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 s usceptibility to a nuclear polyhedrosis virus (LdNPV), but the relationship of susceptibility to chemistry is poorly understood. We investigated the e ffects of genetic and resource-mediated variation in phytochemistry on vira l pathogenicity. Trees were grown in pots in a common garden. Disks were pu nched from aspen leaves, inoculated with LdNPV and fed to third instars. Ad ditional leaves were analyzed for levels of nitrogen, starch, phenolic glyc osides, and condensed tannins. Despite marked variation among trees in leve ls of phenolic glycosides and tannins, we observed minimal variation in lar val susceptibility to LdNPV. Viral pathogenicity was only weakly (inversely ) correlated with tannin concentrations in one of two experiments. These re sults 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 n atural enemy.