Hemlock Woolly Adelgid

Adelges tsugae

Images of this species:

Common look-alikes:

Identification

Hemlock woolly adelgid (HWA) is a small, aphid-like insect that feeds on hemlock species in North American. It has "wool" attached to the twig of hemlock trees, not attached to the needles. HWA is immobile when covered in wax - they are waxy, not silky or stretchy. HWA is wispy like a cotton ball and does not look painted on like pine sap.

signs and symptoms

White, cottony balls at the base of hemlock tree needles
Yellowing needles, needle fallout, branch dieback, and crown thinning

See image slideshow above for signs and symptoms.

Biology

Origin

The hemlock woolly adelgid (HWA), Adelges tsugae, is a tiny insect from east Asia that attacks forest and ornamental hemlock trees. 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).

Life Cycle

The life cycle of HWA is complex. It involves asexual and sexual reproduction and two different host tree species. On Eastern hemlock trees, HWA produces two generations per year: a spring generation (progrediens) and a winter generation (sistens). The progrediens has two forms, a wingless form that remains on hemlock trees and a winged form (sexuparae) that searches for a suitable host spruce tree upon which it will start a sexual reproductive cycle. Each generation has six stages of development: egg, four juvenile (or nymph) stages, and adult.

The life cycle of the HWA.

ECOLogical threat

HWA 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 a shelter for wildlife and their ability to shade streams. 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.

Biological controls are being used to reduce the impact of hemlock woolly adelgid across the landscape. Two biological control agents have been used in Vermont: the Laricobius nigrinus beetle and the Lecanicillium muscarium fungus.

Management Options

Look for hemlock woolly adelgid on hemlocks purchased for planting, and wherever hemlock is growing, especially in southern Vermont. We are still tracking this recent invader, so please report suspects.
Don’t move infested or potentially infested material. Some states, including Vermont, have a hemlock woolly adelgid quarantine that regulates the movement of hemlock plants and wood products.
Slow the spread of HWA by removing bird feeders from early April to August. Birds offer easy and convenient transportation for the HWA. This is presumably how HWA first spread to southern Vermont.
High-value trees can be treated. Insecticides don’t provide long-term protection, so treatments will need to be repeated.
Where hemlock woolly adelgid has not been found, there is no need to alter forest management in anticipation of the insect. Given Vermont’s cold climate, it is unclear how extensive its impact will be. We do know that it takes years to affect tree health, and that even unthrifty hemlocks provide habitat benefits that will disappear if the trees are removed.
For more information, consult:

Hemlock Woolly Adelgid in Vermont: Recommendations for Landowner Response

Managing Hemlock in Northern New England Forests Threatened by Hemlock Woolly Adelgid and Elongate Hemlock Scale