Hemlock Woolly Adelgid in the Finger Lakes: Saving Your Eastern Hemlocks

By Linda Marsh, Pests & Diseases. Last updated: June 25, 2026

If you have noticed tiny white cottony tufts on the undersides of your hemlock needles, you are not imagining it. Hemlock woolly adelgid (HWA) has spread into the Finger Lakes gorges and surrounding woodlands, and it is one of the most serious threats facing eastern hemlock in Upstate New York. The pest is small, but the damage is severe and often fatal if left untreated. The encouraging part: hemlocks respond well to treatment, and the window to act is wider than for most tree pests.

What does hemlock woolly adelgid look like?

The clearest sign is what gives the insect its name: small, round, white woolly masses about the size of a pencil tip, clustered at the base of the needles on the undersides of twigs. These are not eggs or fungus. They are the protective waxy coating the adelgid spins over itself. People often search for "white spots on hemlock needles" after spotting them in late fall through spring, which is exactly when the wool is most visible.

The adelgid itself is a soft-bodied insect smaller than a poppy seed. It feeds at the base of needles, drawing nutrients from the young twigs. Over time the tree responds by dropping needles, the canopy thins from the bottom up and the inside out, and new growth stalls. If you are seeing a hemlock dying in Upstate NY with grayish, thinning foliage, HWA is the first thing to rule out.

Why is HWA spreading through the Finger Lakes gorges?

Eastern hemlock is the defining tree of the Finger Lakes gorges. It shades the cool, steep ravines at Watkins Glen, Letchworth-area streams, and the shaded gullies that thread through Monroe County. That dense, moist habitat is exactly where hemlocks thrive, and unfortunately it is also where HWA spreads efficiently from tree to tree.

The adelgid moves on wind, birds, deer, and human activity. Our region historically got some protection from very cold winters, which kill back adelgid populations. As Zone 5b-6a winters have trended milder and less reliably frigid, more adelgids survive into spring, and infestations that once stayed knocked back are now persisting and expanding. The New York State Hemlock Initiative at Cornell has documented this northward and inland march for years.

Is your hemlock worth treating?

Almost always, yes. Eastern hemlock is slow growing and effectively irreplaceable on a human timescale. A mature hemlock screening your property line or anchoring a shaded slope took many decades to reach that size, and no nursery tree will replace it quickly. Hemlocks also hold soil on the steep, erosion-prone slopes common around the Finger Lakes and along glacial-clay ravines.

Compare that to the cost of treatment. Unlike emerald ash borer, where many ash trees are too far gone to save, HWA-infested hemlocks respond well to insecticide treatment even when they look stressed, as long as the canopy is not already mostly dead. A tree with green foliage remaining is a strong candidate. If you are weighing the decision and the same questions come up that homeowners face with ash, our guide on emerald ash borer in Monroe County walks through the same treat-or-remove logic that applies to hemlock.

How do you treat hemlock woolly adelgid?

There are three broad approaches, and they differ a lot in cost, longevity, and who should apply them.

Systemic options (soil drench and trunk injection) are the workhorses for saving valuable hemlocks. They move the active ingredient throughout the tree so the adelgid is poisoned as it feeds, and a single treatment can protect a tree for years. Trunk injection is often preferred near streams and the gorge-bottom seeps common in our region, because it places the product inside the tree rather than in the soil. These applications are best handled by professionals who can calculate dose by trunk diameter and follow ANSI A300 and the product label.

Can you treat HWA yourself with horticultural oil?

For a small, reachable hemlock with a light infestation, yes, horticultural oil or insecticidal soap can knock back the adelgid by smothering it. Timing is everything in Zone 5b-6a. The most effective sprays target the dormant or near-dormant window and again as activity resumes, with thorough coverage of needle undersides where the wool sits. Because contact sprays do not move through the tree, you must coat every infested twig, which is impractical on a tall tree.

If you are deciding whether a sprayed-on fix is enough or whether the tree needs a managed program, our explainer on what plant health care covers lays out how arborist-led insect management works for pests like HWA. The honest limitation: DIY oil buys a season, not years, and missed coverage lets the population rebound. For a large or high-value hemlock, systemic treatment by an arborist is far more reliable.

When should you call a certified arborist?

Call when the tree is large, when it sits near water, when the infestation is widespread, or when you simply want a treatment that lasts. A certified arborist can confirm the diagnosis (HWA can be confused with other needle problems), measure the canopy you have left to work with, and choose between soil drench and trunk injection based on your specific site. Our guide on when to call an arborist for tree insects lays out the warning signs that mean it is time to bring in a pro rather than spray again. And if your hemlock or other conifers already look gray and thinning, see why blue spruces lose their needles in Rochester to sort needle pests from disease and other causes of conifer decline.

FAQ

Will my hemlock die if I do nothing? Most likely, yes, over a span of years. Untreated HWA infestations typically kill eastern hemlocks as the canopy thins from the bottom up, though decline can take several years in our climate.

How long does one treatment last? A systemic soil drench or trunk injection can protect a hemlock for multiple years, depending on the product, the tree, and reinfestation pressure. An arborist will set a monitoring and re-treatment schedule.

When is the best time to treat in Upstate New York? Systemic soil drenches are typically applied in spring or fall, when soils are moist and the tree is actively taking up the product. DIY horticultural oil targets the dormant or near-dormant windows. Local timing varies, so confirm with an arborist.

Can I save a hemlock that already looks gray and thin? Often, if meaningful green canopy remains. A tree that is mostly dead is usually past saving, but partially declined hemlocks frequently recover after systemic treatment.

Sources

• New York State Hemlock Initiative, Cornell University: https://blogs.cornell.edu/nyshemlockinitiative/
• NYS Department of Environmental Conservation, Hemlock Woolly Adelgid: https://dec.ny.gov/nature/animals-fish-plants/insects-and-other-species/hemlock-woolly-adelgid
• USDA Forest Service, Biological Control of Hemlock Woolly Adelgid: https://research.fs.usda.gov/nrs/projects/biological-control-hwa
• Monster Tree Service of Rochester (Plant Health Care): https://www.monstertreeservice.com/rochester/