Non-Native Insect Threatens Health of Popular and Beneficial Trees
Hemlock Woolly Adelgid

Robert H. Tichenor, Maryland Department of Agriculture | 410-841-5920

Photo: Dennis J. Souto, USDA Forest Service

ANNAPOLIS, MD (March 13, 2003) - A small insect from eastern Asia is severely impacting Maryland's hemlocks, an important evergreen tree. The hemlock woolly adelgid, Adelges tsugae, native to China and Japan, is a serious threat to the health and sustainability of hemlocks in eastern North America.

The hemlock woolly adelgid has been identified by the Maryland Invasive Species Council (MISC) as one of their "Invasive Species of Concern." These invasive species are widely recognized by biologists and resource managers to have the potential to damage natural ecosystems or harm native species. In the first in a series of monthly publications by MISC, this report seeks to increase public awareness of invasive species.

Hemlocks are important in urban and recreational areas, and are one of the species most commonly planted by homeowners. Although not a common or widespread species in Maryland forests, hemlocks are ecologically very important. The long-lived and shade tolerant hemlock provide food and shelter for many species of wildlife. Maryland's hemlocks are often associated with streams, help maintain cool water temperatures and prevent erosion.

Adelgids suck the sap from young hemlock twigs (they may also inject a toxic saliva), causing needles to lose moisture, turn pale green and drop from the tree. Buds also may die and, in heavy infestations, dieback of major limbs or the entire tree may occur.

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 in the spring when egg masses are present. Other stages in the life cycle are much harder to see, as fully grown adults are only about the size of a period on a printed page.

Adelgids have two generations per year. The cool weather species completes most of its development from October through May. Overwintering adults lay eggs in April and May under the white woolly mass. Nymphs (or crawlers) hatch and within a few days settle on twigs. They feed on the twig through their maturation into adults in late May.

Wingless adults settle on hemlocks to lay eggs. Crawlers hatch by July, settle on the new growth and become dormant until October. Nymphs then resume feeding and develop during the winter, maturing by spring.

Hemlock woolly adelgids were first found in the eastern United States in Virginia in the 1950's. They have since spread to more than 11 states. They can be found on both eastern hemlock, Tsuga canadensis and Carolina hemlock, T. caroliniana.

Hemlock woolly adelgid can now be found in most Maryland counties where hemlocks are planted or grow naturally. In Garrett County, where hemlocks are a common and ecologically important species, adelgids were first found in late December, 2001. Since then several additional isolated infestations have been identified.

Both landscape and forest hemlocks can become infested with adelgids. Hemlocks under stress, such as those in areas badly affected by drought, are more likely to decline and die. Several stands in Maryland have shown signs of decline.

An important part of hemlock woolly adelgid management is early detection; control will be more successful if done before adelgid populations reach damaging levels. The treatment of landscape hemlocks is much easier and more likely to succeed than the treatment of forests areas. Chemical control is often the best option for controlling adelgids in the landscape. Dormant oils can be used from November to March, and insecticides or insecticidal soap can be used from July through October. Whatever treatment is used it is most important to get thorough coverage of all infested parts of the tree. (For insecticide recommendations contact the University of Maryland, Cooperative Extension Service.)

Until recently there were few options available for controlling the adelgid in forests. Hemlocks in Maryland are usually found in inaccessible areas, such as along streams, where chemical control is often impractical or impossible, due to the chance of chemical drift into the water. Tree injections with insecticides is a new alternative that may allow valuable trees in riparian areas to be treated. MDA also is participating with the US Forest Service in evaluating the utility of biological control agents as a management tool. In Maryland, there have been two releases of beetles that feed on adelgids. MDA is still evaluating the success of these releases, and may do additional releases this spring.

MDA and DNR are cooperating to assess the adelgid impacts in Garrett County. Several state agencies are identifying at risk stands of hemlock that will be surveyed by MDA staff for the occurrence of the adelgids. Control options specific to the hemlock stand will be developed to minimize hemlock and habitat loss.

For additional information, contact MDA's Forest Pest Management at 410-841-5922, or visit the US Forest Service HWA website.

A copy of MISC's "Invasive Species of Concern in Maryland" is available on the internet at http://www.mdinvasives.org, or contact the MDA Plant Protection and Weed Management Section at 410-841-5920.