Larval defenses protect slug caterpillars from multiple generalist predators but may increase their risk of parasitoid attack

Predators have played a significant role in the evolution of herbivorous insects and we can observe a wide variety of larval defense mechanisms in nature. Slug caterpillars (Limacodidae) are known for their unusual morphologies, including stinging spines, which suggest that their evolution has been shaped by their interactions with predators. We tested the hypothesis that spiny larvae (with stinging spines) would suffer less predation from generalist predators than un-spiny larvae. In a series of bioassays, we tested the preferences of a suite of invertebrate predators and found that all of the predators preferred un-spiny prey species over spiny limacodid species. Although stinging spines may protect herbivorous larvae from generalist predators, we predicted that parasitoids might actually target these physically-defended larvae. Parasitoid larvae may actually benefit from the physical defenses of their herbivorous host because these defenses, while no longer offering a fitness advantage to the parasitized herbivore, may still protect the parasitoid larva from generalist predators. We analyzed four years of data on the parasitism rates of limacodid larvae in the field and found that spiny larvae are significantly more likely to be parasitized than non-spiny larvae. These results support the hypothesis that parasitoids may select larval hosts that offer enemy-free space from their own natural enemies.