Hemlock Woolly Adelgid

Description
The hemlock woolly adelgid is less than 1/16-inch (1.5-mm) long and can barely be seen with the naked eye. The  white fluffy "wool", or ovisac, created by adult adelgids are the main indication of an infestation. Eggs are brownish-orange and wrapped in the white fluffy "wool" secreted by an adult female. After hatching, the nymphs are reddish-brown, becoming reddish-purple upon reaching the adult stage. During the adult stage of its life an adelgid produces a covering of the wool-like wax filaments that protect itself and its eggs from natural enemies and prevent them from drying out.

Origin
The hemlock woolly adelgid, Adelges tsugae, is native to Asia where it is not a problem to native hemlocks. It was introduced to the United States in the 1924 to the Pacific Northwest, and was first reported in the Eastern United States in 1951 near Richmond, Virginia.

More about the origin of woolly adelgid

Host/Diet
Adults, as well as the nymphs, suck sap from the base of needles on hemlock trees, causing the needles to dry out and drop.  They may also inject toxins into the tree as they feed, accelerating needle drop and branch dieback. This defoliation can cause the hemlock tree to die in only a few years. The hemlock woolly adelgid develops and reproduces on all species of hemlock, but most species are resistant to its attack, only the eastern and Carolina hemlock lack resistance. The range of eastern hemlock stretches from Nova Scotia to northern Alabama and west to northeastern Minnesota and eastern Kentucky. Carolina hemlock occurs on dry mountain slopes in the southern Appalachians of western Virginia, North and South Carolina, Georgia, and Tennessee.

Predators
The hemlock woolly adelgid lacks natural enemies in North America. This factor contributes to the woolly adelgid's slow but steady expansion in North America.  There are a number of defenses that help hemlocks in Asia cope more successfully.

Reproduction
The hemlock woolly adelgid is parthenogenetic (all individuals are female and reproduce asexually) and has six stages of development: the egg, four nymphal instars, and the adult. The adelgid completes two generations a year on hemlock. The winter generation, the sistens, develops from early summer to midspring of the following year (June–March). The spring generation, the progrediens, develops from spring to early summer (March–June). The generations overlap in mid to late spring. The adult females produce between 50-300 eggs in a lifetime.

Range in North America
As of 2005 the hemlock woolly adelgid was established in portions of 16 States from Maine to Georgia, where infestations covered about half of the range of hemlock. The predicted spread rate is about 20 miles per year.

Adelges tsugae at Coweeta
Because the Hemlock is a keystone species in the Coweeta Basin the hemlock woolly adelgid is being studied extensively by scientists at the Coweeta Hydrologic Laboratory, USDA Forest Service, and their collaborators.  Some of Coweeta's publications regarding this species and its impact on Hemlocks include:

Brown, J. 2004. Impacts of hemlock woolly adelgid on Canadian and Carolina hemlock forests. pp. 19-36 In Proceedings, Land use change and implications for biodiversity on the Highlands plateau: A report by the Carolina Environmental Program: Part A, 10 December 2004, Highlands, NC. Highlands Biological Station, Highlands, NC.

Ford, C.R., and J.M. Vose. 2006. Eastern hemlock transpiration: patterns, controls, and implications for its decline in southern Appalachian forests. pp. 181-187 In Fowler, D.L. (ed.), Second interagency conference on research in the wetlands. USDA Forest Service, Southern Research Station, Coweeta Hydrologic Laboratory, Otto, NC.

Ford, C.R., and J.M. Vose. 2007. Tsuga Canadensis (L.) Carr. mortality will impact hydrologic processes in southern Appalachian forest ecosystems. Ecological Applications. 17(4): 1156-1167.

Ford, C.R., J.M. Vose, M. Daley, and N. Phillips. 2007. Use of water by eastern hemlock: implications for systemic insecticide application. Arboriculture & Urban Forestry. 33(6): 421-427.

Knoepp, J.D., J.M. Vose, K.J. Elliott, B.D. Clinton, C.R. Ford, and B.D. Kloeppel. 2005. Hemlock Woolly Adelgid Research at the Coweeta Hydrologic Laboratory. In Proceedings, Save our hemlocks third symposium on hemlock woolly adelgid in the eastern United States.

Nuckolls, A.E. 2007. The effects of hemlock woolly adelgid ( Adelges Tsugae) damage on short-term carbon cycling in southern Appalachian eastern hemlock (Tsuga Canadensis) stands. University of Georgia, Athens, GA.

The Participants:  Eastern Hemlock | Woolly Adelgid | Human Intervention
The Story: History | Present | Future
Research: Research at Coweeta LTER | Other Research

Related Resources | Resources for Teachers | Maps of Wooly Adelgid Distribution