Bartlett Tree Experts: Hemlock Woolly Adelgid

Adelges tsugae

Tiny "cotton balls" on the underside of hemlock branches signal infestation by hemlock woolly adelgid.

The hemlock woolly adelgid (HWA) is a serious pest of both eastern hemlock (Tsuga canadensis) and Carolina hemlock (T. caroliniana). Heavy infestation by these insects can lead to tree death in just a few years. Although HWA is too small to be seen with the naked eye, infestation is quite obvious due to the white, woolly covering of the egg sacs, which look like tiny cotton balls lining the underside of branches at the base of the needles. The damage to infested trees is characterized by off color needles, needle drop, and eventual twig and branch dieback.

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Affected Areas

Originally from Asia, hemlock woolly adelgid was discovered in Virginia around 1950. Although the insect has been present longer in the Western U.S. and Canada, the predominant hemlocks located there, western hemlock (T. heterophylla) and mountain hemlock (T. mertensiana), sustain little damage due to their natural resistance and local predators.

In the east, the pest has spread from Georgia to New Hampshire and westward to Michigan, causing major mortality, especially in forest areas that are impossible to reach with control measures.

LIFECYCLE

Thinning limbs with yellowing needles is often a sign of hemlock woolly adelgid.

Populations of this adelgid are all female and reproduce asexually. Crawlers emerge in the spring and can move to other parts of the tree, or can be moved to new trees by wind, birds, and mammals. Once these crawlers settle on a feeding site, they become immobile feeding nymphs. When they reach adulthood, each hemlock woolly adelgid can lay up to 300 eggs and start a new generation.

Symptoms

The first sign of hemlock woolly adelgid damage is usually fading, thinning, and dying limbs near the base of the hemlock tree. The insects typically begin their infestation in the lower branches and can easily be identified by the cottony masses attached at the needle base.

Control

Without treatment, hemlock trees will continue to lose vitality. There are several options available for controlling hemlock woolly adelgid, including application of horticultural oils and soaps. If hemlocks are monitored and treated as necessary, the health and beauty of the tree can be maintained.

Hemlock Woolly Adelgid