Many heads are better than one: Head capsule stacking alters feeding behaviour in gum-leaf skeletonizers

The fate of most insect herbivores in the field is to end up as prey for one of the many predators encountered while feeding on plants.  The enormous predation pressure these animals face has been the selective driver of a evolution of vast array of defensive strategies in herbivorous insects, integrating chemical, behavioural and morphological defences.  We investigated how the practice of head capsule stacking in gum-leaf skeletonizers, Uraba lugens (Lepidoptera:Nolidae) influenced their mortality in the field, behavioural responses to predation risk and feeding behaviour.  This practice, believed to have an anti-predator function, is a result of larvae retaining head capsules after molting. Capsules build upon each other with each molt, so that larvae end up with a vertical tower of 5 to 7 stacked head capsules on their heads.  Larvae that had their stacks removed suffered significantly higher levels of predation in the field than those retaining their head capsules.  We also found that larvae exposed to simulated predation adopted cautious behaviours, being slower to initiate feeding bouts, choosing the feed in shorter intervals and undertaking fewer feeding bouts during feeding 3hr trials. These differences were only observed in the short term, as larvae adopted normal feeding behaviours 24hr after simulated attacks were applied. Finally, larvae who had their stacks removed adopted feeding behaviours similar to those used by larvae exposed to failed predation attacks. The behavioural strategies adopted by larvae upon the loss of their morphological adornments supports the putative defensive function of head-capsule stacking, identifying a novel defence in larval Lepidoptera.