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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Flocked hemlocks - hemlock woolly adelgid, Adelges tsugae


Eastern hemlocks are beautiful native trees well suited for home landscapes.


Eastern hemlock is a wonderful landscape tree found in the wild stretching from the shores of the Chesapeake to the Blue Ridge Mountains. In neighborhoods it graces landscapes as an evergreen specimen or screen along property lines. More than five decades ago, the hemlock woolly adelgid appeared near Richmond Virginia. It likely entered this country on infested nursery stock from Japan. For many years this pest made its presence known mostly in home landscapes and parks where it often disfigured and sometimes killed hemlocks. As it spread to the Appalachian Mountains, it devastated magnificent stands of eastern hemlock, leaving thousands of dead trees in its wake.

The hemlock woolly adelgid has killed thousands of eastern hemlocks such as these in Shenandoah National Park.

From Maine to Georgia it now threatens eastern hemlock in the north and its rarer cousin, the Carolina hemlock, in the south. This tiny insect spends most of the summer and early autumn hunkered down as an inconspicuous immature stage, called a nymph, on the bark of the hemlock near the base of the needles. When the cold winds of winter blow in late October and November, the nymphs resume development and mature in mid-winter. During this time, they produce large amounts of the white, woolly wax which gives the adelgid its name.

White bundles of fluffy wax are a sure sign of a woolly adelgid infestation on hemlocks.

This waxy cover provides protection for the adelgid and for the eggs she will lay within it in late winter. Between March and June, a second generation of adelgids will hatch out and mature, and then the cycle begins again with females producing the next batch of nymphs on hemlock twigs that summer. All the while, the developing nymphs and egg-laying females feed by inserting hypodermic-like mouthparts through the bark of the twigs. The long, sucking mouthparts search along the tree’s vascular system and eventually find specialized tissues called parenchyma cells of the xylem rays. The mouthparts are inserted into these cells and the adelgid robs the tree of its stored nutrients. Heavily infested trees decline in vigor, turn a sickly grayish/green color, lose their needles, and may die in five to ten years, if the adelgids are not controlled.

Beneath its waxy cover the adelgid robs the hemlock of vital reserves.

To reduce the risk of death or damage to your hemlocks, try to keep them as healthy as possible. Be sure they are planted in loose, organic soils with room for the roots to grow. In times of drought, irrigation may be helpful. Inspect your hemlocks carefully at least twice a year, once in December and again in May, to catch an adelgid infestation early. Sometimes a small, isolated infestation can be nipped in the bud by simply pruning out an infested branch or two and destroying them. Almost ten years ago I spotted adelgids infesting hemlocks that separated my insect preserve from my neighbor’s backyard. I treated the trees with an insecticide and this chemical fix held the adelgid at bay for many years. Some of these potent insecticides can be purchased over the counter and applied through the soil. Heavily infested or very large trees may require the care of a licensed and certified arborist. They have the tools and knowledge to deal with adelgids.

The United States Forest Service is spearheading several projects to help defeat the adelgid throughout the range of our hemlocks. Projects include methods to rapidly detect forest trees infested with adelgids, evaluations to identify species and varieties of hemlocks that resist adelgid attack, and searches to discover biological control agents that kill and eat adelgids. Several species of lady beetles attack the adelgid in its home range in Asia. Lady beetles imported from Asia and released in our area have shown promise in reducing adelgid numbers.

So, with approach of the holiday season, take a moment to don your parka, go outdoors, and commune with your hemlock. Give it a check-up and detect those dastardly adelgids before any damage is done.


For more information about hemlock woolly adelgid and its management, please visit the following web sites: