our findings indicate that bat calls detectable by moths, whether or not the bats actually hunt them, create a soundscape of fear with negative impacts on moth behavior and life history, and likely make a valuable contribution to pest control.