Eastern hemlock trees are some of the largest and most common trees in the Great Smoky Mountains. They play an ecologically vital role in cooling mountain streams and providing habitat for many other species. Unfortunately, they are under attack from a non-native insect called the hemlock woolly adelgid (Adelges tsugae). Without successful intervention, the hemlock woolly adelgid is likely to kill most of the hemlock trees in the park.
Called the "redwood of the east," eastern hemlocks (Tsuga canadensis) can grow more than 150 feet tall on trunks measuring six feet in diameter. Some hemlocks in the park are over 500 years old.
Over 800 acres of old-growth hemlock trees grow in the Smokies-more than in any other national park. Younger hemlock forests of 75-100 years in age cover an additional 90,000 acres of land in the park. Originally discovered here in 2002, adelgid infestations have now spread throughout the park's hemlock forests. In many areas infested trees have now died.
Since its arrival in the U.S. in the 1920s the hemlock woolly adelgid has rapidly colonized parts of New England and the Mid-Atlantic States, where it feeds on eastern hemlock. In the south, it also feeds on Carolina hemlock. The insect is easily dispersed by birds and wind but travels most rapidly as a hitchhiker on infested horticultural material.
The hemlock woolly adelgid has infested hemlocks on the Blue Ridge Parkway for about 10 years and in Shenandoah National Park since the late 1980s. In these areas as many as 80 percent of the hemlocks have died due to infestation.
Hemlocks play an important role by providing deep shade along creeks, maintaining cool micro-climates critical to survival of trout and other cold water species. The impact of widespread loss of hemlock could trigger changes more significant as those that followed the demise of the American Chestnut in the 1930s and 40s.
What to Look For
The hemlock woolly adelgid feeds on the sap at the base of hemlock needles, disrupting nutrient flow and causing the needles to change from deep green to a grayish green, then fall off. Without needles the tree starves to death, usually within three to five years of the initial attack. Infestations often start in large, mature hemlocks, but the insect also attacks and kills younger trees as well.
What is the National Park Doing?
• Foliar Treatments
• Systemic Treatments
• Predator Beetles
You Can Help
Efforts to control hemlock woolly adelgids are being funded through the Save the Hemlocks initiative of the Friends of Great Smoky Mountains National Park, a nonprofit organization. For additional information, call (865) 932-4794.
Last updated: June 18, 2015