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Hemlock Woolly Adelgid

Hemlock woolly adelgid was accidentally brought to eastern North America and is a threat to eastern hemlocks. Learn where this pest is now, what it looks like and how to prevent its spread to new areas.


A pest of hemlock trees, hemlock woolly adelgid (HWA) is native to the Asian continent. It has not yet been found in Wisconsin, but has become well-established in several counties in western Michigan [exit DNR]. HWA currently ranges from northern 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.

View a map [PDF exit DNR] showing areas where hemlock woolly adelgid is regulated.

How hemlock woolly adelgid spreads

So far, hemlock woolly adelgid is established in only part of eastern hemlock's 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. In Asia, the insect survives very cold temperatures, so it is likely to endure conditions in North America.

Hemlock woolly adelgid is classified as a prohibited species in Wisconsin's invasive species rule NR 40, Wis. Admin. Code.


Adelgids are related to aphids and have a similar, complex life cycle. HWA (Adelges tsugae) is parthenogenic, which means all individuals are female and produce offspring without mating. In North America there are two generations per year.

In the spring, adelgids hatch from a white, cottony egg sac that may contain as many as 300 eggs. The adelgids crawl around until they settle at the base of a needle and begin to suck nutrients from it. Their saliva is toxic to the tree and eventually causes the needles to drop and the twigs to dieback. They will usually remain at that feeding site for the rest of their lives. Each adult produces an egg sac in June and July.

Eggs sacs of hemlock wooly adelgid on hemlock twigs.
Woolly egg sacs on hemlock twigs. Photo by Bill McNee, taken in Maryland.

The second generation hatches from these eggs, begins to feed and then enters a dormant period for the rest of the summer. Being a cool weather species, feeding starts again in October and continues as weather conditions allow. These immature adelgids become adults during late winter and early spring. Some of these adults are wingless and remain on hemlock trees. Others fly away to lay their eggs on spruce trees. There are no suitable spruce species in North America so these offspring do not develop successfully here.

Impact in Wisconsin

Eastern hemlock (Tsuga canadensis) is the only hemlock species present in Wisconsin, and it has little resistance to HWA. Heavy infestations can kill trees within four to 10 years, during which time infested trees will likely be further weakened by other insects and diseases. Although some trees recover, the mechanisms that allow for this are poorly understood. Most trees eventually die. More information on eastern hemlock in Wisconsin is available from the University of Wisconsin-Green Bay Herbarium [exit DNR].

Hemlock is a long-lived, ecologically important tree that provides habitat for many wildlife species. It also helps to control stream erosion and is an important ornamental tree. In dying hemlock forests in the eastern United States, additional environmental impacts are becoming common, such as altered forest structure, degraded fish habitat and increased invasion of non-native plants.

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 in late winter and early spring. Hemlocks that are infested will develop needles that yellow and eventually fall off, leaving dead, bare branches and thin crowns. Infested trees decline and die over several years.

A stand of hemlock trees, some browned and dying.
Hemlock tree mortality from HWA. Photo by Jason Van Driesche,

It is important to report trees that are suspected of having HWA. If you suspect that a tree is infested, please contact the Wisconsin Department of Agriculture, Trade and Consumer Protection (DATCP) Pest Reporting Hotline at 1-866-440-7523 or For more information, visit the DATCP HWA webpage [exit DNR].


The best way to prevent or slow the spread of hemlock woolly adelgid is to make rigorous efforts to avoid moving it to new areas. Public awareness and cooperation play important roles, and quarantines and mandatory inspections are also used to prevent hemlock woolly adelgid’s spread on potentially hazardous hemlock wood and products. Suspect materials include hemlock nursery stock, logs and firewood from eastern states where hemlock woolly adelgid is present. Learn more about the hemlock woolly adelgid quarantine in Wisconsin [exit DNR].


In infested areas, the insect can be managed on individual trees through the use of insecticides, horticultural oils and insecticidal soaps. Keeping ornamental hemlocks well-watered and healthy can help them to withstand an infestation.

In forests

Widespread insecticide treatment in forests is not practical and salvaging dead or dying trees is the most common management technique. Some infested states apply limited insecticide treatments at sites where hemlocks have especially high ecological value.


One approach to managing hemlock woolly adelgid has been to introduce natural enemies from the insect's native range in Asia. One predatory beetle (Sasajiscymnus tsugae) has been mass reared and released in the eastern U.S. Its adults and larvae prey on hemlock woolly adelgid and help to reduce its numbers. In North America there are a few native predators but they do not eat enough of the adelgids to prevent damage to hemlocks. Laricobius nigrinus, a predatory beetle found in the western U.S., also preys on hemlock woolly adelgids and is currently being released in the eastern states.