Mercury vapour lamps interfere with the bat defence of tympanate moths (Operophteraspp.; Geometridae)

Abstract

Bats often forage near streetlamps, where they catch moths in particular. At least two hypotheses may explain the apparent increase in the availability of moths to bats feeding around streetlamps: (1) the moths become concentrated near the light and therefore more profitable to exploit; and (2) the light interferes with the moths' evasive flight behaviour. We tested the second of these hypotheses by exposing flying male winter moths,Operophteraspp., to bursts of ultrasound (26kHz, 110dB sound pressure level) from an electronic source. The light from a 125W mercury vapour lamp had a quantitative effect on the moths’ evasive flight response at close range (within ca4m), inhibiting it totally in nearly half (43%,N=125) of the cases. By contrast, moths flying in the surrounding woodland and without interference from the lamp always responded to the sound. Streetlamps of the mercury vapour type (white lamps) thus interfere with the defensive behaviour of moths and presumably increase their vulnerability to echolocating bats. This may have implications for the conservation of both moths and bats.