Hemlock Woolly Adelgid

Originally from Asia, Hemlock Woolly Adelgid (HWA) was first discovered in New Hampshire in Portsmouth in 2000. This small, wingless insect uses its piercing mouth-parts to feed in the xylem of the small hemlock twigs. HWA, left untreated, can kill a tree in 4 to 10 years. Untreated outbreaks of HWA weaken the tree and leave it susceptible to damage from other pests, such as elongate hemlock scale and hemlock borer. Maintaining trees in a healthy condition lessens damage by other pests.

Hemlock woolly adelgid

Hemlock woolly adelgid, Elizabeth Willhite, USDA Forest Service, Bugwood.org

HWA populations can be found in more than half of the towns in New Hampshire and every county except Coös. This pest can be introduced into new areas by birds and other wildlife, and through human activities, such as the movement of infested nursery stock and forest products. Spread of HWA on nursery stock and forest products is prohibited by existing nursery stock (RSAs 433:28-433:30) and forestry regulations (RSA 227-K:17) which prohibit the sale of nursery stock and forest products infested with HWA. HWA is also a prohibited invasive insect in the state (Agr 3802.01), so that it is not permissible to transport live HWA except for purposes of destruction. 

N.H. Division of Forests and Lands has an action plan in response to HWA.

There are extensive best management practices for nurseries to prevent the spread of HWA and elongate hemlock scale.

Best management practices for forest products include:

  • Completely delimb all hemlock roundwood.
  • Power wash all equipment between jobs at sites where hemlock was harvested.
  • Consider documenting your use of BMPs with a compliance agreement with the N.H. Division of Forests and Lands.

NH Towns with Detections - Alphabetical List
NH Towns with Detections - Map by Year
NH Towns Surveyed for Hemlock Woolly Adelgid in 2022

Hemlock Tree Treatment Options

A licensed pesticide applicator can be hired to treat your hemlock trees.  When seeking the help of an arborist, you can ask if they are licensed by State of NH Division of Pesticide Control or contact the State of NH Division of Pesticide Control at (603) 271-3550  to find licensed applicators in your area. Resources for selecting an arborist in NH and a link to the NH Arborists Association can be found by clicking below.


Harvard Forest ecologist David Orwig discusses threats to our native forests.

Harvard Forest ecologist David Orwig discusses threats to our native forests.