USDA Forest Service

Gypsy Moth In North America


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Evaluation of Forest Susceptibility to the Gypsy Moth across the Coterminous United States


Assessment of forest susceptibility was based upon existing forest inventory data collected throughout the coterminous US. In the eastern United States, all inventory data were obtained from the USDA Forest Service Forest Inventory and Analysis (FIA) (Hansen et al. 1993). In the eastern US, FIA inventories federal land as well as privately held land. In the East, these inventories are usually conducted every 5 to 15 years. Each state typically contains over 1,000 irregularly spaced Forest Inventory and Analysis (FIA) plots. In the western US, FIA does not inventory National Forests. Therefore data on western forests was obtained as a mixture of FIA data as well as inventory data collected by individual national forests.

Sampling methods used for inventorying forest resources varied among regions and organizations conducting inventories ( Table 1). All inventory data contained data on individual trees, as well as data about plots. Individual tree records were used to sum total basal area by each species for each plot. These plot records were then expanded (using appropriate expansion factors) to county level estimates of basal area per acre.

Inventory data were available from most portions of the coterminous US (Fig. 1). However, inventories were not available from other portions. State and private land in the western 2/3 of Oklahoma and Texas are not inventoried by FIA and therefore these areas were missing. FIA data were available from every state in the country but in the west, FIA does not inventory National Forests and in some cases, National Forest inventory data were not available.

National Forest inventory data were occasionally incomplete. For example, all NFS data from California (Region 5) did not include either districts or counties. Therefore, it was necessary to randomly assign plots within a given National Forest to counties (weighted by the proportion of the national forest in each county). In portions of the southwest (Region 3), county was not included and similar assignments were made to counties within a district.

We adopted proportion of basal area represented by preferred species as the measure of forest susceptibility. While other variables (e.g., proportion chestnut oak) may help to explain more variation in susceptibility, these models are less likely to be successfully applied outside the range of data originally used to calibrate them. Montgomerys (1990) 3-way classification (preferred, susceptible, immune) was used to classify each tree species as preferred by the gypsy moth. This classification was based on a summary of field and laboratory studies, as well as extrapolations based upon taxonomic affinity, and is described in detail elsewhere (Liebhold et al. 1995).

In order to validate the models and data used in this analysis, we compared county-level predictions of susceptibility with historical defoliation observed in currently infested areas. Specifically, we computed total basal area of preferred species, proportion of stand basal area in preferred species, proportion of land area covered by susceptible stands (> 20% of basal area in preferred species), proportion of land area covered by highly susceptible stands (> 50% of basal area in preferred species), and proportion of land area covered by extremely susceptible stands (> 80% of basal area in preferred species) for each county in MA, CT, NJ, and PA. We examined the correlation of each of these variables with average defoliation in each county. Average defoliation was computed by first scanning historical aerial sketch maps and then overlaying all years to obtain a historical defoliation frequency (Liebhold et al. 1994). Defoliation frequency in each 2 x 2 km raster was averaged for each county.

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Last modified 11-03-03 by Sandy Liebhold .

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