Hemlock Woolly Adelgid
The Hemlock Woolly Adelgid (Adelges tsugae), often abbreviated as HWA, is a very small (1/32″ long) invasive Hemipteran (‘true bug’) that feeds on sap that it sucks from hemlock trees (Tsuga spp.). It was accidentally introduced to western North America from its native Asia in 1924, and eventually–decades later–worked its way to the eastern United States, where it has caused widespread mortality of hemlock trees. Over one-third of the native range of hemlock in the eastern US is now infested with HWA. A quarantine has been established for Maine, New Hampshire, and Vermont. Nevertheless, southern areas of all three of these states now have pocket HWA infestations. Note: The revision of the Hemlock Woolly Adelgid Quarantine Rule went into effect as of 9/25/2013.
The desiccation caused by HWA feeding causes the tree to lose its needles and not produce new growth. In the northern portion of the hemlock’s range, the color of the hemlocks infested by HWA will typically change from their healthy, dark green color to a sickly, grayish-green color after just a few years, and death typically occurs four to ten years after an initial infestation. Any trees that do manage to survive the direct effects of a HWA assault are usually weakened to the point where they may die from secondary causes.
HWA reproduces asexually and, in North America, goes through two generations per year. The egg sacs are what people look for when searching for signs of HWA infestation, because they are very distinctive, resembling small tufts of cotton that cling to the underside of the hemlock branches. Between 100 and 300 eggs are laid in each one of these sacs! Larvae emerge in the spring and can spread on their own or with the assistance of wind, birds and/or people or other mammals. Avoid placing bird feeders in or near hemlocks to discourage birds from landing there and potentially spreading HWA larvae.
Photos and Additional Information:
- Hemlock Woolly Adelgid (Maine Forest Service) — see also Hemlock Woolly Adelgid in Maine
- Hemlock Woolly Adelgid (US Forest Service) (includes a video: Impacts of the Hemlock Wooly Adelgid)
- Hemlock Woolly Adelgid Publications US Forest Service, Northeastern Area)
- Hemlock Woolly Adelgid (University of Vermont)
- Hemlock Woolly Adelgid (University of Kentucky)