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New York State Hemlock Initiative

Keeping the legacy alive

HWA: Basics

Photo by Connecticut Agricultural Experiment Station,

Hemlock Woolly Adelgid (HWA)

The hemlock woolly adelgid (ah-del-jid) (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.

Appearance: HWA looks like a tiny ball of cotton attached to twigs at the base of needles on hemlock trees. It is important to note that HWA is not found on needles, but on the twigs of hemlock trees. HWA is a 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.

Biology: Interestingly, HWA actually grows during the winter and through to early summer. During summer and fall HWA 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 November to April. You can find it by looking at the undersides of hemlock branches. For a more in-depth look at HWA, you can take a look at the USDA’s HWA Booklet on our HWA: In Depth page.

You may also view or hemlock and HWA identification guide for scouting in your own neck of the woods.

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

Why is HWA a threat?

HWA has caused widespread mortality in eastern and Carolina hemlock populations in the United States due to a lack of predators and low resistance among the trees.  This drastic decline in hemlocks poses a problem for forest composition, as many species of wildlife rely on hemlocks for food and shelter. It also may cause a decrease in water quality for human drinking water supplies. For more information on why conserving hemlocks is important in our forests, click here.

Where is HWA in New York?

HWA has moved through southern and western New York.  It was discovered in the Adirondacks in late July 2017 on Prospect Mountain near Lake George.  Click here to view a time-lapse map from the New York State Department of Environmental Conservation showing the spread of HWA in New York since 1985.  Originally, it was hoped that cold winters would prevent HWA from spreading north or into the Adirondacks, but the insect has demonstrated resistance to cold, reaching as far north as Nova Scotia.  Even if there is a temperature limit to HWA’s range, it will continue to spread and cause widespread mortality among hemlock populations of New York unless effective identification, treatment, and prevention protocols are adopted.

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