UT's Energy, Environment
and Resources Center Presents
Smoky Mountain
Resource Issues in Great Smoky Mountains National Park
SIGHTLINE
FALL/WINTER 2002
Vol. 3 No. 2
***
Editor: David Brill; Assistant Editor: Constance Griffith; Writers: Kris Christen, Lisa Byerley Gary, Elise LeQuire, Dennis McCarthy, and Becky Nichols. Graphic Designer: Lisa Byerley Gary.
SIGHTLINE is published on behalf of
Great Smoky Mountains National
Park
by the
Energy, Environment and Resources
Center (EERC)
at the
University of Tennessee.
EERC conducts research designed to promote real-world solutions to problems in the fields of energy, environment, technology, and economic development.
For additional information, write EERC, 311 Conference Center Building, The University of Tennessee, Knoxville, TN 37996-4134, call 865-974-4251, or visit our Web site at http://eerc.ra.utk.edu.
SIGHTLINE is sponsored by:
and
Saving Grace
The
Park Service is armed and ready to do battle with a tiny insect that
threatens to fell the hemlock, one of the Smokies’ most graceful and
beloved trees.
by David Brill
Adult hemlock adelgids resemble the tops of cotton swabs and cluster at the bases of the hemlock's needles.
Few trees native to the forests of Great Smoky Mountains National Park (GSMNP) are as graceful or evocative of place as the hemlock. These plentiful evergreen trees provide nesting habitat for birds, shelter trout streams from the summer sun, and mottle hillsides.
Hemlocks are the dominant tree species in about 5,000 acres in the Park—or about 1 percent of the GSMNP’s half-million acres—but they are widely distributed elsewhere in the Park.
Though some of the Park’s hemlocks are 400 years old, the entire population is under threat from an exotic insect pest that’s much smaller than the head of a pin but destructive enough to decimate the Smokies’ hemlocks.
The hemlock woolly adelgid (HWA) or more technically, Adelges tsugae Annand, an aphid-like insect that’s native to Japan and China, sucks the sap from the bases of hemlock needles, usually beginning on the undersides of the branches. As the HWA feeds, it disrupts the nutrient supply to the needles. The loss of foliage eventually kills the tree.
In May 2002, the Park Service confirmed GSMNP’s first HWA infestation three miles north of Fontana Dam in Swain County, North Carolina. A short time later, Park Service personnel identified a second infestation site about a mile from Cades Cove in Blount County, Tennessee. As of press time, the Park Service had located 43 additional areas of infestation.
The adult HWA is covered with a white waxy wool resembling the tip of a cotton swab. Infested trees can succumb to the pest within three to five years of the initial attack.
The HWA is closely related to the balsam woolly adelgid, which has killed more than 90 percent of the Park’s Fraser firs at high-elevation sites like Clingmans Dome and Balsam Mountain.
Expected (but Unwelcome) Visitor
The HWA was first detected in the Pacific Northwest in the 1920s. The western hemlock, like the hemlocks native to China and Japan, is tolerant of the pest, and the HWA did little damage.
By the early- to mid-1950s, the HWA had made its way East, and observers first detected it in a large municipal park in Richmond, Virginia.
Once established in Richmond, the insect was then positioned to ravage a tree species vulnerable to its feeding habits: the Eastern hemlock (Tsuga canadensis), which is plentiful in GSMNP. By the 1970s and ’80s, distribution of the HWA had spread rapidly, moving west onto the Blue Ridge and north, reaching as far as Massachusetts.
Currently, the HWA infests about one-half of the area where hemlocks occur in the Eastern United States. In Shenandoah National Park about 80 percent of the park’s hemlock trees are now infested, and most are expected to die.
Between 1999 and 2001, the HWA jumped the border from Virginia into North Carolina and moved south toward GSMNP.
Though hemlock trees are of little commercial value, they play a vital role in the maintenance of healthy ecosystems in GSMNP, according to Glenn Taylor, a GSMNP forestry technician.
“Many neotropical migrant birds, like the wood thrush, come to the Smokies from Central and South America to nest in the hemlock,” says Taylor. “The trees also shade streams and keep water temperatures low, which increases dissolved oxygen and helps sustain cold-water fish like trout.”
Pesticidal Mission
While techniques for fighting the HWA were not fully developed in time to save the hemlocks in Shenandoah National Park or other sites of early infestation, the Park Service, in tandem with the U.S. Deaprtment of Agriculture’s National Forest Service and the University of Tennessee (UT), is taking aggressive action in GSMNP to combat the pest.
The Park’s pesticidal arsenal comprises three proven weapons. The first involves use of a pesticidal soap solution that’s sprayed onto the branches of infested trees. The soap, which contains potassium salts of fatty acids and alcohol, dissolves the waxy wool and kills the insects by drying them out. For the spray to be effective, says Taylor, the entire tree must be saturated. Sprayers powerful enough to reach the upper branches of the hemlock trees must be pulled behind vehicles, which limits use of this option to accessible sites, like Park visitor centers, with high-value trees.
A second treatment option—which must also be administered by hand—is a chlorinated nicotine compound that’s injected into the soil at the base of the tree and is then taken up by the tree’s roots. The compound kills the HWA as it feeds on the needles of treated trees. The injection treatments last longer than the soap—about a year—but are slower acting and more costly.
According to Taylor, treating a single tree with the injector can cost $75 or more, while soap treatment costs about $30 per tree.
“The soap and pesticide must be applied by hand, so it is not practical to treat large or isolated stands,” says Kristine Johnson, GSMNP supervisory forester. “But in developed areas or with smaller outbreaks, we may be able to keep an outbreak in check.”
The Park Service’s most effective treatment method may involve inviting one of the HWA’s natural enemies to join the fight.
In the HWA’s home in Japan and China, natural predators keep the pest’s population under control. Among these predators is a tiny lady bug beetle, Pseudoscymnus tsugae, which feeds almost exclusively on adelgid species, including the HWA and the balsam woolly adelgid. The all-black P. tsugae is about one-tenth of the size of the familiar red-and-black lady bug.
In 1992, on a trip to Honshu, Japan, Mark McClure, a scientist with the Connecticut Agricultural Experiment Station in Windsor, Connecticut, observed P. tsugae feeding on the HWA and noted that the beetle killed 86-99 percent of all adelgids present in its feeding range. McClure collected several of the beetles and returned with them to the United States.
McClure’s research
indicated that the beetle feeds exclusively on adelgids. Thus, the
distribution of P.
tsugae will likely be limited to areas where adelgids are available
as a food source.
Voracious Feeders
Between 1995 and 1999, 120,000 adult P. tsugae were released in hemlock forests in Connecticut, New Jersey, and Virginia. According to a paper titled “Is Pseudoscymnus tsugae the Solution to the Hemlock Woolly Adelgid Problem,” written by McClure and Carole Cheah and Timothy Tigner, “P. tsugae reproduced, dispersed, overwintered, and showed remarkable short-term impact on [the HWA] by reducing adelgid densities 47 to 88 percent in only five months....”
Taylor points to the beetles’ voracious appetites as the key to their success in controlling HWA infestation. “Each beetle in development to adulthood eats about 100 adelgids,” he says. “Though P. tsugae will feed on all life stages of the adelgid, adult beetles need the nutrient-rich adelgid eggs, which are generally available in spring, to reproduce.”
In June, the Park Service began the experimental release of 10,000 P. tsugae in four highly infested locations in GSMNP: the Cataloochee area in North Carolina and Panther Creek, Laurel Falls, and Stony Branch areas in Tennessee. Each location received about 2,500 beetles, the minimum number to be effective, says Taylor. The National Forest Service provided the beetles, which were acquired from a private lab in Pennsylvania, at a cost of nearly $2 apiece.
Four months after the release, Paris Lambdin, a professor in UT’s entomology and plant pathology department, returned to the Laurel Falls release site and confirmed that the beetles had survived and established themselves on the area’s heavily infested hemlock trees.
The Park Service plans additional releases of P. tsugae in the future. According to Taylor, an entomology laboratory at the New Jersey Department of Agriculture may provide the beetles for $1 apiece. The North Carolina Department of Agriculture has also started hatching the beetles at a laboratory on the North Carolina State University campus.
NATIONAL PARK SERVICE
Park Service personnel saturate an infested hemlock with insecticidal soap.
Long-term Fight
Taylor concedes that, despite their best efforts, Park Service personnel will likely never completely eradicate the HWA from the Park, in part because the pest is so widely distributed.
“This will be a long-term
battle that we may never fully win,” he says. “We regard fighting the
HWA as an ongoing maintenance task, and maybe the best we can hope for is
to keep the pest in check.”
For more information, contact Kristine Johnson, Great
Smoky Mountains National Park, 107 Park Headquarters Road, Gatlinburg, TN
37738, or call 865-436-1707.