The dreaded hemlock woolly adelgid has finally made its appearance in Asheville. They’ve actually been here for a while, but I’ve now seen them with my own eyes (or, rather, I’ve seen the waxy white “wool” these tiny insects create to shield themselves and their eggs: minute tufts of cottony material glowing against the base of dark-green hemlock needles). In any case, they’re here — and let’s hope they’re not unstoppable.
“We’re getting calls from every corner of the county,” says Linda Blue of the N.C. Cooperative Extension Service’s Buncombe County office. “A year ago there were just a couple of pockets of adelgids in Montreat and in Asheville, [but] in the last few months we’re getting calls from Barnardsville, Asheville and Fairview.”
In fact, the hemlock woolly adelgid is now prevalent in about half of the hemlock’s natural range across the eastern U.S., reports Rusty Rhea, an entomologist with the U.S. Forest Service. More than 90 percent of the hemlocks in the Shenandoah National Park have died due to adelgid infestation, he notes. Native to Asia, the hemlock woolly adelgid (Adelges tsugae) was first noticed in the Pacific Northwest in the 1920s, and then spotted in Richmond, Va., in the early ’50s, according to Save Our Hemlocks, a nonprofit based in Knoxville, Tenn. The Asian, western and Pacific hemlocks all appear to have natural defenses against the blight. But no innate defense or native predator protects the eastern hemlock (Tsuga canadensis) or the Carolina hemlock (Tsuga caroliniana) from this deadly invader.
The adelgid is parthenogenetic, meaning that there are no male adelgids, only females that reproduce asexually, creating identical copies of themselves. Each female produces up to 300 eggs a year, and the hemlock woolly adelgid spreads rapidly, dispersed by wind and carried by migratory birds and mammals. Humans spread the pest via infested nursery stock.
The adelgid feeds on the sap at the base of each needle; as the infestation spreads, the needles turn from green to gray and eventually fall off. In time, the tree starves to death, usually within a few years (though it may take up to 10 years, depending on the tree’s age and vitality). Other stressors, such as drought, can also hasten an infected tree’s demise, so watering hemlocks during extended dry spells can be helpful.
What can you do to combat this invader? First, monitor your trees frequently and take preventive measures, such as keeping bird feeders well away from hemlocks. If the infestation is detected early, small trees can be treated with horticultural oil sprays, and an arborist can treat large trees. Blue recommends trying horticultural oils first on small trees, because “these are the safest products to use — safest for the handler and the environment.” And don’t fertilize infected trees, because the adelgid actually feeds on the nitrogen the hemlock takes up from the fertilizer.
If the infected tree is more than 50 feet away from the nearest stream, an arborist may inject imidacloprid (known commercially as Merit) into either the tree or the soil. This systemic insecticide kills the hemlock woolly adelgid, but it can have a negative impact on beneficial insects.
Hemlocks play a vital role in cool-water ecosystems, both in the Southern Appalachians and throughout the eastern United States. By creating dense shade above cove mountain streams, hemlocks support trout and other animals that inhabit those waters. There’s no other evergreen species in these forests that can fill that role — so if we lose the hemlock, the nature of cove forests and their streams will be irreversibly altered. Hemlock woolly adelgid infestations were found in the Pisgah and Nantahala national forests in 2001 and in the Great Smoky Mountains National Park in 2002, Save Our Hemlocks reports.
In an attempt to control the pest, scientists and land managers are experimenting in large backwoods areas with imported predators that feed only on the adelgid. “We have found several insects — most have been lady beetles — and three that are adelgid-specific … are originally from Asia,” notes Rhea. “We are now rearing and releasing them in the forest. We are using several insects that work together, and we are still exploring foreign soils for pathogens and controls.”
It’s too soon to tell how successful these efforts will ultimately be, however. “The first release of these beetles was in Connecticut in 1996, and we have had some good results,” Rhea reports, cautioning, “In most biocontrol programs, you have to wait five to 10 years before you see any changes.”
Already, our forests have lost the chestnut, and the balsam woolly adelgid (imported from Europe) has practically wiped out the Fraser fir. And now the hemlocks are dying. On the other hand, efforts to combat the gypsy moth using biological controls have been relatively successful, says Rhea.
To learn more about the hemlock woolly adelgid, check the Save Our Hemlocks Web site (www.saveourhemlocks.org).
[Freelance writer Megan Shepherd is garden manager at the North Lodge Bed and Breakfast in Asheville.]