Hemlock Wooly Adelgid

A pest of hemlock trees, hemlock woolly adelgid (HWA) is native to the Asian continent. HWA has not yet been found in Wisconsin but has become well-established in several counties in Western Michigan. HWA currently ranges from northern Alabama/Georgia to southern Maine and from northern California to southeastern Alaska.

In Asia, hemlock woolly adelgid does not cause much damage to native hemlocks, even when adelgid populations are high. Asian hemlock trees have developed natural resistance over time, and there are natural enemies in place that help regulate HWA populations.

There is some evidence to suggest that HWA has been in western North America long enough for western hemlock tree species to become resistant to the insect. Eastern hemlock trees do not have this resistance and are highly susceptible to HWA.

At present, hemlock woolly adelgid is established in only part of the eastern hemlock range in North America. On average, the insect spreads about 15-20 miles per year, the result of dispersal by wind, people, birds, and other animals. HWA survives very cold temperatures in its native range.  However, HWA spread and population buildup appear to be slower in colder parts of the northeastern U.S.

SIGNS AND SYMPTOMS

The white, cottony egg sacs of the hemlock woolly adelgid can be seen on the undersides of hemlock branches at the base of needles year-round. Hemlocks that are infested will develop needles that are yellow and eventually fall off, leaving dead, bare branches and thin crowns. Infested trees decline and die over several years. It is important to report trees with suspected HWA.