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Hemlock Woolly Adelgid

By Bailey Walten 1994 Newsletter

Japanese invaders have struck again! We are still fighting the Japanese Beetle but another bug from Japan has arrived and is targeting the eastern (Canadian and Carolina) hemlocks. The Hemlock Woolly Adelgid, Adelges tsugae, is the culprit this time. We had mentioned this problem in our August, 1991 newsletter when we were just beginning to see an upsurge in the problem. It has now become such a serious pest in this area that we felt a full-length article was needed to alert our customers of this minute menace.

This tiny insect, about 1/16 inch long, causes severe desiccation and eventual death to their hosts by sucking sap after injecting a saliva toxic to our eastern varieties of hemlock. It has not posed much of a threat to western hemlocks which are apparently resistant to the poisonous spittle. The hemlock woolly adelgid was first spotted in North America back in 1922 out west up in Vancouver, British Columbia. By 1953, it had migrated east and was identified in northern Virginia. Although it thrives on our eastern hemlocks, the adelgids’ progress was initially slowed by the inhospitably hot, dry summers. Being a cool weather insect, its progress really accelerated once it moved north into the eastern mountain chains. Now it ranges from Tennessee to Massachusetts and is encroaching on Vermont and New Hampshire. They are spread by wind, birds, forest animals, commercial logging and nursery operations and during recreational activities. Here to stay, it is destroying thousands of hemlocks in our forests and in landscapes and continues to spread like wildfire.

Recent newspaper articles have portrayed this insect as an uncontrollable menace and once hemlocks are infested with them, the trees are doomed. Sensational and hopeless as that sounds, the reality is arborists are having success in controlling this threat, at least on those trees accessible to spray equipment. Our own experience in treating this problem for almost four years has shown timely and persistent applications with “Dormant Oil” or “Merit” are quite effective in controlling this insidious pest. Using oil in early spring and late fall will smother most of the insects and eggs. Some have recommended summer use of oil but this can have adverse effects on hemlocks if they are suffering from drought stress. That is why we advocate the use of a contact/systemic insecticide like “Merit” during the warm months from May to September. The foliage and insects must be thoroughly soaked to insure good control which necessitates using the proper spray equipment. The key is several doses a season over a few years and keeping the trees on a good preventive maintenance program. Spraying once or twice is not sufficient!

The hemlock woolly adelgid has enjoyed remarkable success here because it has no natural enemies in our country and is an incredible breeding machine. It is parthenogenic species (reproduces with unfertilized eggs) where ALL the adults are female. Mating is unnecessary so males are nonexistent. These mamas could write the book on exponential population growth as each individual can lay 50 to 300 eggs and there are two generations per year. When spraying, at least 95% of the population must be killed on contact just to maintain their numbers at the same level.

In Maryland, eggs are laid in March and April and hatch in April and May. The crawlers (remember, all female) dispersed mainly by wind, find their way to young hemlock branches and after settling down, begin feeding on sap. By the end of June a second generation is produced consisting mostly of wingless adults. The few winged adelgids must migrate to spruce to reproduce. Fortunately for us, they do not multiply on our eastern varieties of spruce. The non-winged adelgids will settle down and lay dormant during the hot summer weather until late October. Being a cool season insect they wake up and begin feeding again and can remain active through the winter from November till March.

They are not hard to spot if one knows what to look for! Each adult female and its eggs are encased in white wax and resemble tiny cotton balls. These are attached to the twigs at the base of the needles. From a distance, it almost looks like snow. Uncontrolled, these toxic suckers can kill trees in 1 to 3 years. As the insects feed on the twigs, the stems dry out and no longer produce new growth. Shortly afterward, the needles will lose color and begin to shed. Research has only recently revealed that fertilizing may not help. Apparently the adelgids relish Nitrogen, the primary ingredient contained in most fertilizers. We feel that as long as the insects are being sprayed and kept under control, it would still be beneficial to at least fertilize with a low-nitrogen formulation.

Because large woodland tracts of hemlocks are being decimated, the environment is being impacted negatively in several ways. Resultant erosion and heating up of streams destroys fish, other wildlife and watersheds. As previously mentioned, hemlocks used in landscape plantings are suffering also. Locally, the Harvey Ladew Topiary Gardens in Monkton have lost many of its hemlocks used extensively as living walls complete with doorways, windows, gables and other sculpturing. Old age, constant shearing, soil compaction, and drought stress have exacerbated the actions of low level adelgid populations to the point where plans are being made to replace all of the hemlock topiary in the next ten years. Therefore, it is better to allow hemlocks to grow more naturally since high maintenance can intensify the hemlock woolly adelgid’s detrimental effects.

The future for woodland hemlocks in the northeast does not look bright at this point because they are inaccessible to high-volume hydraulic sprayers. Research is underway in Japan to find native biological agents that keep them in check. A Japanese mite is being studied that feeds on and dislodges the cottony covering that protects the egg sacks. Until natural control methods are discovered, eastern hemlock forests are in peril. Fortunately, we can protect our cherished landscape specimens with early recognition of the problem and swift action. Properly timed, thorough applications with the right chemicals are our best defense against the Hemlock Woolly Adelgid.

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