The Hemlock Woolly Adelgid

Bending in the wind with its twigs and branches flexing at its rounded top, the Eastern Hemlock (Tsuga canadensis) tree is an iconic species inhabiting the eastern United States. Similar to firs, hemlock trees grow into some of the most beautiful forest trees in mature forests and ravines. The range of the Eastern Hemlock stretches from the northern areas of Georgia reaching as far north as Maine and spreading westward into Minnesota. Found in lower as well as higher elevations and growing in acidic to neutral soil, the hemlock tree provides wildlife with habitat and food, and the forest streams with shade from its canopy. In the past, through current times, the hemlock has had to fight humans for territory, being cut down to provide wood for building and the pulp and paper industry. However, the hemlocks of the eastern United States face now perhaps a greater threat, an invasive species named the Hemlock Woolly Adelgid.

The Hemlock Woolly Adelgid (Adelges tsugae) is a small, aphid-like insect and is considered a pest to the forests of North America. At about .8mm in length, the oval-shaped, brown-red colored adelgid can be identified on the underside of the hemlock needles wrapped by the white, dry, woolly covering it produces on its body and masses of eggs. This covering is present throughout the life of the adelgid.

Hemlock Woolly Adelgids are parthenogenetic, meaning that all adelgids are female and reproduce asexually, going through two developmental stages a year, the first generation developing from March until June. When the first generation reaches maturity in mid-June, they lay a second batch of eggs that hatch in early July. These new crawlers find another hemlock to settle on and become dormant until October. Within their individual cases, the adelgids feed on the sap produced by the tree and inject toxic saliva through thread-like mouthparts on the underside of the adelgid’s body. They sustain themselves throughout the winter on the sap of the tree until they are ready to lay eggs again and the cycle repeats. The feeding of the adelgids on hemlock trees causes rapid, but not immediate, declines in tree health. Generally, death of the infested tree comes within four years, less if the adelgids are completely uncontrolled.

Adelgids are native to Japan and parts of Asia and were originally introduced into the eastern U.S. in the 1950s. They inhabit hemlocks, spruces, and other ornamental trees in these areas but are kept in check by natural predators and host tree resistance, both which are lacking in the eastern United States. Also contributing to the rapid and widespread success of the adelgid is their reproductive behavior. Two hatches every year causes very high and fast growth rates with every individual having the capability of producing up to 300 eggs. Adaptations to temperature and elevation have also fueled the species. Dispersal can happen in a variety of manners from wind, birds and other animals, to humans, and from tree to tree. In the past decade it is thought that the rate of spread of the Hemlock Woolly Adelgid has been 15 miles per year. This invasion is ecologically devastating for the forests that these hemlocks thrive in. Not only do the hemlocks suffer as a species, but other biotic factors are affected too.

The Eastern Hemlock provides suitable habitat to many species that depend on the canopy such as wild turkeys, grouse, deer, and rabbits as well as bird species that roost, nest and eat within the trees. Largely affected by the loss of the trees, are the streams that hold hemlocks within their riparian zones. The shade from the dense canopy cools the water that is necessary for the survival of much aquatic life like trout and insects. Loss of volumes of hemlock trees would cause a cascade of problems disrupting the entire ecosystem of eastern forests, starting with the hydrologic cycle.

The future of the hemlocks looks bleak; however efforts to control the adelgids are being researched and implemented. The National Park Service uses methods including foliar treatments, systemic treatments, and predator beetles. Treatments are controversial although they are environmentally friendly insecticides and horticultural oils. They are extremely expensive, but both have proven effective against the adelgid. The predator beetles have been used as a bio control since 2002 by the National Park and other organizations to combat the pest but success of this method cannot yet be assessed. Laricobius nigrinus, or the Tooth-Necked Fungus Beetle, and Pseudoscymnus tsugae, the Japanese Ladybug, are two species released as predators of the Hemlock Woolly Adelgid. However, introduced species can have unforeseen effects on ecosystems.

It is unclear exactly how the first Woolly Adelgids made their way from Japan to the eastern United States to be found near Richmond, Virginia, in the 1950s. Since that time the hemlocks have declined in numbers from logging and this invasive pest. Efforts made to halt the adelgids have slowed the rapid spread of the insect but the hemlocks are disappearing. If the insecticides, horticultural oils and beetles do not wipe out the Hemlock Woolly Adelgid the forests from Georgia to Maine will look and act as an ecosystem drastically differently than in the past.

Audubon is located at 1600 Riverside Road, just off Route 62 between Warren and Jamestown. The trails are open from dawn to dusk as is Liberty, the Bald Eagle. The Center is open from 10 a.m. until 4:30 p.m. daily except Sunday when we open at 1 p.m. For more information you may visit or call 569-2345.