Bat Predation and Flight Timing of Winter Moths, Epirrita and Operophtera Species (Lepidoptera, Geometridae)

Abstract

In Scandinavia ca 15 species of geometrids including Epirrita spp. and Operophtera spp. typically emerge and mate in October-November or, in some of the species, in March-April. The males of these cold adapted moths have very low wing loadings (3-5 N m -2 ), while females of most species are flightless. We examined the hypothesis that these 'winter moths' achieve protection from insectivorous bats by temporal isolation. The moths were strictly nocturnal, being most active during the first hour after dark and on mild evenings (5-9°C), when aerial-hawking bats (Eptesicus nilssonii and Vespertilio murinus) were also active. The males flew slowly (ca 0.5 m s -1 ) along highly predictable flight paths. Flying males showed highly adaptive evasive responses to ultrasonic pulses. We conclude that the cold adapted geometrids are nocturnal, in contrast to many other geometrids, because their ears provide them with an energetically cheap mode of predator defence, which, however, only works against bats, i.e. during the night.