Despite the large volume of research examining the role of tannins as an anti-herbivory mechanism for plants, there is still conflicting evidence regarding their effects on insect herbivores. Furthermore, their effects could be mitigated by other factors such as ecosystem engineering. Leaf-tying caterpillars on oak trees act as ecosystem engineers and can facilitate insect populations. We investigated the effect of two species of leaf-tiers, Pseudotelphusa sp. and Psilocorsis reflexella, on the intensity of herbivory on eight species of oaks by placing these caterpillars in artificial ties. We also examined the correlation of herbivory with foliar phenolics, including condensed tannin, hydrolyzable tannin, and total phenolics of the host plant, and their interactions with the leaf-tier treatment. We quantified herbivory intensity by measuring percent skeletonization of leaves collected from the trees at the end of the season.

Results/Conclusions The presence of leaf ties significantly increased skeletonization (by leaf-tying caterpillars) in the leaves. Leaf ties occupied by the larger of the two tiers (Psilocorsis reflexella) supported the highest rate of skeletonization. Overall, skeletonization was positively correlated with the concentrations of hydrolyzable tannin and total phenolics. There were two significant interactions between the leaf-tier treatments and tannin concentration on herbivory intensity. Skeletonization was negatively correlated with total phenolics concentration in leaf ties containing Pseudotelphusa caterpillars, and initially unoccupied artificial ties exhibited a negative correlation between skeletonization and condensed tannin concentration. Our results suggest that the ecosystem engineering effect of the leaf-tying caterpillars modifies the response of insect herbivores to foliar phenolics. Because total herbivory intensity was not negatively associated with the concentration of any of the phenolics, our findings do not support their role as an anti-herbivory agent.