Landscape Characterization of Forest Susceptibility to Gypsy Moth Defoliation

Bethesda

1994

S. 18-29

27 Lit. Ang.

Bandaufführung

7380

64704

This paper seeks to relate defoliation by the gypsy moth, Lymantria dispar (L.), to components of the landscape in order to elucidate processes operating at the regional scale. Aerial sketch maps of gypsy moth-caused defoliation in Pennsylvania from 1969-1989 were assembled in a raster-based geographical information system (GIS). These images were manipulated using Boolean algebra to determine the total defoliation frequency for each 2x2km grid cell over the 21-yr study period. Because the gypsy moth became established in different parts of Pennsylvania at different times, the defoliation frequency for each cell was corrected by dividing the raw frequency by the total number of years at risk to defoliation. The number of years at risk to defoliation was calculated as the number of years each area was considered within the generally infested area minus a constant lag term, to correct for the period in years between first designation of quarantine until first defoliation. For Pennsylvania the lag was estimated as 5 yr. Average defoliation frequencies were calculated for each of six major forest groups occurring in Pennsylvania. Pine, oak-pine, and oak-hickory type groups were the most susceptible to defoliation; maple-birch-beech, aspen- birch, and nonforest-type groups were the least susceptible. The high defoliation frequency in pine types reflects heavy feeding on oaks in these areas since gypsy moth larvae rarely feed on most pine species. Forest type group areas were further cross-tabulated by elevation classes. The three most susceptible forest type groups exhibited decreased susceptibility at elevations lower than 200m.