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USDA Forest Service - Forest Health Protection - Hemlock Woolly Adelgid

Rutgers Cooperative Research and Extension Fact Sheett

Hemlock Woolly Adelgid

The Hemlock Woolly Adelgid, a tiny aphid-like insect from Asia, was first discovered in the Pacific Northwest in the 1920’s. By the early 1950’s it was discovered in Virginia and has since been found as far north as Rhode Island. Its preferred host tree is hemlock, but it may also attack spruce.

Tree Effects
The Hemlock Woolly Adelgid nymphs and adults feed on sap from the tree’s twigs. The tree drops its needles and, if left uncontrolled, the adelgid can kill a tree within a year.


A tree infested with Hemlock Woolly Adelgid will exhibit gray-green needles and cotton-like wool tufts under the needles. By frequently inspecting trees for signs of Hemlock Woolly Adelgid, a homeowner can intervene in a timely manner and possibly prevent the tree from dying.

A homeowner can mechanically remove both eggs and adults by spraying the twigs with water or pruning the most heavily infested branches from the tree. An infested tree can be successfully treated any time except when needles emerge. Spray the affected tree thoroughly with horticultural oil. Horticultural oils are highly effective in killing adelgids, yet relatively safe to the applicator, beneficial insects, and the environment.

To prevent infestation, homeowners should place all bird feeders away from hemlock trees because birds, as well as squirrels and deer, can spread the adelgid from tree to tree. Since the adelgid affects stressed trees more quickly, homeowners should water their hemlocks in drought.

Hemlock Wooly Adelegid Crawler
Cotton-like Tufts
Hemlock Woolly Adelgid Control