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Gypsy Moth In North America


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


Table 2 lists the top 20 preferred species, ranked on their total basal area over the inventoried area. Out of the highest ranking 10 species, only one species, quaking aspen, occurs in the western US. Some caution should be used in interpreting this ranking because the lack of inventory data in certain western counties (Fig. 1) resulted in a bias favoring eastern species. Nevertheless, these data indicate that most of the susceptible basal area (which is closely correlated to foliage area) is concentrated in the eastern US.

White oak was the top-ranking susceptible species (Table 2). Figure 2 shows the distribution of this species. High concentrations of white oak exist throughout the east, but the highest concentrations exist in the Ozarks, Cumberland Plateau, and the southern Appalachians. Most of these areas are currently beyond the expanding range of the gypsy moth. Sweetgum was the 2nd most common susceptible species (Table 2). This species is common throughout the Piedmont from North Carolina to Louisiana, and also exists largely beyond the current range of the gypsy moth. Quaking aspen was ranked number 3 (Table 2). It is one of only a few tree species whos range extends across the eastern and western portions of the continent though it is most common in the northern portions of the lake states (Fig. 4). Most of these areas of high aspen concentration are beyond the current range of the gypsy moth. Northern Red oak was ranked no. 4 in terms of its total basal area (Table 2). This species is common throughout the northeast and is common in portions of the lake states. Much of the range of this species occurs in areas already infested by the gypsy moth. The ranges of the other most common preferred tree species are pointed to in (Table 2).

Over-all forest susceptibility was quantified using five measures: average basal area per acre 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) (Fig. 24), and proportion of land area covered by extremely susceptible stands (> 80% of basal area in preferred species) for each county. All three measures yielded maps that indicated similar distributions of susceptible forests over the coterminous US. The areas with the highest concentration of susceptible forests were in the central and southern Appalachians, the Cumberland Plateau, the Ozark Mountains, and the northwestern lake states. Comparison of these maps with the know distribution of individual susceptible species (see table 2) indicates that oaks are the major component of susceptible forests in the Appalachian, Cumberland, and Ozark areas but quaking aspen is the major susceptible species in the northwestern lake states. One interesting note is that even though sweetgum is the second most abundant susceptible species (Table 2), it is apparently not abundant enough to cause high levels of stand susceptibility; it is rarely associated with enough other susceptible species to make the stands in the Piedmont highly susceptible.

Statistics showing correlations of the 5 different measures of susceptibility with historical defoliation are shown in Table 3. When data were combined over all 4 states, the highest correlation with defoliation was with proportion of land area with greater than 20% of stand basal area in preferred species, though basal area / acre and proportion of land area with greater than 50% of stand basal area in preferred species had correlation coefficients that were nearly as big. In general, correlations were higher in PA and CT than in NJ and MA (Table 3, Fig. 22). There are several possible sources for variation in defoliation frequency that is not explained by forest composition. First, urban forests were not inventoried; even though substantial defoliation may have been recorded in urban forests, these forests did not appear in the inventory data. Secondly, there are ecological factors, other than proportion of basal area in preferred species, that may contribute to explaining forest susceptibility (Bess et al. 1947, Valentine and Houston 1979, Herrick and Gansner 1986. It is possible that these other sources of variation may have been more pronounced in MA and NJ than in PA and CT, thus explaining the lower correlations with defoliation in these states.

Table 5 shows the number of acres classified as suscpetible in each state.

There are several caveats that should be attached to the interpretation of these data. As mentioned above, inventories were not available from any urban forests and inventories were missing form several forested areas in the west. Also, it is important to keep in mind that susceptibility assumptions are based upon several assumptions that have not been completely proven. First, the suitability of many tree species to the gypsy moth is in many cases, based upon incomplete information For many species, feeding trials have not been performed and for other species for which there is some laboratory data available, data on susceptibility to defoliation in natural forests is unknown (Liebhold et al. 1995).

Despite these limitations, these results should be useful in planning for the future. The finding that the gypsy moth has not yet invaded most of the susceptible forests in the US suggests that there still may be considerable value in limiting the future spread of the gypsy moth. It also indicates that both the impacts of defoliation and costs of gypsy moth management are likely to increase in the future.

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Last modified 11-03-03 by Sandy Liebhold .

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