The induction of phenolic glycosides in Populus tremuloides by the aspen leaf miner

Quaking aspen (Populus tremuloides) produces several defenses against herbivory, including phenolic glycosides and extrafloral nectaries (EFNs), which occur on only a subset of leaves.  The Interior of Alaska is currently experiencing an outbreak of the aspen leaf miner (Phyllocnistis populiella, Lepidoptera: Gracilliadae) which mines the epidermal leaf tissues. In this study we investigated a) whether mining damage causes induction of phenolic glycosides in P. tremuloides and b) whether leaves with and without EFNs differ in chemical defense.  Using an insecticide, we experimentally reduced leaf miner damage on replicate ramets within a single putative genet.  Two weeks after insecticide application, unsprayed trees were heavily-mined relative to sprayed trees.  Concentrations of the phenolic glycosides tremulacian and salicortin in mined leaves were significantly higher than those of sprayed leaves, indicating that epidermal mining induces phenolic glycoside production.  Leaves with EFNs had significantly higher phenolic glycoside concentrations than leaves lacking EFNs.  There was no difference in the ability of leaves with and without EFNs to induce phenolics in response to mining.  Several species of chewing insects have been shown to induce phenolic glycosides however; this is the first such study to show induction in response to epidermal leaf mining. Our results suggest aspen leaves within aspen trees vary considerably in the degree to which they are defended: a subset of leaves contain both high concentrations of chemical defenses and EFNs, which attract predacious hymenopterans.