Image by R. Childs, UMass

Close-up of the orange colored HWA eggs within a woolly wax mass

Hemlock Woolly Adelgid

The Hemlock Woolly Adelgid (HWA) is an aphid-like invasive insect that is native to Japan that affects both forested and ornamental hemlock and spruce trees. HWA was first detected on the east coast of the United States in the 1950s in Richmond, Virginia. Since then, it has spread to 18 states on the east coast from Maine to Georgia affected both Native eastern and Carolina hemlock trees. HWA was first detected in New York State in the 1980s and has spread throughout the state on Long Island, the Hudson Valley, Rochester, the Catskills and most recently, into the Finger Lakes regions.

HWAs have a complex life cycle with six different stages of development: four nymph stages and an egg and an adult stage. Adult females found in the winter are black, oval and soft-bodied and about 2mm long. HWAs are generally found within the white woolly masses of wax they secrete from their glands that are about half the size of a cotton swab tip. The eggs, laid in the spring, are brownish to orange in color and almost microscopic at 0.25mm long by 0.15mm wide. Newly hatched nymphs are reddish brown with a speck of white on the edge and are generally less than 0.5mm in length.

The hemlock woolly adelgid feeds deep within the plant tissues of hemlock tree needles by inserting their mouthparts into the underside of the base of the needles. These mouthparts drain the tree of its food storage cells which the tree responds to by walling off the wound and disrupting the flow of nutrients to the effected needles and twigs. These effected needles will dry out and loose color shortly before falling off the tree. Dieback of major limbs of hemlocks with HWA presence can occur within two years of infestation and will generally affect the tree from the bottom upward.

Eastern Hemlock trees play a unique ecological role in eastern forests by creating a cool, damp and shaded microclimate to support plant communities and provide important winter habitats for wildlife. Research in 2005 found that declines in hemlock caused by HWAs can result in drastic changes to the ecosystem including the loss of plant and animal species presence.

Currently, HWA management and management of HWA infestations include the use of chemical insecticides and the use of a biological control, or introduction of a natural predator species to the HWA.


New York State Hemlock Initiative, This website from Cornell University focuses on current research, management and community science initiatives to conserve NYS’s hemlock populations from threats, particularly hemlock woolly adelgid (HWA).

Hemlock Woolly Adelgid Species Profile, This webpage from the New York Invasive Species Information website details science-based information about this invasive species which includes information about the origins, biology, potential impacts, current management practices and the distribution of this species throughout New York State.

DEC – Hemlock Woolly Adelgid, This webpage from the New York State Department of Environmental Conservation, highlights actions that can be taken to prevent the spread of HWA and report sightings of the species in your local community. It also details the current biological and chemical controls of this species and the risk it poses to the New York State Ecosystem.

Hemlock Woolly Adelgid Fact Sheet (PDF), This two-page fact sheet created by the DEC details the common frequently asked questions about HWA and includes identifying information through descriptions and imagery as well as information on what to do if you encounter HWA infestations.

USDA Forest Service- HWA, This webpage produced by the United States Department of Agriculture Forest Service through the Northern Research Station explores the Forest Disturbance process that the HWA contributes to. Here you can find research by scientists and other collaborators from the Northern Research Station focused on four major areas: risk, detection and spread; biology and ecology; control and management; and effects and impacts

Last updated December 13, 2023