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Hemlock woolly adelgid

Hemlock woolly adelgid was accidentally brought to 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.


Where hemlock woolly adelgid has been found

A pest of hemlock trees, hemlock woolly adelgid (HWA) is native to the Asian continent. It has not yet been found in Wisconsin, but small localized infestations were recently discovered in Michigan in 2017. HWA was first detected in British Columbia, Canada in 1922 and in Oregon in 1924. It was later detected near Richmond, VA in 1951. It currently ranges from northern Georgia to southern Maine and from northern California to southeast Alaska.

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.

Native range of eastern hemlock (shown in green) and areas where hemlock woolly adelgid is present
and regulated as of October 2018 (shown in purple). Printable version of map.


Hemlock woolly adelgid’s life cycle and how it attacks trees

Adelgids are related to aphids and have a similar, complex life cycle. Hemlock woolly adelgid (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. They become adults and then produce an egg sac in June and July.

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 long as weather conditions allow. These immature adelgids become adults during late winter and early spring. Some of these adult adelgids are wingless and remain on hemlock trees. Others have wings and fly away in search of a spruce tree in which to lay their eggs. In North America the offspring of these winged adults don't develop successfully because there are no suitable spruce species present.

Woolly egg sacs
Woolly egg sacs. Wisconsin DNR photo by Linda Williams
Adult and eggs
Adult and eggs. Photo by Michael Montgomery, USDA Forest Service, from [exit DNR]


Preferred hosts of hemlock woolly adelgid

There are two hemlock species in eastern North America - eastern hemlock (Tsuga canadensis) and Carolina hemlock (T. caroliniana). Both have little resistance to hemlock woolly adelgid. Heavy infestations of the pest can kill trees within 4 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 because of infestation.

The hemlocks in western North America, western hemlock (Tsuga heterophylla) and mountain hemlock (T. mertensiana), are more resistant to hemlock woolly adelgid than eastern hemlock species. 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.

Impact of hemlock woolly adelgid

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

Signs and Symptoms

Hemlock woolly adelgid - what to look for

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.

Damaged Hemlock Healthy Hemlock Woolly egg sacs


How to prevent the spread of hemlock woolly adelgid

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.


On single trees

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.


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 negrinus, a predatory beetle found in the western U.S., also preys on hemlock woolly adelgids and is currently being released in the eastern states.

Sasajiscymnus larva attacks adelgid nymphs
Sasajiscymnus larva attacks adelgid nymphs. Photo by USDA Forest Service

Sasajiscymnus adult attacks adelgid nymphs
Sasajiscymnus adult attacks adelgid nymphs. Photo by USDA Forest Service

Last revised: Monday July 22 2019