Out on a limb: Are insect herbivores at greater risk of predation on young foliage?

Background/Question/Methods

Herbivorous insects often exhibit a preference for young foliage relative to old foliage. This preference is generally explained by higher nitrogen and water concentrations and reduced fibre and toughness in young foliage. Although feeding on young foliage often enhances performance, it may also increase predation risk for herbivores by influencing the probability of detection by natural enemies. For example, young foliage typically grows at the ends of branches where herbivores are likely to be more apparent to visual predators. Additionally, young foliage can contain greater concentrations of volatiles which, when released in response to herbivore feeding, may act as host/prey location cues. We tested the hypothesis that herbivores on young foliage are at greater risk of predation than those on old foliage by assessing attack rates on caterpillars made from modelling clay that were associated with either young or old Eucalyptus foliage. We also identified the predators responsible for attacks on model caterpillars as bird, mammal or arthropod to consider whether attack rates on young and old foliage differed according to predator type.

Results/Conclusions

Overall, 32 % of the model caterpillars showed evidence of predation, with slightly higher attack rates on caterpillars associated with old compared to young foliage. Foliage age did not influence the probability of attack overall, however, investigation of predator specific attack rates revealed that model caterpillars associated with old foliage were significantly more susceptible to predation by birds. This suggests that herbivores associated with young eucalypt foliage may not be at increased risk of predation and may even experience reduced predation risk if their main predators are birds. Current experiments using various predator exclusion treatments will compare the survival and growth of actual caterpillars placed on young or old eucalypt foliage in the field. This will not only take into account any influence of differences in the plant volatiles released from young and old foliage in response to herbivore feeding, but will also indicate whether there exists a trade-off for herbivores between maximising performance and minimising predation risk when utilising foliage of different age.