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Hemlock Woolly Adelgid

Photo by Elizabeth Willhite, USDA Forest Service,

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Common Name: Hemlock woolly adelgid (HWA)
Scientific Name: Adelges tsugae
Origin: Asia


HWA is a tiny, aphid-like insect that was first reported in the eastern United States in the 1950’s. HWA is less than 1/16″ long, and is dark reddish-brown to purplish-black in color. Its name derives from a white “wool-like” covering, called an ovisac, on mature egg-laying adults.


HWA develops and reproduces on all hemlock species, but attacks the eastern and carolina hemlocks only.


The adelgid causes hemlock decline and mortality within 4-10 years of infestation in its northern range. During the fall and winter months, developing adelgids feed on young twig tissue, including starch reserves critical to the tree’s survival.


Moving bird feeders away from hemlocks, removing infested trees from woodlots, and state quarantines may help to slow the spread, while insecticides and biological controls can be used for local eradication.


The white, woolly ovisacs can be easily identified on the undersides of hemlock branch tips from late fall to early summer. Infested trees may appear grayish from a distance.

Regional Distribution

Early Detection

WNY PRISM Priority

Tier 3 – Containment

Invasive Species Priorities – Tier Chart


Hemlock woolly adelgid was introduced to the United States through the Nursery and Landscaping industry, through contaminated plant shipments. The insects are spread by attaching to boots, clothing and birds.


Hemlock woolly adelgid is a prohibited species in New York State – for more information on Prohibited and Regulated Species, visit


New York State Hemlock Initiative