Other Exotic Forest Threats - Hemlock Woolly Adelgid

Hemlock woolly adelgid, Adelges tsugae, was accidentally introduced to North America and is currently a great threat to eastern hemlocks. Native to Asia, the first North American reports were in British Columbia, Canada in 1922 and in Oregon in 1924. Hemlock woolly adelgid was found near Richmond, Virginia in 1951. The insect is now found from northern Georgia to southern Maine, and from northern California to southeast Alaska.

Currently, 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. Thus, it is likely to colonize most or all of the range of the eastern hemlock species.

Life Cycle:

Adelgids are related to aphids and have a similar, complex life cycle. Hemlock woolly adelgid is parthenogenic, meaning that all individuals are female and produce offspring without mating. In North America there are 2 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 needle drop and twig 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.

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Woolly egg sacs
WDNR Photo by Linda Williams

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

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

Impacts on hemlock:

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Damaged Hemlock
Photo by James Johnson, Georgia Forestry Commission, from www.forestryimages.org

Healthy Hemlock
Photo by Paul Bolstad, University of Minnesota, from www.forestryimages.org

The two hemlock species found in eastern North America, eastern hemlock (Tsuga canadensis) and Carolina hemlock (Tsuga caroliniana), are susceptible and seem to have little resistance to hemlock woolly adelgid damage. Heavy infestations can lead to the death of the tree within 4 - 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. Back in Asia, hemlock woolly adelgid does not cause significant 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.

Hemlock is a long-lived, ecologically important species that provides habitat for many species of 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.

Management:

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Sasajiscymnus larva attacks adelgid nymphs
Photo by USDA Forest Service

Sasajiscymnus adult attacks adelgid nymphs
Photo by USDA Forest Service

Wisconsin does not have hemlock woolly adelgid, and we would like to prevent it from coming here. Quarantines and inspections are in place to stop the movement of potentially infested articles, and people should not move hemlock nursery stock, logs, or firewood from the eastern states where hemlock woolly adelgid is present. The pest was shipped to Michigan several years ago on infested nursery stock, but it was successfully eradicated.

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. 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.

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