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5.7 Non-native Insects

5.7.1 Hemlock Woolly Adelgid

The hemlock woolly adelgid (Adelges tsugae), an insect species native to Asia, was first identified in the Eastern United States in the early 1950s in Richmond, VA. It has recently expanded into the Southern Appalachians and threatens to spread throughout the ranges of eastern and Carolina hemlock. In the South, it is currently established in the mountains around the Shenandoah Valley, and it is spreading southward along the Blue Ridge (Figure 16).

Eastern hemlock is an important component of riparian ecosystems, providing cooling shade for streams, contributing nutrients for streams through litterfall, and providing winter shelter for wildlife. It may also be important as a feeding and nesting niche for neotropical migrant birds (Rhea and Watson 1994). The ecology of Carolina hemlock is less understood. It generally occupies more xeric sites on ridges and rock outcrops, but also probably provides cover and nesting sites for birds and small mammals.

Once infested by the adelgid, hemlocks are weakened, gradually defoliate, and become unable to refoliate or to produce cones. The adelgid causes mortality in all ages of both species. Mortality occurs after complete defoliation, generally within 5 years of initial infestation (McClure 1987).

Both eastern and Carolina hemlock are threatened. The adelgid could eliminate the limited population of Carolina hemlock within the next 2 decades.

There is suspected but unconfirmed genetic resistance to adelgids in both of the eastern hemlock species. Resistance is known to occur in hemlocks native to Asia and in the two species native to the Western United States. There are no known silvicultural strategies to prevent adelgid-caused impact or mortality. Chemical spraying or soil treatment can protect individual hemlock trees, but such treatment is impractical for forest trees (Rhea 1996). Results of recent attempts at biocontrol of this pest are inconclusive. It appears that all untreated hemlocks, with the possible exception of small geographically isolated populations, could eventually be killed by the adelgid (Rhea 1996).

5.7.2 Balsam Woolly Adelgid in the Southern Appalachians

The impacts of balsam woolly adelgid (BWA; Adelges piceae) were first documented in 1957 on Fraser fir in the Southern Appalachians. There are five major areas of spruce-fir forest in North Carolina, Tennessee, and Virginia (Figure 14). The majority of this forest type is on federal land and is maintained for public use. These forests occur at high elevation and are highly valued scenic and recreation areas that attract several million visitors annually. The balsam woolly adelgid has infested Fraser fir in all five areas and impacts are evident.

Several laws have been enacted that direct the management of the Fraser fir and help resource managers make decisions dealing with the future of this tree. These laws help maintain the limited or threatened ecosystems and are key to the preservation of the spruce-fir forests. Fraser fir is under consideration for inclusion on the Federal Endangered Species List.

Several species of flora and fauna rely on mature spruce-fir habitat for survival. Many of these plants and animals are found only in this environment. Damage caused by the adelgid has put these species at greater risk.

The Fraser fir forests of the Southern Appalachians are declining (Dull and others 1988, Nicholas and Zedaker 1990). The BWA has eliminated 95 percent of the mature fir from the forest, fir mortality attributed to the BWA continues at a steady rate, and the residual fir population consists of trees generally less than 40 years old.

Ground-applied chemical controls have proven effective against BWA but none are economically or environmentally feasible in a forested situation. Aerial application of chemicals has proven ineffective.

Biological controls for the adelgid have been extensively studied, but so far no effective biocontrols have been found. In addition, natural enemies have had little effect on the thriving adelgid population.

Cultural control methods have also been attempted without success.

There is some speculation that BWA may ultimately eliminate Fraser fir by destroying its reproductive capacity. Reproduction of this species does occur, but much less frequently than before BWA was present. Fraser fir survives to more than 40 years even when under pressure from the BWA, and at present it appears that the BWA will not eliminate spruce-fir forests at the high elevations of the Southern Appalachians. However, there remains the possibility that species dependent on mature fir canopies may be lost or that an additional stressor may cause the loss of the Fraser fir forest type.