Eastern hemlock is a wonderful tree native to North America found in the wild stretching from the shores of the Chesapeake to the Blue Ridge Mountains and beyond. In neighborhoods it graces landscapes as an evergreen specimen or screen along property lines. More than five decades ago, the hemlock woolly adelgid, a dastardly sucking insect kin to an aphid, 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 in Shenandoah, Blue Ridge, and Smokey mountains. A recent study estimated government and household expenditures to control hemlock wooly adelgid and lost property values when hemlocks died exceed $214 million annually in the United States. From Maine to Georgia this pest threatens eastern hemlock in the north and its rarer cousin, the Carolina hemlock, in the south.
This mini-monster 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 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 from which the woolly adelgid derives its name. The waxy cover provides protection for the adelgid and for eggs she will lay in the woolly sac 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 that summer on hemlock twigs. Except during the summer season of dormancy, developing nymphs and egg-laying females feed by inserting hypodermic-like mouthparts through the bark of 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 parenchyma 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.
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.
Pulling back the wax reveals wiggling legs and rows of wax pores lining the underside of the bizarre looking adelgid.
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 breed varieties of hemlocks that resist adelgid attack, and searches to discover biological control agents that kill adelgids. Several species of predatory 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. One species of predatory beetle found murdering adelgids on the west coast of the U.S. has been imported and released in adelgid - infested forests in the east. The hope is that these agents of biological control can reduce populations in a long term, sustainable way.
So, as the holiday season arrives, take a moment to don your parka, go outdoors, and commune with your hemlock. Give it a check-up and detect those diabolical adelgids before any serious damage is done.
Bug of the Week wishes everyone a joyous Holiday Season.
Two great articles, “Economic impacts of non-native forest insects in the continental United States” by J. E. Aukema and colleagues, and “Biology and Control of Hemlock Woolly Adelgid” by N.P. Havill, L. C. Vieira, and S. M. Salom, were used to prepare this article.