Phytochemical Variation in Quaking Aspen: Effects on Gypsy Moth Susceptibility to Nuclear Polyhedrosis Virus

The performance of gypsy moths (Lymantria dispar) feeding on quaking aspen (Populus tremuloides) is strongly influenced by host foliar chemistry and susceptibility to a nuclear polyhedrosis virus (LdNPV), but the relationship of susceptibility to chemistry is poorly understood. We investigated the effects of genetic and resource-mediated variation in phytochemistry on viral pathogenicity. Trees were grown in pots in a common garden. Disks were punched from aspen leaves, inoculated with LdNPV and fed to third instars. Additional leaves were analyzed for levels of nitrogen, starch, phenolic glycosides, and condensed tannins. Despite marked variation among trees in levels of phenolic glycosides and tannins, we observed minimal variation in larval susceptibility to LdNPV. Viral pathogenicity was only weakly (inversely) correlated with tannin concentrations in one of two experiments. These results suggest that differential defoliation of aspen by gypsy moths in the field is due to the direct effects of host chemistry on larval performance rather than to the indirect effects of host chemistry on efficacy of this natural enemy.