Hemlock Woolly Adelgid

Hemlock Woolly Adelgid

(Adelges tsugae)


Get Involved! Join the SLELO PRISM invasive species Volunteer Surveillance Network (VSN) and survey for HWA, or other invasive species. CLICK HERE to join.

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

Current NY State Hemlock Initiative Volunteer Positions. Help locate HWA infestations and healthy hemlock stands, monitor HWA phenology, host an HWA insectary CLICK HERE TO VIEW


Description: 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. It feeds on the tree’s  food storage cells. The hemlock “walls off” the area where the HWA is feeding, causing a disruption in nutrient flow and, eventually, killing the needle. Needles will dry out, gray, and fall off. Over several years, whole limbs will die back, killing the host tree from the bottom up. Stressed hemlocks are more likely to become infected with mortality typically occurring within 4 to 10 years after infestation by hemlock woolly adelgid.


Appearance: Adult HWA are 2mm long with a black, oval-shaped soft body.



Nymphs: Also called crawlers, they cause the most damage to host trees as they attach specialized mouth parts to the base of hemlock needles leading to leaf loss. Pictured below are HWA nymphs in their dormant phase which occurs during most of the summer months. They are tiny, flat, black and oval with a ring of white “fringe” around their perimeter.



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 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.

Photo Credits: Title photo: Connecticut Agricultural Experiment Station Archive, Connecticut Agricultural Experiment Station, Bugwood.org. Adult, nymph, and egg photos:UMassAmherst 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.

 For more information visit:

Informative Video Short: Spread by trade and climate, bugs butcher America’s forests

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

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: Catksills Hemlock Health-TNC Final Report 2014

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


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

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