Native to Asia, the hemlock woolly adelgid, or HWA, is an invasive, aphid-like insect that attacks North American hemlocks. HWA are very small (1.5 mm) and often hard to see, but they can be easily identified by the white woolly masses they form on the underside of branches at the base of the needles.
In winter, HWA produce white, cotton-like "woolly" masses at the base of hemlock needles. You can find these woolly masses throughout the year.
Juvenile HWA, known as crawlers, can usually be found at the base of the needles. They insert their long mouthparts and begin feeding on the tree's stored starches. HWA remain in the same spot for the rest of their lives, continually feeding and developing into adults. Their feeding severely damages the canopy of the host tree by disrupting the flow of nutrients to its twigs and needles. Tree health declines, and mortality usually occurs within 4 to 10 years.
All species of hemlock are vulnerable to attack, but severe damage and death typically occurs in eastern (Tsuga canadensis) and Carolina (Tsuga caroliniana) hemlocks only. Eastern hemlock is the most common species of hemlock in New York State.
Signs Of An Infestation
- White woolly masses (ovisacs) about one-quarter the size of a cotton swab on the underside of branches at the base of needles
- Needle loss and branch dieback
- Gray-tinted needles
Removal of hemlocks from New York state ecosystems can dramatically change ecosystem processes and may result in the loss of unique plants and wildlife.
Hemlocks are ecologically important due to the unique environmental conditions they create under their dense canopies. These cooler, darker and sheltered environments are critical to the survival of a variety of species that rely on them for food, protection, and ideal growing conditions. Moose, black bears, salamanders, and migrating birds, as well as unique lichen and plant communities, are all closely associated with the hemlock ecosystem.
Well-suited for growing on steep slopes where not many other species can grow, hemlocks stabilize shallow soils and provide erosion control. In addition, they are often found along streams, where their shade helps moderate water temperatures, maintaining a suitable environment for cold-water species such as trout. Removal of hemlocks from NYS ecosystems can dramatically change ecosystem processes and may result in the loss of unique plants and wildlife.
Distribution and Habitat
HWA was introduced to the western United States in the 1920s. It was first observed in the eastern US in 1951 near Richmond, Virginia after an accidental introduction from Japan. HWA has since spread along the East Coast from Georgia to Maine and now occupies nearly half the eastern range of native hemlocks. HWA was first discovered in New York State in 1985 in the lower Hudson Valley and on Long Island. Since the initial infestation, HWA has continued to spread north to the Capital Region and west, through the Catskill Mountains and the Finger Lakes Region, into western NY. Most recently, HWA was found in the Adirondack Park.
Several insects from Asia have been studies, approved, and successfully introduced in HWA-infested areas. These insect predators serve as biological controls as they feed on HWA and reduce their populations. Three insect species from the Pacific Northwest continue to be introduced and their effectiveness as long-term solutions continue to be evaluated.
The beetle Laricobius nigrinus has been released at various locations in the Finger Lakes and Catskills Regions beginning in the 2000s, and has generally been more successful at establishing than Sasajiscymnus tsugae. The silverflies, Leucopis argenticollis and Leucopis piniperda, are a more recent addition to the arsenal. Since 2015 just a handful of releases of these species have occurred in New York. These release sites are still being monitored for establishment and effectiveness.
DEC is partnering closely with the New York State Hemlock Initiative to continue to research and apply the use of insect predators of HWA by:
- providing funding for a biocontrol research and rearing laboratory at Cornell;
- sharing HWA survey data;
- evaluating potential release locations; and
- collaborating on outreach and control projects.
Specific chemical insecticides can be used to treat an already infested tree or as a preventive measure in a high-risk infestation area. They are useful for treating individual, ornamental, or high-value trees, but are not practical or economical in a forest setting.
Two insecticides that have shown promising results are Imidacloprid and Dinotefuran. Both must be applied by a licensed pesticide applicator, and either can kill HWA on its own. Applying both insecticides to an infested tree, however, combines the immediate effectiveness of the fast-acting Dinotefuran with the long-term protection of Imidacloprid, leaving the tree adelgid free for up to seven years. For more information about chemical control of HWA, please see NYSDEC's Frequently Asked Questions document (PDF).
Things to Know Before Treating Your Hemlock Tree
Before beginning treatment:
- It is important to confirm your trees are infested with HWA to make sure your treatment is going to be successful, you should know it is HWA first.
- Report HWA findings to DEC
- Visit the Hemlock Initiative website for tips on how to manage the infestation with chemical treatment.
While DEC cannot recommend specific businesses, we do provide tips for finding a professional arborist or tree service, some of whom have a licensed pesticide applicator on staff.
Integrated Pest Management
The most effective management strategy for controlling HWA combines the short-term protection of insecticides with the long-term solution of biological control agents. As research continues the effectiveness of natural enemies to control HWA populations, chemical insecticides can keep trees alive and free of infestation until natural enemies take over.
How You Can Help
You can help slow the spread of HWA in our forests by cleaning equipment or gear after it has been near an infestation, and by leaving infested material where it was found.
How to Report
If you believe you have found HWA in a town where it is not yet known to be (see map on iMapInvasies):
- Take pictures of the infestation signs as described above (include something for scale such as a coin or ruler).
- Note the location (intersecting roads, landmarks or GPS coordinates).
- Email report and photos to DEC Forest Health at [email protected]. Photos are critical for helping us identify if it is HWA. You may choose to call the Forest Health Information Line at 1-866-640-0652 instead.
- Additionally, you can report the infestation using the IMapInvasives tool, or contact your local Partnership for Regional Invasive Species Management (PRISM).
Other ways you can report:
- Report the infestation using the iMapInvasives tool.
- Contact your local Partnership for Regional Invasive Species Management (PRISM).