Part 1: Can we save our Hemlocks
from the Wooly Adelgid?
Story by Sam Calhoun
It’s potentially even more devastating than the chestnut blight of the early twentieth century, and efforts so far to arrest its spread and its deadly infestation of indigenous hemlocks have failed. It’s the hemlock wooly adelgid (HWA)—a tiny insect that is killing trees thousands of times its size.
The comparison of the HWA’s impact with that of the chestnut blight is an apt one with one exception, according to Rusty Rhea, an Asheville-based entomologist with the USDA Forest Service who is assigned to Forest Health Protection. Rhea points out that when the chestnut trees died, oak trees partially filled the resulting void in the ecosystem through mast production (acorns). “But when we lose the hemlocks,” Rhea said, “we have nothing to fill the void. In some ways it will be worse than the chestnut blight. As a result of the chestnut blight, the ecosystem was altered; the result of the hemlock wooly adelgid is that an ecosystem will be lost.”
There’s no inventory of the number of hemlocks in the Southeast, but Rhea says that we have millions of them. Every third or fourth tree in the Smoky Mountains is a hemlock, he said. And those trees have no protection—no natural resistance to the insect. Rhea has been working with HWA for the past 10 years and has seen no evidence of resistant trees.
Where Did HWA Come From?
The HWA is an introduced pest that is native to Japan and China. It is a common inhabitant of the forest and oriental hemlocks in Japan, but these trees are resistant to its damage.
Its introduction into the eastern United States was likely through Asian nursery stock. The HWA was first collected from Eastern Hemlocks in 1951 near Richmond, Va. It was first reported in North Carolina in 1995.
According to Grandfather Mountain Park Naturalist Jesse Pope, the rate of infestation of the HWA in our region can be traced to wind, migratory birds, animals and roads as the HWA can attach itself to vehicles and travel.
What Do HWA Look Like?
The HWA is a tiny reddish-purple, aphid-like insect 1 to 2 mm long and about 1/32-inch thick. As it matures, it produces increasing amounts of a protective white wooly wax. Infested branches become covered with circular, fluffy white blobs that usually collect near the base of the needles.
What Kinds of Trees Are Affected?
HWA infests the Eastern Hemlock (common throughout the eastern states) and Carolina Hemlock (found only in the southern Appalachian Mountains)—species that have shown little or no resistance to HWA.
How Quickly Is HWA Spreading?
Estimates range from 17 kilometers (10.5 miles) to 30 kilometers (roughly 18.6 miles) per year, but isolated infestations have been discovered far ahead of the main front. As Pope points out, wind, birds, deer and vehicles are helping to spread the infestation.
What Does HWA Do to Trees?
The insect feeds at the base of the needles, causing them to dry out and take on a gray cast. Through its feeding, the insect depletes the food reserves from the tree’s storage cells. The trees lose their needles and are unable to produce new buds.
“As long as they have a food source, they’re going to do well,” Pope added.
How Serious Is the Problem?
Heavy infestations can kill trees in as little as four years, but some trees have survived for more than ten. The dieback of major limbs can occur within two years.
One-half the range of hemlocks in the eastern United States is infested with HWA, affecting 16 states from northeastern Georgia to southeastern Maine.
To highlight the problem, the hemlocks in northern Virginia “are pretty much history,” Rhea said. Over the past 12 years, HWA has killed 95 percent of the region’s hemlocks.
The owners at Grandfather Mountain, along with other concerned naturalists in the High Country, became aware of the problem last year and have taken measures to fight it.
Although some treatment methods have worked for Pope on Grandfather, the fight has only just begun.
“It’s still a big problem,” Pope said, who used intravenous chemical treatments in the most heavily infested areas on Grandfather last year.
In those areas, “it’s the same if not better,” he said, but Grandfather’s Boone Bowl section, which was not treated last year due to low populations of HWA, has gone from a trace of HWA to a noticeable infestation.
Pope explained that the greatest infestation of HWA last year was along Grandfather’s Profile Trail near Highway 105. He attributes the situation to the HWA hitching rides on cars and then finding a new food source along the trail. And although the Profile Trail was treated last year, the HWA is back again this year.
According to Pope, the HWA starts its life as an egg sack that looks like a cottony puff. Once these sacks hatch, they become “crawlers.” As a crawler, the insect may either choose to stay with the food source that it already inhabits or move to another food source. The crawlers suck the sap out of the hemlock until they’re full. Then they can reproduce asexually.
That’s where the infestation takes hold. One HWA can produce up to 300 eggs in one sack, which, according to Pope, means that a handful of the insects can completely kill a tree. The insects repeat this life cycle every six months—adding to the problem.
Local weather may also be adding to the problem. Below-average cold temperatures in the winter—such as repeated temperatures in the teens at night—can kill the insects, but mild winters—like the one the High Country just experienced—do nothing to eradicate the insects.
The reason this infestation seems to be spiraling out of control is an issue of balance, according to Pope. HWA has existed for years in the northwestern United States, but hasn’t had nearly as devastating an effect as in the east. This difference is because of a species of beetles that is predatory exclusively on the HWA. HWA still exists in the northwest, but the beetles keep the relationship in balance.
Over the past few years, experiments have been done with the release of predatory beetles, such as on Grandfather Mountain and on the campus of Lees-McRae College.
But, Pope said, concerned experts from the Blue Ridge Parkway and National Forest Service all agree that the beetle method is a long-term solution that will only work following the short-term solution of chemical treatments.
According to Pope, the idea is that chemical treatments—given intravenously through pressurized capsules and injections—will save the trees quickly and then the “goal is to get predatory biological weapons (beetles) to sustain.”
In the next few weeks, High Country News will take a look at the various solutions for the HWA.
In the end, this problem will affect much more than the local hemlock population; it will and is affecting area headwaters and wildlife. Infected hemlocks near water can disturb the ecosystem. The death of trees and their branches decreases the shade of regional creeks and rivers is. Reduced shade raises the temperature of the water, thus affecting the native trout population, and the dead trees can no longer provide refuge for significant numbers of wildlife species.
“If we don’t save the hemlocks, then you can say goodbye to trout and other cool water fish and invertebrate species; also over 90 species of birds nest in hemlocks, not to mention their value as wildlife cover and forage,” said local entomologist Richard McDonald.
TO PART 2
If you have comments or experiences that you think will help with this community education project, send them via email to sam@highcountrynews.info.
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