Hemlock Woolly Adelgid

(Adelges tsugae)

The hemlock woolly adelgid has been known in the United States since about 1927 It is thought to have been introduced from Asia. It is a serious problem on both eastern (Tsuga canadensis) and Carolina hemlocks (Tsuga carolina).

It has been a pest in the Philadelphia region for nearly fifty years, and has been slowly spreading south, west and north. It is now found in the Shenandoah Valley, north to the mid?Hudson region of New York, and into New England. It is also reported in California and the Pacific Northwest, but the western hemlock seems to be more resistant.

This insect is a serious pest of the hemlocks in the eastern United States. The adult stage is covered with a white, cottony mass of filaments. Heavy infestations give the trees a whitish cast, and may look like powdery snow on the branches. The damage is caused by the insect sucking sap and nutrients from the phloem, or inner bark of the tree's twigs. The insect is prolific, and can cover a large tree in just two or three years. When populations are heavy, the tree quickly declines, experiencing loss of vitality, needle shedding, and twig and branch dieback. Left unchecked, the adelgies can kill a mature hemlock.

Life Cycle

The adults spend the winter on the twigs. The eggs are set in the spring, in March and April, but hatching is dependent on weather. The hatching period begins in May in the mid-Atlantic region, but may be prolonged into late June or even July. The hatchlings, referred to as crawlers, are very mobile, and wander over the twigs and needles seeking suitable feeding sites. They may be picked up by squirrels, birds, and the wind, and carried to other trees long distances away.

After crawling over the plant for a few days, the nymphs settle at the base of a needle, usually in the leaf axil, and insert a long stylet into the soft phloem. They begin feeding on the sap of the tree, drawing out copious quantities. Some researchers suspect that they may inject a substance toxic to the tree, resulting in more rapid decline. The nymphs soon start exuding the white waxy material around the fringes of their bodies. It appears as a skirt under magnification with a hand lens. Up to this stage, the insect can be controlled by pesticide applications.

A second batch of eggs, produced during the summer by maturing females, begins hatching in September and October. This second generation matures quickly in the cool fall weather, and will spend the winter on the twigs to start the cycle again the following spring.


Control of the pest depends on two factors: thorough coverage with appropriate pesticides, and proper timing. Coverage is important because of the prolific breeding of the pest. Since there are no naturally occurring controls , their populations increase rapidly.

Timing of applications should be made a few weeks after hatching. Monitoring the trees with a hand lens for crawlers and settled nymphs will help assure better control. The exact timing will depend on location and local climatic conditions, but usually sprays can be applied in mid to late May or June. A second treatment three to four weeks after the first will improve control. A fall application, from mid September to mid October, will further reduce the population. Dormant applications of horticultural oil will also have some effect on reducing populations, when applied in early spring or late fall, during the dormant season.

Recommended treatment products include diazinon, Mavrik, horticultural oil, Sevin, and insecticidal soap. Merit, a new product available in 1995, is labelled for the pest, but field results have not yet been reported. Horticultural oil should not be used more than once in a 60 day period. The treatments should be repeated on an annual basis until the pest populations have been significantly reduced. Two years of treatments are usually needed, but more may be necessary, especially if there are untreated hemlocks nearby.

Other Pests

There are several other pests of hemlocks that may cause significant harm to the trees. The elongate hemlock scale (Fiorinia externa), the spruce spider mite (Oligonychus ununguis), and the hemlock rust (eriophyid) mite (Nalepella tsugifolia), can all injure the hemlocks. The woolly adelgid is very distinctive, however, and is not easily mistaken for another pest.

Cultural Treatments

Once the tree is infested by the hemlock woolly adelgid, it is weakened and low in vigor. Fertilization of the infested tree will not improve the tree, however, until the pest is controlled. Laboratory and field research have shown that fertilization, especially with nitrogen compounds, serves to improve conditions favoring the insect. Fertilization should therefore be postponed until the populations decline.

Watering of the tree remains a necessity, however. The tree should have at least one inch of water per week, whether from rainfall or irrigation. Soil conditions should be checked and the pH, or acidity, maintained at about pH 5.5 to 6.0. Applying mulch to the area around the trees will also help the trees during stress periods. An organic mulch should be used, placed about 3 inches deep. The mulch should not be placed against the bark of the trees. It should be spread to at least the dripline of the trees.


Whenever using pesticides or chemical treatments of any kind, read the label first. Be sure to follow all label warnings and instructions. Apply the materials only at labelled rates. Higher mix ratios will not improve efficacy of the pesticide, and may cause harm to the trees and the environment. Use approved personal protective clothing and equipment, as specified on the label.

© Copyright 1996, Russell E. Carlson

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This page was last updated on June 11, 2003.