Hemlock Woolly Adelgid (Adelges tsugae)

Invader Type: 





What does Hemlock Woolly Adelgid look like? Learn here.


Pest Overview

The hemlock woolly adelgid  (HWA), Adelges tsugae, is a tiny insect from east Asia that attacks forest and ornamental hemlock trees. It feeds on young twigs, causing needles to dry out and drop prematurely. Trees may die in four to six years. Some survive, but with sparse foliage, losing value as shelter for wildlife and their ability to shade streams.

HWA was introduced to the United States in the 1920s to the Pacific Northwest, and in the early 1950s to the Washington DC and Richmond, Virginia areas. Since its introduction, HWA has spread throughout the eastern United States via wind, birds, mammals, human activities, and the transport of infected nursery stock, creating an extreme amount of damage to natural stands of hemlock, specifically eastern hemlock (Tsuga canadensis) and Carolina hemlock (Tsuga caroliniana).  

Hemlock is Vermont’s 7th most common tree. Unless it is kept in check by our cold winters, this insect will have a major impact as it spreads within Vermont.

What is being done about it?

Tactics used to combat hemlock woolly adelgid infestations include release of natural enemies like predatory beetles and pathogenic fungi, preserving genetic resistance, regulations to prevent the movement of infested hemlocks, and pesticide treatments.

What are we doing in Vermont?

The Vermont Department of Forests, Parks & Recreation and the Agency of Agriculture, Food, and Markets are collaborating to manage the insect. The U.S. Forest Service helps support these projects.

  • Surveys—Citizen volunteers have been trained and are assisting with detection surveys in Bennington, Windsor and Windham Counties.

  • Quarantines—A quarantine regulates the movement of hemlock from infested counties.

  • Control—We are participating in forest impact and management research projects in cooperation with the U.S. Forest Service and local universities.  A cold-hardy predatory beetle from the northwestern U.S. has been released in infested stands in Windham Countyand Bennington Counties.  We have also used pathogenic fungi and chemical treatments to slow the spread of the insect.

Reproductive Strategy / Lifecycle: 

To correctly identify and manage for hemlock woolly adelgid, it is important turn learn HWA biology.

Hosts and Signs / Symptoms: 

Hemlock woolly adelgid only affects hemlock trees, making identification important. Learn how to identify HWA.

Other pests affect also attack hemlock trees. Learn the signs and symptoms on HWA.



  • Familiarize yourself with hemlock woolly adelgid and look for potential infestations. The most obvious sign is a white, woolly mass found on the underside of twigs at the base of needles. If you’re interested in doing more, volunteer to participate in the citizen monitoring program.    

  • Remove bird feeders and baths from early April until August, to reduce spread by birds.              

  • Purchase trees from local nurseries that have been inspected by the VT Agency of Agriculture, Food, and Markets.        

  • If shipping hemlock products, stay abreast of changing quarantine regulations. 

  • Don’t rush to salvage hemlock. Our cold winter temperatures can be lethal to hemlock woolly adelgid.

  • Take action. Vermonters can learn more about how to get involved.



View the most current distribution map of Hemlock Woolly Adelgid  in North America.  Hemlock woolly adelgid was observed in Virginia in the early 1950’s and has now spread from Georgia to Maine. In 2007, it was found on native trees in Vermont for the first time. It is now known to occur in Windham and Bennington Counties.  It has also been detected in Windsor County.