The effects of defoliation caused by three foliage feeding insects, the gypsy moth (Lymantria dispar), the cherry scallopshell moth (Hydria prunivorata), and the elm spanworm (Ennomos subsignarius), on tree mortality and crown conditions were evaluated using data collected from 1984 to 1999 in the Allegheny National Forest located in northwestern Pennsylvania. While previous studies have focused on the effects of defoliation on trees in individual stands, this study differed in that it used exhaustive maps of defoliation and an areawide network of plots to assess these efects. A geographic information system was used to map the coincidence of USDA Forest Service Forest Inventory and Analysis and Forest Health Monitoring plot locations with defoliation polygons derived from aerial surveys to calculate cumulative years of defoliation for each pest. Over 85% of the Allegheny National Forest land area was defoliated at least once during the 16-year period from 1984 to 1999. Frequency of defoliation by specific defoliator species was closely associated with the dominance of their primary hosts in stands. Frequency of defoliation was often associated with crown dieback and mortality, but these relationships were not detectable in all species. These results suggest that when impacts are averaged over large areas (such as in this study) effects of defoliation are likely to be considerably less severe than when measured in selected stands (as is the approach taken in most previous impact studies).
Morin, Randall S.; Liebhold, Andrew M.; Gottschalk, Kurt W. 2004. Area-Wide Analysis of Hardwood Defoliator Effects on Tree Conditions in the Allegheny Plateau. Northern Journal of Applied Forestry. 21(1): 31-39.