Hemlock Woolly Adelgid

The hemlock forests of Maryland are part of a unique and often fragile habitat. Because of their shade tolerance and intolerance of fire, hemlocks are usually found growing in riparian areas or in steep cove forests in the northern and western tier counties of Maryland. It’s estimated that more than 42,000 acres of such forests exist in Maryland.

The health of Maryland’s hemlocks, and the associated ecosystems, is being threatened by the hemlock woolly adelgid (HWA). This small, exotic insect is native to Asia, and was first found in North America in British Columbia in the1920’s. It was reported in Richmond, Virginia in 1951, and spread northward into Maryland by the 1980’s.

Heavy infestations of HWA may result in decline of tree health and eventual mortality. The severity of decline and mortality is often hastened by drought, or other pests, such as elongate hemlock scale and hemlock borer. Several stands in Maryland, which have been infested with HWA for more than 10 years, have extensive decline and some mortality.

Landscape hemlocks in the Baltimore – Washington area were infested in the late 1980’s and natural stands in the area became infested by 1990. The infestation steadily moved westward through native stands of hemlock in Frederick and Washington Counties in the mid-1990’s, Allegany County in 1999 and Garrett County in 2001.

While adelgid populations were moving through Maryland in the 1980’s and 1990’s, there were very few management tools available to stop its spread. Although treatment options for HWA are still being developed, there are now more tools available than there were just 10 years ago.

The Maryland Departments of Agriculture and Natural Resources have developed a Hemlock Wooly Adelgid Management and Suppression Plan. This plan sets forth management options for HWA on public lands across the State. Treatments began in the fall of 2004 and have continued through the present.

Hemlock woolly adelgids are most easily recognized by the white “woolly” wax they produce on young hemlock twigs. The “wool” is present all year, but is most abundant and conspicuous during the spring and fall when egg masses are present. Most other stages in the life cycle are much harder to see. Fully grown adults are only about the size of a period on a printed page. For more detail on the biology and identification of HWA click on the links on the right side of this page.

For information about controlling HWA on your landscape trees contact the University of Maryland Home and Garden Information Center at 1-800-342-2507.​​​