Eastern hemlock is a wonderful landscape tree. It is 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 three 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. From Maine to Georgia it now threatens eastern hemlock in the north and its rarer cousin, the Carolina hemlock in the south.
The hemlock woolly adelgid has killed thousands of eastern hemlocks such as these in Shenandoah National Park.
Almost five years ago I discovered adelgids on hemlocks separating my insect haven from my neighbor's backyard. I treated the trees with an insecticide and this chemical fix held the adelgid at bay for a goodly long time. On New Year's Eve I noticed the rascally adelgid staging a comeback. 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.
White bundles of fluffy wax are a sure sign of a woolly adelgid infestation on hemlocks.
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 white, woolly wax which gives the adelgid its name. This wax provides protection for the adelgid and later the eggs she lays within. Between March and May adelgids complete a second generation and females produce the next batch of nymphs that summer on the twigs. All the while the developing nymphs and egg-laying females, insert 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.
Beneath its waxy cover the adelgid robs the hemlock of its vital reserves.
Heavily infested trees decline in vigor, turn a sickly 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. Fortunately, potent insecticides are available that can kill adelgids infesting your hemlocks. Some of these can be purchased over the counter and applied through the soil. Heavily infested 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. They have identified several species of lady beetles attacking the adelgid in its home range in Asia. One lady beetle has already been released in our area and has shown promise. A second species discovered in the Yunnan province of China is scheduled for release later this year. So, on a blustery winter day, 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.
References:
For more information of hemlock woolly adelgid and its management, please visit the following web sites.
http://na.fs.fed.us/fhp/hwa/
http://www.agnr.umd.edu/users/hgic/pubs/online/hemlock_woolly_adelgid_pfv.pdf
http://www.dcnr.state.pa.us/forestry/woollyadelgid/index.aspx