In a 1997 article Liebhold et al. described the susceptibility of U.S. forests to severe defoliation by the gypsy moth, a problem that results in decreased growth, dieback, and tree mortality (Sharov et al. 2002). Pointing out that damage susceptibility is closely tied to the proportion of basal area represented by the gypsy moth's preferred species, the authors noted the large proportion of white oak in the east-central U.S., the preponderance of sweetgum in forests of the southeastern U.S., and the high concentrations of aspen across the extreme northeastern states. On a list of twenty preferred species that occur in abundance in eastern forests were thirteen species of red and white oak. One conclusion reached by Liebhold et al. is that "it is inevitable that the gypsy moth will continue to spread south and west over the next century." Indeed, between 1900 and 1994 the pest had spread across much of the northeastern U.S. (Figure 74), and was clearly gaining momentum as populations grew (Sharov et al. 2002).

 

Figure 74
The Dynamics of Gypsy Moth Spread During the 20th Century

1900        1934        1965        1994

Source: www.fs.fed.us/ne/morgantown/4557/gmoth/spread/

Despite the ominous projections of Liebhold et al. and others, remarkable progress in gypsy moth control has been made through the Slow-the-Spread (STS) Project of the U.S. Forest Service's Regions 8 and 9 which was begun in 1999. Through the use of the biological agent Bacillus thuringlensis, spectacular results in reducing gypsy moth infestation throughout the Northeast, North Central, and Southern Regions have been reported (USDA-Forest Service 1998; Sharov et al. 2002). The Forest Service estimates that the program will markedly affect the rate of spread in the decades ahead (Figure 75). A downside to this effort is that the gypsy moth control program consumed over 64 percent of the USFS Cooperative Pest Support Funds in FY 2000 (Wilson 2001).

Bacillus thuringlensis is most effective in wet years and less so in dry or drought years. Thus, an extended period of dry summer weather could reinvigorate gypsy moth populations.

Figure 75

Estimated Spread of Gypsy Moth in the United States With and Without the STS Project

With STS Program        Without STS Program

Source: www.fs.fed.us/ne/morgantown/4557/gmoth/spread/