Hemlock Woolly Adelgid

Pest: Hemlock Woolly Adelgid (Adelges tsugae Annand)

Order: Homoptera
Family: Adelgidae

Description:

Hemlock woolly adelgid, Adelges tsugae, was accidentally introduced to North America and is currently a great threat to eastern hemlocks. Native to Asia, the first North American reports were in British Columbia, Canada in 1922 and in Oregon in 1924. Hemlock woolly adelgid was found near Richmond, Virginia in 1951. The insect is now found from northern Georgia to southern Maine, and from northern California to southeast Alaska.

This bug is an example of how the import of pests into our country, in this case from Asia, can have devastating results to our forests and landscapes. Since there are no indigenous predators for this insect, it has spread unchecked throughout our landscapes and is threatening the native stands of Hemlock throughout the east. The adelgid itself is a small black insect that attaches to the under- side of hemlock twigs, where it remains and feeds. As the insects develop, they secrete the characteristic white wax that gives the bug its name. A mature infestation of adelgid looks like small cotton balls have been pasted to the underside of the twigs. The hemlocks are one of the most versatile and beautiful of our native evergreens. The Hemlock Woolly Adelgid attacks both the Carolina and Eastern hemlock and is capable of severely weakening and killing its host plants. Healthier plants, prior to infestation, may endure longer but previously stressed plants may die in 3-5 years.

The Pest:

The Hemlock Woolly Adelgid (HWA) is a tiny insect that is closely related to the aphids. Adelgids, in general, are associated with conifers and produce galls and / or woolly masses. The HWA only produces woolly masses.

This insect is a typical of most insect species in the east in that it is inactive for much of the growing season and very active throughout the winter. From mid-July until mid-October, they are immatures (nymphs) and are flat, black, oval, and ringed with a fringe of white waxy strands. These very tiny nymphs can be found at this time settled on the stems of the host at the base of the needles. They neither feed nor develop during this time period. In mid-October, they resume feeding by using their piercing-sucking mouthparets to remove plant sap from the twigs of the host plant. While they feed, they develop into adults with new egg-masses appearing beginning in March. These are the most noticeable stage of the pest and have the appearance of small, white cotton balls lined up at the base of the needles. From then until June there can be many life stages present at one time and much feeding and injury can occur. There are two generations per year. Magnification is required to see all of the individual life stages.

Plants growing in stressful sites (compacted soils, ledgy soils, poor drainage, drought prone, etc.) are much more apt to succumb within 3-5 years from this pest’s invasion. Hemlocks in more natural (healthier) settings, such as the forest, have been shown to withstand infestations longer.

Impacts on hemlock:

The two hemlock species found in eastern North America, eastern hemlock (Tsuga canadensis) and Carolina hemlock (Tsugacaroliniana), are susceptible and seem to have little resistance to hemlock woolly adelgid damage. Heavy infestations can lead to the death of the tree within 4 - 10 years, and trees are also weakened and made vulnerable to attack by other insects and diseases. Some trees recover, although the reasons are not well understood.

The hemlocks in western North America, western hemlock (Tsuga heterophylla) and mountain hemlock (Tsuga mertensiana),are suitable hosts but are much more resistant to damage than the hemlock species in eastern North America. Back in Asia, hemlock woolly adelgid does not cause significant damage to hemlock even though adelgid populations may become very high. This is due to host resistance and possibly natural enemies that help to keep the adelgid population down.

Hemlock is a long-lived, ecologically important species that provides habitat for many species of animals, birds, fish, and other plants. It is also an important ornamental species and helps to control erosion along streams. In areas of the eastern United States where much of the hemlock is dead or dying, there are additional environmental impacts such as altered forest structure, degraded fish habitat, and increased invasion of non-native plants.

Proliferation:

Once this pest is in any given area, it will constantly pose a threat to all hemlock. Wind and birds are the primary means for moving this pest from one area to another but humans can also move this pest on plants if care is not taken. Certain species of asian hemlocks are resistant to this pest and nurseries are beginning to grow and sell those in the United states. The western hemlock (Tsuga heterophylla)is a native species and is highly resistant to this pest but it does not grow well at all outside of its native range of the northwestern USA. One approach to managing hemlock woolly adelgid has been to introduce natural enemies from the insect's native range in Asia. One predatory beetle (Sasajiscymnus tsugae) has been mass reared and released in the eastern U.S. Its adults and larvae prey on hemlock woolly adelgid and help to reduce its numbers. In North America there are a few native predators, but they do not eat enough of the adelgids to prevent damage to hemlocks. Laricobius negrinus, a predatory beetle found in the western U.S., also preys on hemlock woolly adelgids and is currently being released in the eastern states.

Images:

Recent News About Wooly Adelgids in the National Forest

Beetle release could save Cherokee Forest trees

By Ron Clayton, Correspondent
ERWIN, Tenn. - About 3,000 tiny predator beetles were released at the Rock Creek Recreation Area near Erwin this week in an experiment to try to control the hemlock woolly adelgid, a pinhead-sized bug that is destroying hemlock trees in the Cherokee National Forest. If the beetles reproduce and successfully combat the adelgid infestation, they could be used to battle the hemlock pests in the Tellico/Hiwassee Ranger District, Forest Service officials said. "This is a study," said Ed Brown, a silviculturist who cares for the Cherokee's trees. Forest Service spokesman Terry McDonald said the service also uses insecticides against the adelgids, but the beetles will be key to the battle because the outbreaks are widespread. Laboratories for rearing the beetles have been established in North Carolina, South Carolina and Tennessee. Releases already have occurred in the Carolinas and in the Great Smoky Mountains National Park. In an earlier interview, wildlife biologist Mary Dodson said the adelgid has been found in "two or three places in the Tellico." She said hemlock is a major component along streams. Losing the trees to adelgids could mean a rise in water temperature and potential harm to aquatic life, especially in trout streams, she said. The adelgids eat the sap from the trees. According to Forest Service publications, they appear to release a toxin that weakens the trees and prevents them from forming new cones. It takes three to five years for adelgids to kill a tree, so quick use of the beetles can save forests, Mr. Brown said. Mr. McDonald said any treatment in the Tellico/Hiwassee District probably would center on recreation areas first. He said the most likely areas are those where the trees have not been overstressed and where there are enough adelgids for the beetles to feed on. E-mail Ron Clayton at jwalton@timesfreepress.com
This story was published Friday, June 17, 2005

Click here for information about the hemlock wooly adelgid (HWA), Adelges tsugae.

Click here to learn more about HWA and treatment of infected trees.

USDA Hemlock Wooly Adgid website

Canadian HWA information site

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