Hemlock Woolly Adelgid

Early autumnal hillside with dead hemlock populations
Dead hemlocks seen in a distant forest

NPS Photo- Chris Ulrey


Our mountain forests are changing and the evidence is visible all along the Blue Ridge Parkway. The once lush foliage of the hemlocks has given way to dead and dying trees under attack by an invasive insect. Eastern and Carolina Hemlocks provide nesting sites for birds and shade critical to the survival of many species on the forest floor and in streams.The hemlock's range and populations have ebbed and flowed over the millennium due to retreating glaciers and drought, and previous insect attacks. Another attack is underway and we are working to keep this tree alive.

Full eastern hemlock form, and closeup of needle detail for eastern and Carolina hemlocks
Left: Eastern hemlock form; Upper right, eastern hemlock close-up; Lower right, Carolina hemlock close-up

Left: Forestryimages.org- David Stevens; Upper right, NPS Photo- Carol Petricevic; Lower right- Forestryimages.org- William Ciesla

The Cause and Effect:

The eastern hemlock (Tsuga canadensis) is a lacy needled, but stately and very tall conifer that ranges from the Southern Appalachians and north up through the mid-Atlantic and New England states, and into southeastern Canada. It is also found in the northern Great Lakes region. The hemlock is most commonly found in cool, damp coves and along creeks. It is often called the "redwood of the east." The Parkway is also home to the rare Carolina hemlock (Tsuga caroliniana) and is most common on damp rocky slopes and outcrops in in the southern Appalachians. The northernmost population of this species is found on the Parkway.

Hemlock wooly adelgid under microscope, teardrop shaped insect with small legs and a sucking mouthpart
Close-up of an adult hemlock woolly adelgid under microscope

Forestryimages.org- Kelly Oten

The hemlock wooly adelgid is an insect native to Japan and was originally introduced to the eastern United States to Richmond, Virginia, in the 1950s. It is found in the mid-Atlantic states and down the Appalachians. Range expansions are occurring to the north, into New England, but is slowed by the colder northern temperatures. The adelgid has been found in the Pacific Northwest since the 1920s, but it is unknown whether it is native or introduced to the region, because there are native adelgid predators.Discovered on the Blue Ridge Parkway in 1984, the Hemlock Woolly Adelgid has devastated the trees in Southern Appalachia. Without treatment, the remaining infested trees will also be lost.

Hemlock branch covered in the cotton ball cocoons of the hemlock wooly adelgid
A branch covered in the "cotton ball" cocoons of the hemlock wooly adelgid

Forestryimages.org- US Forest Service, region 8

Damaged trees appear greenish gray in color and have thinning crowns. Tiny white "cotton balls", containing hundreds of adelgid eggs, can be seen on the branches throughout much of the year. Wooly adelgids undergo two generations in a year, during the spring and fall. All adelgids are female and they reproduce parthenogenically, which means that they are all essentially clones. Upon hatching in the fall, the adelgids feed on the needles, siphoning off nutrients and eventually killing the trees. In its native range, there is a third type of insect in this species- a winged adelgid that flies to, and feeds on spruce trees. Luckily, they do not eat our native spruce trees (Picea rubens).
Pesticide root drench of hemlock tree
An ACE (American Conservation Experience) crewmember pours imidiclopryd on the roots of a hemlock tree.

NPS photo- Chris Ulrey

What are we doing?

Land managers from the National Park Service and U.S. Forest Service along with many partners have been working to reduce the impacts of the aldegids' spread by using chemical and biological controls. In the short term, many trees have been saved with application of a pesticide, imidacloprid to their roots, which the hemlock takes up to its leaves. When the insect takes in the chemical, it dies. Dinotefuron is also used, as a faster working chemical, when a tree is very close to death. Both chemicals are also used as flea treatments for cats and dogs.

Hemlock wooly adelgids and predator beetles, Sasajiscymnus tsugae
Hemlock woolly adelgid and predator beetles, Sasajiscymnus tsugae

NPS photo courtesy of Great Smoky Mountains National Park

The long term plan is for predator beetles to help keep our hemlocks alive. In Japan (Sasajiscymnus tsugae) as well as the Pacific Northwest (Laricobius nagrinus), there are small predator beetles that feed solely on hemlock adelgids, and nothing else. It is hoped that with a predator that can eat the adelgids, a balance can be found for long term survival. The Sasajiscymnus tsugae beetles were released in the Linville Falls and Laurel Springs areas of the parkway in 2006, but nearby areas in National Forests have also had releases, so the beetles are nearby and continuing to eat adelgids. Laricobius has been found on the parkway but was never released here, so they do get around.

Hope for tomorrow

The disappearance of the hemlocks would have devastating impacts to vital habitats, water and air quality, and soil conservation. Studies have shown that the evergreen hemlock works like a blanket or shade over streams and protects native brook trout populations. Loss of hemlocks have made streams warmer in the summer and cooler in the winter, making it harder for the trout and other associated species to survive. In some places, rhododendrons have increased in density, making thickets that are impenetrable.

Although the adelgid will not be eradicated, biologists believe that trees saved today will provide seeds for tomorrow's hemlock stands. With continued funding, staffing and committed partners, research and restoration efforts can be possible.

Last updated: October 8, 2015

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