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The Health of Southern Forests, USDA Forest Service, Forest Health Protection, Southern Region

Other Stresssors


Hemlock Woolly Adelgid

First discovered in the United States in 1924, the hemlock woolly adelgid (HWA) is believed to be an Asian introduction. The insect is a serious pest of both eastern hemlock and Carolina hemlock. In the South, infestations are most prominent in the Shenandoah Valley of Virginia.

Figure 31 shows the range of hemlock in the South, plotted from FIA information. Figure 32 shows HWA occurrence in Virginia by county. The line of infestation was drawn by using the most recent survey data from both the Virginia Division of Forestry and the USDA Forest Service.

Figure 31. Hemlock range.

Figure 31. Hemlock range.


Figure 32. Occurrence of hemlock woolly adelgid in Virginia by county.

Figure 32. Occurrence of hemlock woolly adelgid in Virginia by county.


The HWA is similar in biology to its relative, the balsam woolly adelgid (BWA). An excellent indication of infestation is the presence of white cottony sacs at the base of the needles (Figure 33). These sacs are reminiscent of the tips of cotton swabs. Present throughout the year, they are most prominent in the spring.


Figure 33. Cottony masses on hemlock stems indicate infestation by the hemlock woolly adelgid.
Figure 33. Cottony masses on hemlock stems indicate infestation by the hemlock woolly adelgid.

The HWA feeds during all seasons, with the greatest damage occurring in the spring. The tiny insect is dispersed by wind, birds, and mammals.

The adelgid feeds by inserting its stylet into young twigs, sucking sap, and retarding the growth of its host. Needles become discolored and change from deep green to grayish green, eventually dropping off prematurely. The loss of new shoots and needles seriously impacts tree health, and death can occur within a few years.

The hemlock woolly adelgid will continue to spread southwest along the Blue Ridge Mountains, causing devastation of the hemlock resource in its wake. The number of infested counties in North Carolina continues to grow, and South Carolina recorded its first infestation in the extreme northwestern part of the State in late 2001. Many entomologists fear the worst for both eastern and Carolina hemlock, believing that HWA holds the potential for wiping out both tree species.