Hemlock Woolly Adelgid

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 injecting 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 within four to ten years, depending on infestation density and the overall health of the host tree prior to infestation.

Signs of HWA Infestation:

Calling all Hikers and Showshoers, We Need Your Help!

Help keep an eye out for hemlock woolly adelgid (HWA) while you’re out hiking and snowshoeing. The story-map below showcases hiking trails where there are hemlock trees to monitor for signs of HWA. Detecting hemlock woolly adelgid early on will aid spread prevention and control efforts for this invasive pest.

If you see signs of HWA presence, 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 


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

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



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.

For more details about chemical treatments visit, 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. 

For more details about biocontrol for HWA visit, NYS Hemlock Initiative: Biocontrol Research

More Information:

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.


Prevent the introduction of invasive species into the SLELO PRISM.

Rapidly detect new and recent invaders and eliminate all individuals within a specific area.

Share resources, including funding personnel, equipment, information, and expertise.

Collect, utilize, and share information regarding surveys, infestations, control methods, monitoring, and research.

Control invasive species infestations by using best management practices, methods and techniques to include: ERADICATION (which is to eliminate all individuals and the seed bank from an area), CONTAINMENT (which is reducing the spread of established infestations from entering an uninfested area) and SUPPRESSION which is to reduce the density but not necessarily the total infested area.

Develop and implement effective restoration methods for areas that have been degraded by invasive species and where suppression or control has taken place.

Increase public awareness and understanding of invasive species.

Develop and implement innovative technologies that help us to better understand, visualize, alleviate or manage invasive species and their impacts or that serve to strengthen ecosystem function and/or processes.

Rob Williams
PRISM Coordinator

Megan Pistolese
Outreach and Education

Brittney Rogers
Aquatic Invasive Species

Robert Smith
Terrestrial Invasive Species