Distribution of First Instar Gypsy Moths (Lepidoptera: Lymantriidae) Among Saplings of Four Tree Species Common in the Great Lakes Region

Abstract

We examined the inter-tree distribution of first instar gypsy moth larvae under natural dispersal conditions in the field in Michigan in 1991. The study focused on saplings of northern red oak (Quercus rubra), white oak (Q. alba), red maple (Acer rubrum), and witch-hazel (Hamamelis virginiana), which are common understory components of forests in the Great Lakes region. Large-volume trees with foliage that is well-developed early in the spring should provide an excellent surface area for catching newly-hatched gypsy moth larvae, which are randomly dispersed by wind in the spring around the time of leaf flush or shortly thereafter. Comparing across tree species in our study, red maples had the largest crown volumes and wellflushed leaf material, but very few larvae were found on these trees at both sampling times in mid-May. Despite the lack of larvae on red maple, these trees were liberally covered with gypsy moth silk, indicating that many larvae landed on these plants but left rapidly when they apparently found the trees to be an unacceptable food source. Among the other three tree species, patterns of larval distribution reflected levels of host phenological development during both sampling periods, with more insects occurring on tree species that had more advanced leaf development. The highest numbers of larvae were found on northern red oak and witch-hazel. Intermediate num­bers of insects were present on white oak. Trends in insect distribution also paralleled patterns seen in tree phenology across slope gradients. Defoliation ratings corresponded well to measures of first instar presence for each tree species. Northern red oak and witch-hazel trees experienced more early defoliation on average than did red maple and white oak. For those tree species that are acceptable hosts of the gypsy moth, average level of phenological development for a given tree, species, or forest stand at the time of larval dispersal can be an important predictor of plant or stand susceptibility to gypsy moth establishment and subsequent defoliation