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Hemlock woolly adelgid

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

Distribution

Map of hemlock woolly adelgid distribution
Hemlock woolly adelgid distribution.
Click for larger version
Map credit: USDA Forest Service

Where hemlock woolly adelgid has been found

Hemlock woolly adelgid has not been found in Wisconsin as of February 2010. This pest is native to Asia and the first North American reports of it were in British Columbia, Canada in 1922 and in Oregon in 1924. Hemlock woolly adelgid was found near Richmond, Va. in 1951. Today the insect is found from northern Georgia to southern Maine, and from northern California to southeast Alaska.

How hemlock woolly adelgid spreads

So far, hemlock woolly adelgid has only invaded part of the range of eastern hemlock in the United States and Canada. On average, the insect has spread about 15-20 miles per year. Wind, birds, animals, and accidental movement by people cause this rapid spread. In Asia, the insect is found in very cold climates so it is likely to live in most or all of the range of the eastern hemlock species.

Hemlock woolly adelgid is classified a prohibited species in Wisconsin under Chapter NR40.

Biology

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. WDNR Photo by Linda Williams
Adult and eggs
Adult and eggs. Photo by Michael Montgomery, USDA Forest Service, from www.forestryimages.org

Impact

Trees that hemlock woolly adelgid attacks

The two hemlock species found in eastern North America, eastern hemlock (Tsuga canadensis) and Carolina hemlock (Tsuga caroliniana), can be attacked and have little resistance to hemlock woolly adelgid damage. Heavy infestations can kill the tree within four to 10 years, and trees are also weakened and made vulnerable to attack by other insects and diseases. Some trees recover, although the reasons are not well understood.

The hemlocks in western North America, western hemlock (Tsuga heterophylla) and mountain hemlock (Tsuga mertensiana), are suitable hosts but are much more resistant to damage than the hemlock species in eastern North America. In Asia, hemlock woolly adelgid does not cause much damage to hemlock even though adelgid populations may become very high. This is due to host resistance and possibly natural enemies that help to keep the adelgid population down.

Impact of hemlock woolly adelgid

Hemlock is a long-lived, ecologically important species that provides habitat for many animals, birds, fish, and other plants. It is also an important ornamental species and helps to control erosion along streams. In areas of the eastern United States where much of the hemlock is dead or dying, there are additional environmental impacts such as altered forest structure, degraded fish habitat, and increased invasion of non-native plants.

Damaged Hemlock
Damaged Hemlock. Photo by James Johnson, Georgia Forestry Commission, from www.forestryimages.org
Healthy Hemlock
Healthy Hemlock. Photo by Paul Bolstad, University of Minnesota, from www.forestryimages.org

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 / 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
Damaged Hemlock. Photo by James Johnson, Georgia Forestry Commission, from www.forestryimages.org
Healthy Hemlock
Healthy Hemlock. Photo by Paul Bolstad, University of Minnesota, from www.forestryimages.org
Woolly egg sacs
Woolly egg sacs. WDNR Photo by Linda Williams

Prevention

How prevent the spread of hemlock woolly adelgid

Keeping hemlock woolly adelgid from traveling to new areas seems simple – do not move infested hemlock wood or products.

Wisconsin does not have hemlock woolly adelgid, and we want to prevent it from coming here. Quarantines and inspections are in place to stop the movement of potentially infested items, and people should not move hemlock nursery stock, logs or firewood from the eastern states where hemlock woolly adelgid is present. Michigan found hemlock woolly adelgid at several sites in 2010. The state is attempting to eradicate the insect from those areas.

Management

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.

Biocontrols

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: Wednesday September 05 2012