Hemlock Woolly Adelgid

Adelges tsugae

Protect hemlock trees, join the SLELO PRISM invasive species Volunteer Surveillance Network (VSN)

Hemlock woolly adelgid (HWA) (Adelges tsugae) is a small aphid-like insect native to Asia that is now threatening eastern hemlock (Tsuga canadensis) and Carolina hemlock (Tsuga caroliniana).

Impacts: HWA threatens hemlock trees, genus Tsuga. HWA feeds on hemlock tree sap by attaching their specialized mouthparts into the base of hemlock needles. This triggers a defense in host trees to “wall off” these incisions (similar to a scab growing over a cut) and over time the feeding of HWA  weakens the host tree and leads to tree mortality. Infested trees will display signs of distress such as needle loss, lack of regrowth in the spring, and limb damage or loss. Hemlocks that experience environmental stresses, such as drought, are more susceptible to an HWA infestation. In general, untreated hemlock infestations will cause host trees to die off between four to ten years, depending on infestation density and the overall health of the host tree prior to infestation.

Signs of Infestation: The seasonal signs of HWA described below may be more or less apparent depending on the location of the host hemlock(s). HWA populations on cooler sites, such as on north-facing slopes, or in higher elevations/latitudes, may awaken from their dormant aestivation stage later than those on sites with other aspects, or in lower elevations/latitudes.

SLELO PRISM HWA seasonal signs damage and hemlock ID guide
From late spring to early fall look for small black dots with white halos around them at the base of needles where the needle attaches to the twig (view left photo below).
From late fall to early spring look for white woolly masses at the base of hemlock tree needles where the needle attaches to the twig (view right photo below).

If you see these signs (white woolly masses or HWA crawlers), report observations via www.iMapinvasives.org

or contact the NYSDEC Forest Health Information Center: 1-866-640-0652
*Record the GPS coordinates of the location & take clear photos of the suspected signs


Appearance: Adult HWAs are 2mm long with a black, oval-shaped soft body and a white “halo” or fringe around their bodies.


Eggs: Very small, brownish-orange, laid in white, woolly masses of wax



Stress on infested hemlocks can be reduced by pruning dead or dying branches and watering trees during dry periods. This will reduce the chances of infestation by HWA. Note: Avoid fertilizing hemlocks with a nitrogen fertilizer as it will encourage HWA reproduction and survival.


Individual trees can be treated with a systemic insecticide, applied as a soil drench or basal bark spray. Once applied, the treatment can provide years of protection for the tree.

Hemlock foliage may also be treated with horticultural oils and insecticidal soaps as long as the trees are small enough to be saturated, ensuring that the HWA has been exposed.

NYS Hemlock Initiative: HWA Management


Several HWA predators, including beetles and fungal pathogens, are being tested for effectiveness in treating infested hemlocks on a large scale. While more research is needed to assess long-term feasibility, studies have shown this to be a promising method of controlling HWA.


More Information:


Learn about state-wide efforts to manage HWA
 New York State Hemlock Initiative  

Current NY State Hemlock Initiative Volunteer Positions. CLICK HERE TO VIEW




Mountain Lake PBS covers HWA found in the Adirondacks for the first time

Spread by trade and climate, bugs butcher America’s forests (Informative Video)



Field Guides/Survey Protocols: 

SLELO PRISM’s Guide to Seasonal Signs of HWA & Hemlock ID.

Hemlock Initiative HWA Survey Protocol

Hemlock Initiative HWA Field Guide

Hemlock Initiative Land Owners Guide to HWA


Suggested Survey Sites: 

Suggested Hemlock Woolly Adelgid Survey Sites


Management Guidance: 

Expert Advice on EAB and HWA: Mark Whitmore, Cornell University

The Hemlock Woolly Adelgid in New York State

HWA pesticide treatments summary

USDA Managing Hemlock in Northern New England Forests 

View presentations from HWA workshop(s)

USDA Forest Service: http://na.fs.fed.us/spfo/pubs/pest_al/hemlock/hwa05.htm

Penn State College of Agricultural Sciences: Entomology Department: http://ento.psu.edu/extension/factsheets/hemlock-woolly-adelgid

The Nature Conservancy and Catskills Invasive Species Partnership: Catskills Hemlock Health-TNC Final Report 2014

Eastern Connecticut Forest Landowners Association Wolf Den Land Trust:  http://www.ecfla.org/articles/adelgid.htm

Native Look-A-Likes:

NYS DEC Native Hemlock Pests 

Michigan DEQ Hemlock Woolly Adelgid Look-Alikes



Photo Credits: Title photo: Connecticut Agricultural Experiment Station Archive, Connecticut Agricultural Experiment Station, Bugwood.org. Adult, nymph, and egg photos: UMass Amherst Center for Agriculture Food and the Environment, https://ag.umass.edu/fact-sheets/hemlock-woolly-adelgid. Crawler photo: Shimat Joseph, University of Georgia, Bugwood.org, Pennsylvania  Department of Conservation and Natural Resources – Forestry Archive, Bugwood.org.

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