Managing For Hemlock Woolly Adelgid

Hemlock woolly adelgid infestation. Photo by Chris Evans, courtesy of Bugwood. org.
Hemlock woolly adelgid infestation. Photo by Chris Evans, courtesy of Bugwood. org.

The nonnative invasive insect hemlock woolly adelgid is taking its toll on eastern hemlock trees in the Southern Appalachian region of the United States, where the tree often serves as a foundation or keystone species along mountain streams. A new article by U.S. Forest Service researchers covers the latest in control strategies for hemlock woolly adelgid and the ecological impacts of the widespread death of eastern hemlock.

Authored by Forest Service Southern Research Station scientists Jim Vose, Dave Wear, Bud Mayfield, and Dana Nelson and published March 1 (available now online) in the journal Forest Ecology and Management, the article includes a matrix that provides the range of decisions land managers in affected areas will need to make in the coming years, each decision tied to outcomes and implications.

The following summarizes main points made in the article:

Despite aggressive efforts to control hemlock woolly adelgid infestations, large numbers of hemlock trees in the Southern Appalachian region are dead or in poor health. This loss has important implications. In addition to providing year-round cover for wildlife, hemlock has a strong influence on streamside habitat conditions and stream health. The shade cast by these majestic trees cools the water where brook trout and other stream organisms live; hemlock needles and wood decompose slowly, providing unique habitat for important forest floor organisms such as salamanders.

As hemlock trees die, many of these important functions are changing. Hemlock mortality is adding large quantities of litter to the forest floor and streams; even more will be added as standing dead trees continue to decompose. Hemlock death is also changing basic ecological processes such as the cycling of carbon, nutrients and water. Some studies predict as much as 30 percent increase in streamflow during the winter months in areas where hemlocks dominate.

Many of these changes may prove to be short-term or localized; more significant changes are expected in the coming decades as other species replace hemlock. In areas where the evergreen shrub rhododendron is absent, red maple, sweet birch, and yellow poplar will probably take the place of eastern hemlock. Where rhododendron is already present, the shrub will probably spread, and could limit recruitment of overstory tree species. These changes in species composition may alter habitats and ecological processes required by terrestrial and aquatic animal and plant species.

The good news is that land managers can start implementing control and restoration activities now to prevent undesirable long-term impacts.

Controlling the spread and impacts of hemlock woolly adelgid involves the integrated use of multiple approaches including chemical control, biological control, cultural treatments, host resistance, and host gene conservation. Chemical control has been extremely effective at small scales, but biological control is currently the only viable option for controlling hemlock woolly adelgid across the landscape, though research continues on other possibilities.

Where control efforts fail, land managers should anticipate hazards and other impacts from dead and dying trees. In recreation areas near streams, for example, trees may need to be felled to minimize hazards, and stream crossings should be carefully monitored to ensure that culverts remain clear. In order to maintain or restore the benefits provided by hemlock trees, restoration efforts may require novel approaches such as the introduction of nonnative or hybridized hemlock species, facilitated movement of native species to new habitats, and aggressive management of existing undesirable species to benefit desirable species. In all cases, monitoring will be required to evaluate efforts and guide adaptive approaches.

Access the full text of the article with matrix of management options.

For more information, email Jim Vose at

Access the latest publications by SRS scientists.

Subscribe to our newsletter!

Receive weekly updates