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Invasive Species Spotlight: Hemlock Woolly Adelgid (Adelges tsugae)

July 25, 2008

Although the natural growing range of Hemlock trees is just to the east of Illinois, these plants are quite adapted to landscape use and are popular in many areas within the state. Hemlocks make interesting trees in the cultivated landscape. They provide year-round color and soft textures. The presence of the hemlock woolly adelgid (HWA) is threatening to seriously impair the health and vitality of these often beautiful trees.

This forest pest was first identified in the Pacific Northwest in 1924. In 1951, it was reported in the eastern United States in the Washington, D.C., and Virginia area. Since then, it has spread throughout the Appalachian Mountains region, infesting and devastating natural forest stands and ornamental plantings of hemlock trees.

Infestation of Tsuga canadensis. Taken from Bugwood: Connecticut Agricultural Experiment Station Archive, Conneccticut Agricultural Experiment Station.

An adelgid is an insect similar to an aphid. They are both true bugs in the class Hemiptera, with piercing and sucking mouthparts. Adelgids are mostly associated with coniferous trees. Hemlock woolly adelgids feed solely on hemlock trees. In the eastern United States, there are two species of Hemlock: the eastern hemlock, Tsuga canadensis, being more widespread and commonly seen; and the Carolina hemlock, Tsuga caroliniana, much less common. The adelgid feeds on the sap of these trees, which can deplete their essential fluid and nutrient supplies, and produces saliva that contains toxins harmful to the trees.

Adult and eggs inside an ovisac. Photo courtesy of Michael Montgomery, USDA Forest Service.

Adult HWAs are less than 1/16 inch long, all black in color, and are usually covered in a white puffy wax that progressively becomes thicker during their one-year life stage. The wax serves as a barrier to protect them from predators, as well as to keep them from drying out during low humidity. They are usually found in great numbers almost covering entire sections of tree branches. This particular species often undergoes a brief period of dormancy during the heat of the summer but can still be found on the stems of the plant. Adelgids, like many aphids, reproduce parthenogenetically, that is, the entire population is female and undergoes asexual reproduction. It can reproduce in large numbers and fast. A typical adelgid can produce up to 300 eggs at a time and can reproduce twice in one year. While their populations increase on a tree, growth becomes stunted; needles begin to fade and fall off the tree. Within a few years, a highly infested tree perishes.

Infestation. Photo courtesy of USDA Forest Service, Region 8 Archive.

Because the HWA has no real natural predators in the eastern United States, its populations have grown rapidly, allowing it to inflict large amounts of damage to hemlock stands. Researchers are investigating the benefits and efficiency of introducing predatory insects into infested areas to serve as a control measure in large forested areas. Other treatments successfully used for individual trees include systemic insecticides, horticultural oils, and insecticidal soaps.

For additional information, see

If you suspect hemlock woolly adelgid in Illinois, please contact the Illinois Cooperative Agriculture Pest Survey Program by phone at (217)333-1005 or by email at invasives@inhs.uiuc.edu. For more information, stop by the Illinois CAPS blog (www.illinoiscapsprogram.blogspot.com) for all the latest news on invasive pests in Illinois.

(Mike Garrett)

Author: Phil Nixon


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