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Hemlock Woolly Adelgid

Photo by Mark Whitmore, Cornell University.

The Hemlock Woolly Adelgid programs at Cornell are managed through the NYS Hemlock Initiative. Please visit for more information on HWA in New York, HWA management and hemlock conservation strategies around the state.



The hemlock woolly adelgid (HWA, Adelges tsugae) is an aphid-like, invasive insect that poses a serious threat to forest and ornamental hemlock trees (Tsuga spp.) in eastern North America. HWA are most easily recognized by the white “woolly” masses of wax, about half the size of a cotton swab, produced by females in late winter. These fuzzy white masses are readily visible at the base of hemlock needles attached to twigs and persist throughout the year, even long after the adults are dead.

Hemlock woolly adelgid has been in New York for at least the past 20 years. Originally confined to the lower Hudson Valley, it has since moved north to near Albany and west of Buffalo. It was detected in the Finger Lakes region in 2008; subsequent investigation revealed that it was largely confined to the southern parts of Seneca and Cayuga Lakes. In the intervening years HWA has spread, and is now found at higher elevations south of Cayuga Lake and nearly to the north ends of Seneca and Cayuga Lakes; it’s also found in the Rochester area


HWA on eastern hemlock: closeup. Photo by Mark Whitmore, Cornell University.
Cottony balls produced by HWA on eastern hemlock twig. Photo by Mark Whitmore, Cornell University.

Hemlock Woolly Adelgid (HWA)

HWA looks like a tiny ball of cotton attached to twigs at the base of needles on hemlock trees. This tiny, aphid-like insect produces a dense mass of waxy hairs to protect it during cold weather, which is the cottony ball that’s visible on the twig. An interesting aspect of its biology is that it actually grows during the winter and through to early summer. During summer and fall is very tiny, without the waxy fluff; it finds a place at the base of a needle and waits for the end of fall, when it will begin to grow and produce waxy fibers covering its body. The best time for HWA detection is between February and May. You can find it by looking at the undersides of hemlock branches.


HWA thinning, Robert L Anderason, USDA Forest Service, bugwood
HWA damage causes thinning in hemlock canopy. Photo by Robert L Anderason of the USDA Forest Service, courtesy of

Managing HWA

So what can you do if you have hemlocks on your property? The most important thing is to see if your hemlocks have HWA on them; if you don’t have the bugs, you don’t need to treat. Examine the branches and compare them with the photo included here and others on our website. Once you’ve determined that you have HWA, don’t panic! You have time to act. In many cases even trees whose canopies have thinned can be brought back to full health. We recommend consulting with a licensed arborist and registered pesticide applicator to save your trees, but there is an effective product that homeowners can purchase at the local garden store and use themselves. Always read and follow carefully instructions on the label of any pesticide you are using.

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