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Hemlock Woolly Adelgid

White Wooly egg masses on tree
White woolly egg masses
Photo: Connecticut Agriculture Experiment Station,
Aestivating first instars
HWA adults at the base of needles
Photo: Mark Whitmore, Cornell University

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.

How HWA Damage Hemlock Trees

Juvenile HWA, known as crawlers, search for suitable sites on the host tree, usually 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
juvenile crawler on tree branch
Close-up view of HWA ovisacs and a juvenile HWA
on a hemlock branch
Photo: Pennsylvania Dept. of Conservation
and Natural Resources - Forestry,

Risks to New York State Ecosystems

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.

Confirmed New York State Locations

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.

Prospect Mountain - Lake George, NY

In the summer of 2017, HWA was discovered in the Town of Lake George, the first known occurrence in the Adirondack Park. After several weeks of detailed surveys within a 5-mile radius of the three infested trees, a treatment plan was developed in accordance with the State Environmental Quality Review Act, a summary of which was published in the ENB on 9/20/17. In October, Division of Lands and Forests staff, with assistance from the Adirondack Park Invasive Plant Program (APPIP), conducted the insecticide treatment of 218 trees within a 7-acre area surrounding the original infestation. The objective was to kill HWA on infested trees, protect uninfected trees, and half the spread of HWA to new locations within the Adirondack Park. For more detailed information regarding insecticides, see sections below.

Current Control Efforts

Biological Control

A successful biological control effort in New York will require the establishment of an assortment of different predators, similar to those that exist within HWA's native ranges. Across the range of HWA in the eastern United States, several predators from Asia have been evaluated, approved and successfully introduced in HWA-infested areas. Here in New York State, Sasajiscymnus tsugae was released in the Catskills region beginning in the 1990s, with mixed success. More recently, our focus has turned to three more promising predators from the Pacific Northwest that may be better suited to New York's climate.

silver fly (leucopis species)
Silver fly (Leucopis species)
Photo: Nathan Havill, Research Entomologist,
USDA Forest Service Northern Research Station

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 (leaves DEC website) to further the research and application of biological control to HWA in New York, 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.

Chemical Control

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).

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.

Hemlock woolly adelgid egg masses
Photo: Chris Evans, The University of Georgia

How You Can Help

Preventative Action

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):

  1. Take pictures of the infestation signs as described above (include something for scale such as a coin or ruler).
  2. Note the location (intersecting roads, landmarks or GPS coordinates).
  3. Email report and photos to DEC Forest Health at 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.

Other ways you can report:

What To Do if Your Trees Have HWA

If you have confirmed your trees are infested with HWA, visit the Hemlock Initiative website (leaves DEC website) for tips on how to manage the infestation with chemical treatment. We recommend confirming the identification first 1) so that DEC is notified in case it is a new town detection and 2) because best treatment practices vary greatly among common hemlock issues. In order to make sure your treatment is going to be successful, you should know it is HWA first.

While DEC can not 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).


Progression of Hemlock Woolly Adelgid in NYS (1987 - 2018)