Low autumn activity of bats results in small potential of pest reduction

Abstract In recent years, in combination with climate change and dry summers, geometrids have caused damage to economically important forest trees, particularly oak. We focused on their co-occurring predators, the forest-dwelling bats, and their potential to reduce the number of pest geometrids. To quantify the aggregation response of bats, we used acoustic monitoring of flight activity, availability of moth imagos, and genetic analysis of their faeces. Our results show that even in the presence of massive outbreaks of geometrids, bat foraging activity decreases as winter approaches. Furthermore, our results show that for those bats that were active, the overabundant geometrids were not their main prey. Drymonia ruficornis, Conistra vaccinii and Conistra erythrocephala (Noctuidae) were foraged as often or even more often than overabundant geometrids. Our results show that opportunistic behaviour of temperate bats is not evident at lower temperatures and that reasons other than abundance and accessibility of prey determine their prey preferences. Therefore, season and predator preferences may significantly affect the ecological services provided by bats.