Rapid induced resistance and host species effects on gypsy moth, Lymantria dispar (L.): Implications for outbreaks on three tree species in the Boreal Forest

Field pupal weight, development time, and survival of gypsy moth, Lymantria dispar (L.), larvae on three defoliated (50%) and undefoliated tree species that are common to the forest of the Great Lakes basin were compared for one season in 1988. Host species and defoliation affected female pupal weight; male pupal weight was affected only by host species. The smallest and largest pupae of both sexes, from both defoliated and undefoliated trees, came from larvae that fed on red oak, Quercus rubra, and trembling aspen, Populus tremuloides, respectively; pupal weight of larvae that fed on white birch, Betula papyrifera, were intermediate. Development time was affected only by tree species; the shortest and longest development occurred on trembling aspen and red oak, respectively; development time on white birch was medial. Gypsy moth survival was not affected by defoliation or host species. Superficially, these data obviously suggest that both defoliated and undefoliated trembling aspen and white birch are more nutritious, and will support higher gypsy moth fitness than its traditional hosts like red oak. However, we argue that outbreaks of gypsy moth will not occur in aspen and birch stands because its tri-trophic fitness is lower there due in part to the higher efficacy of certain gypsy moth natural enemies. We hypothesize that outbreaks on these two tree species will be limited by the nuclear polyhedrosis virus, Entomophagus maimaiga, and key physical features (e.g. light trunk color) of the host that deter larval host seeking/accepting behavior. More than 20 years of gypsy moth outbreak records in North America support this hypothesis.