Appalachian forests from North Georgia to Canada are home to eastern hemlocks—graceful evergreens often nestled near streams and brooks. They are also prized ornamentals lending a rich flowing texture to the green fabric of yards, neighborhoods, and landscapes.
Their widespread presence in natural forest settings provides cool shade in summer, welcomed by a diversity of wildlife, including bear, deer, wild turkey, and ruffed grouse. Hemlock’s dense foliage serves as an umbrella over streams and brooks keeping water cool, allowing brook trout and many other water-dwelling creatures to thrive. In winter, hemlocks provide shelter against cold and snow for many members of the forest community. However, hemlocks’ service to the forest goes far beyond seasonal shade and shelter.
Mother bears teach their cubs climbing skills on easy-to-grasp, soft and craggy hemlock bark. Deer mice, and many other animals, depend on hemlock seeds and bark as a source of food. Nine out of ten wood thrushes nest in hemlock trees. A hemlock mini-ecosystem can be associated with nearly one hundred species of birds, almost fifty species of mammals, and two hundred species of insects. All in all, these evergreens are key players in the symphony of forest life.
However, that symphony is being rapidly disrupted. Tiny insects going by the name of the hemlock wooly adelgid, HWA for short—each about half the size of a piece of short-grain rice, are killing thousands of hemlocks. HWA arrived on our shores accidentally, early in the twentieth century as stowaways on plants imported from East Asia and Japan. By mid century they were becoming recognized as a potential threat.The “wooly” comes from the white coverings they produce that contain their eggs. Each white innocent-looking cloak conceals danger within from hundreds of adelgid eggs that will eventually become active as crawlers feeding on sap from the bases of hemlock needles.
Their sharp and piercing mouthparts are hollow needles that penetrate and drain sap from a tree. The sap contains sugar and nutrients vital to a tree’s survival. What Count Dracula is to us, each tiny wooly adelgid is to a hemlock. These vampire-like insects gradually suck the life out of trees they infest. Some experts estimate that a healthy hemlock can live for over 800 years; a tree victimized by thousands of wooly adelgid “vampires” can be killed in as little as four years—long before they can begin to reproduce.
When adelgids entered the forests of the eastern United States, as with most other alien invasive species, they found no natural enemies, and their numbers grew unchecked. They spread from tree to tree as hitchikers on winds, birds, and other animals, including us. Since the early 1950s millions of hemlocks have perished in their wake. Now the attack of the adelgids is spreading even more rapidly than experts had predicted even a few years ago. Evidence indicates that in some locations in the Appalachians, eastern hemlocks could be wiped out in the next decade.
A threat of this magnitude requires, action that will not only be effective but will also be sustained in local settings, as well as more broadly, in forest ecosystems. That has meant finding an agent—a vampire killer—whose special expertise lies in attacking only adelgids.
The vampire killer has turned out to be a species of beetle, each one about three times the size of an adelgid. These insects are specialized killers of adelgids. As players in the forest ecological symphony in the northwestern United States, they keep populations of adelgids in check. Dr. Mark Whitmore, a Cornell forest entomologist in the Department of Natural Resources points out that, “People have been studying [this beetle]for a long time and have established that it will feed and reproduce only on adelgids.”
The problem is that they don’t occur naturally in eastern forests. So, efforts have been underway on many fronts to import and breed them, then release them into infested areas with the goal of establishing stable populations large enough to help control the wooly adegid. Dr. Whitmore and colleagues have been releasing these beetles for a number of years and monitoring their survival and proliferation. Results indicate that the beetles appear to be flourishing in research locations that they have sampled. Locally, a many-tiered program aimed at biological control of adelgids using these beetles is underway.
Dr. Jennifer Boyd, a professor in the Biology Department at UTC, specializes in plant ecology and is engaged in HWA research, as is Dr. Yukie Kajita, Research Assistant Professor in the Biological and Environmental Sciences Department at UTC, who is guiding the beetle release aspects of the effort.
“The overall project also involves UTC students. Lily Wilder, for example, is an undergraduate honors student majoring in biology,” says Boyd. “She is conducting research to examine the potential effects of projected increases in atmospheric carbon dioxide on eastern hemlocks and how this might affect the HWA.”
Jennifer adds that, “Another UTC undergraduate, Travis Harrison, is conducting valuable independent research to map local occurrences of the HWA and quantify the infestation levels of affected trees toward helping to guide control efforts.”
On another level, many citizen volunteers are participating in the control effort. Barbara and Clyde Womack head up the Hemlock Conservation Task Force established under the Signal Mountain Tree Board.
“Working with Tennessee Hemlock Conservation Partnership directed by Heather Slayton and Dr. Pat Parkman, director of the Lindsay Young Beneficial Insects Laboratory at UTK, and Dr. Yukie Kajita at UTC, we released 1,000 predator beetles at Rainbow Lake in November, 2014,” Barbara says. “The goal is to create a field insectary so that the predator beetles can be harvested and relocated. Signal Mountain is the first city in Tennessee to have a privately funded predator beetle release.”
Barbara describes the future of HWA management as an evolving science. Presently the governmental agencies rely on both chemical and biological control. “The use of predatory insects to control HWA is really in its infancy with a bit more than 10 years of experience,” she says. “There have been over 100 releases in Tennessee involving three different species of predatory beetles collected from China, Japan, or from the Pacific Northwest. Some are lab produced and others are collected in the field. The beetles that we released are a winter predatory beetle. They were collected in the field in Washington State.”
Scientists, students, and citizen volunteers agree that if there is any long-term hope of saving the eastern hemlock in Tennessee and elsewhere in the eastern United States, it must be through biological control and a bit of luck for vampire killers!
Contact Barbara Womack at firstname.lastname@example.org to learn how you can help with the Eastern Hemlock Project. Story by Dick Morel, Photography by Deborah Petticord