Hemlock Woolly Adelgid - Forest Landowners Take Note

By Richard Gast, CCE Program Assistant: Horticulture and Natural Resources, Master Gardener

Eastern hemlocks (Tsuga canadensis) are one of the most beautiful conifers found in northern New York forests. It can take up to 300 years for them to reach mature heights of up to 70 feet and diameters of up to 3-feet. They commonly live for 500 years and can live for 800 years or longer. Many are among the oldest trees in the state. In their northern range, they're found at a variety of elevations (sea level to near 5000 ft.) and on a multiplicity of sites (hillsides, valleys, shorelines, glacial ridges). Hemlocks are commonly found growing in mixed stands, with yellow birch, sugar maple, northern red oak, white ash, American beech, and white pine and can be distinguished from pine by their short, flat needles.

Although hemlock has limited value as lumber or firewood, for landowners with strong wildlife interests, managing a hemlock stand as a deer yard may be a priority issue. Hemlocks are considered a 'keystone' species, meaning that they have a critical ecological role. In thick stands, the dense shade that they provide keeps the soil underneath from drying out during the summer months, creating cool and shaded microclimates for many species of plants and foraging animals. In winter, the dense canopy tempers snow-accumulation on the forest floor, providing superior cover for deer, turkey, ruffed grouse, and other wildlife. The needles are a preferred food for deer and selective cutting is a common management practice used to promote natural regeneration and renewal of the food supply.

But, these long-lived giants are being threatened by an invasive insect as small as a radish seed. According to the Cornell Cooperative Extension Invasive Species Program's New York Invasive Species Clearinghouse, the hemlock woolly adelgid (HWA) has had a presence in the American west since 1924. It was first documented in the eastern U.S. in the 1950s, in Virginia. Since then, HWA has made its way north, killing thousands-upon-thousands of hemlocks along the way.

In 1985, HWA was discovered for the first time in New York; in the lower Hudson Valley. It has since been found in the capital district, the greater-Buffalo and greater-Rochester regions, and in the Finger Lakes. In fact, HWA has been found in 30 New York counties. Currently, 25 NY counties have infestations. Mortality has been pervasive in the Appalachian and southern Catskill Mountains. Seventeen other states from Maine to Georgia also have infestations.

On July 18th, HWA was found for the first time in the Adirondacks; discovered on an old-growth Eastern hemlock on Prospect Mountain, on the outskirts of Lake George, by a senior ecologist from Harvard University who was visiting the area on a field trip. HWA was later confirmed by a Cornell University scientist working with survey crews from Cornell's New York State Hemlock Initiative and DEC. The tree had no visible sign of crown-thinning. No less than 10% of the Adirondack Park is made up of Eastern hemlocks.

HWA are aphid-like insects, native to Asia. Eggs and crawlers (very young adelgids) are easily moved by the wind and can be carried by birds and animals or moved on hemlock nursery trees, logs, or firewood. All are female, producing two generations a year asexually. They feed on the sap of hemlock trees, draining stored starches from twigs and branches, causing branch dieback and tree decline, and eventually killing the tree.

On hemlock trees, HWA look like tiny balls of wool or cotton attached to twigs at the base of needles. Hence the name. The adelgids themselves, however, are actually dark-colored and less than 1/16 of an inch long; barely visible to the naked eye. The cottony-appearing substance on the twigs are egg sacs, protected by dense masses of waxy hairs, which they produce as protection; to keep the eggs and crawlers from drying out during cold weather. Interestingly, hatched HWA remain active; continuing to grow throughout the winter, which is why it was once believed that they could not live in northern climates.

It is often easier to see HWA 'wool' on the undersides of shoots, so scouting for the adelgids requires examining the undersides of hemlock branches. It's best to do this in late winter and early spring.

If you believe that you've seen a hemlock tree infested with HWA and would like to confirm your identification, take photos, note the tree's location, and report it to the New York State Department of Environmental Conservation (DEC) forest pest hotline at 866-640-0652 or email your photos to them ([email protected]). Please do not clip infested branch samples and transport them or mail them for identification. You could accidentally spread HWA to areas that are not infested.