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Elevated volatile concentrations in high-nutrient plants: do insect herbivores pay a high price for good food?



1. A tritrophic perspective is fundamental for understanding the drivers of insect–plant interactions. While host plant traits can directly affect insect herbivore performance by either inhibiting or altering the nutritional benefits of consumption, they can also have an indirect effect on herbivores by influencing rates of predation or parasitism.

2. Enhancing soil nutrients available to trees of the genus Eucalyptus consistently modifies plant traits, typically improving the nutritional quality of the foliage for insect herbivores. We hypothesised that resulting increases in volatile essential oils could have an indirect negative effect on eucalypt-feeding herbivores by providing their natural enemies with stronger host/prey location cues.

3. Eucalyptus tereticornis Smith seedlings were grown under low- and high-nutrient conditions and the consequences for the release of volatile cues from damaged plants were examined. The influence of 1,8-cineole (the major volatile terpene in many Eucalyptus species) on rates of predation on model caterpillars in the field was then examined.

4. It was found that the emission of cineole increased significantly after damage (artificial or herbivore), but continued only when damage was sustained by herbivore feeding. Importantly, more cineole was emitted from high- than low-nutrient seedlings given an equivalent amount of damage. In the field, predation was significantly greater on model caterpillars baited with cineole than on unbaited models.

5. These findings are consistent with the hypothesis that any performance benefits insect herbivores derive from feeding on high-nutrient eucalypt foliage could be at least partially offset by an increased risk of predation or parasitism via increased emission of attractive volatiles.