Ground-level predation on artificial caterpillars indicates no enemy-free time for lepidopteran larvae

Lepidoptera is one of the most diverse orders of insects, their larvae are very abundant in many habitats, and frequent prey of various predators. To decrease predation risk, caterpillars developed several means of defence, among them timing their activity to avoid predators (seeking enemy-free time). Although the enemy-free time hypothesis is often invoked to explain caterpillar behaviour, empirical evidence for it is scarce. We tested whether such enemy-free time exists in a temperate forest by comparing predation pressure on artificial caterpillars during day and night on the ground in forest fragments in Denmark. We found a high predation rate, 23.9% d(-1), and higher predation rate at night (30.9% d(-0.5)) than during the day (17.0% d(-0.5)), both by invertebrate (23.3% d(-0.5) vs. 12.4% d(-0.5)) and vertebrate (8.5% d(-0.5) vs. 3.3% d(-0.5)) predators. The most important predators were chewing insects (73.4% of all attacks) and small mammals (19.0%). Attack rates on red caterpillars were higher (30.0% d(-1)) than on green ones (19.5% d(-1)). Overall, these data do not support the idea that night activity can provide enemy-free time for solitary caterpillars on the temperate forest floor.