It is safe to go out on a limb: within‐tree leaf age and location do not alter predation risk for insect herbivores

Abstract

AbstractAnimals must trade off foraging gains against the risk of being preyed upon. For insect herbivores, feeding on young foliage typically enhances nutritional performance, but it may also increase risk of death. Young foliage typically grows at the ends of branches and contains higher volatile terpene concentrations than old foliage. Together, these characteristics may make herbivores more chemically and visually apparent as they eat young foliage, and hence expose them to greater predation risk from natural enemies. We aimed to test this prediction by conducting two field experiments. First, we compared survival of Doratifera casta Scott (black slug cup moth) caterpillars on young and old Eucalyptus foliage using various predator exclusion treatments. Second, we assessed predation rates on plasticine model caterpillars associated with either young or old Eucalyptus foliage. The second aim of our study was to test the validity of using model caterpillars as surrogates for real caterpillars in predation studies. We found that neither caterpillar survival nor attack rates on models were influenced by foliage age, providing no evidence for differential predation risk. Both approaches, however, indicated that crawling arthropods were the most important predator type in this system. Caterpillar survival tended to increase more by excluding crawling invertebrates than by excluding vertebrates; arthropods were responsible for the majority of attacks on the model caterpillars. Our results indicate no conflict between nutrient acquisition and predation risk for caterpillars feeding on young eucalypt foliage. Importantly, the consistency in responses from both our experimental approaches is compelling support for using model caterpillars as surrogates for real ones when studying predation risk.