Saving the Eastern Hemlock by Beetle Power

An army of little insects is sweeping across the southeastern United States, choking every

A healthy eastern hemlock.

hemlock in its path and turning entire mountaintops into wastelands. These Japanese predators known as hemlock woolly adelgids threaten to wipe out the entire population of eastern hemlock, and entomologists plan to introduce yet another exotic predator—a highly controversial and historically unsuccessful idea—to save one of eastern America’s most important trees.

According to research conducted at North Carolina State University, only 20 percent of the 600 species introduced in the U.S. since the early 1900s for biological control proved successful. One of the most well-known examples of biological control failures is the introduction of the cane toad to Australia and the southern United States. Originally introduced as a means of controlling sugarcane pests, the cane toad was unsuccessful because it could not actually live in the sugar cane fields, and it had a nasty habit of eating all the native species’ food. The cane toad is now considered such a threat in Florida that the Florida Fish and Wildlife Conservation Commission recommends people kill them whenever they see them.

Hemlock woolly adelgid feeding on a hemlock branch: Photo credit: University of Kentucky

So despite biological control’s dubious history, most entomologists look to it as the long-term solution to the hemlock woolly adelgid onslaught. The eastern hemlock is an evergreen tree with a habitat ranging west from Minnesota to the east coast and from Quebec to Georgia. The hemlock is one of the most common trees in eastern U.S. forests, and it is a significant source of food for animals like deer and squirrels. It also shades streams—cooling the waters for brook trout and other fish—and provides a home for more than 200 different bird species. A slow-growing tree, a hemlock can mature to over a 100 feet tall and live up to 800 years old—unless the hemlock woolly adelgid finds it.

The hemlock woolly adelgid arrived in 1951 through Asian nursery stocks of hemlock transported to the eastern United States for ornamental gardening. The adelgid quickly migrated to nearby native trees and fed on them, inserting their piercing mouthparts through the bases of needles and sucking the nutrients stored inside the young twigs. Soon, the trees’ needles dried up, and the buds stopped growing. Their deep green color paled before finally falling from the tree, leaving a gray skeleton to succumb to the native predators and elements usually nonthreatening to a healthy hemlock. By the 1980s, the adelgid had caused widespread mortality. As much as 80 percent of total hemlock populations are now dead in areas like the Blue Ridge Parkway and Shenandoah National Park.

Forestry officials started looking for ways to fight the eastern hemlock’s killer,

Graph of hemlock woolly adelgid migration. Graph credit: USDA Forest Service

first with use of chemicals. However, this method quickly proved too costly and dangerous to the ecosystem to continue as a sole solution to the adelgid problem.

“Chemicals do work, but only as a short-term solution,” says Anthony LeBude, nursery extension specialist and assistant professor at North Carolina State University. “Biological control is the only long-term solution that can actually manage the hemlock woolly adelgid population.”

So in 1992, a team led by Mark McClure, chief scientist at the Connecticut Agricultural Experiment Station, discovered China’s lady beetle. The team brought the little black beetle to the U.S., reared it in labs, and first released it in a town park in Windsor, Connecticut, in the spring of 1995. Though a specialized predator in the labs, the lady beetle actually fed on other adelgid species in the southeastern forests. It also never quite established itself as hoped, and its impact on the hemlock woolly adelgid population appeared negligible.

Despite the China lady beetle’s failure to reduce the adelgid population, biological control remains such a popular idea that almost 50 percent of the USDA Forest Service Forest Health Protection’s 2.3 million dollar funding went to biological control alone in 2010. The remaining 1.3 million was divided among chemical control, tree cross-breeding, and hemlock gene preservation.

Laricobius nigrinus may be the best chance for controlling the hemlock woolly adelgid. Photo credit: Cheryl Moorehead

Today, the search for the hemlock woolly adelgid slayer continues, and many entomologists think that a beetle called laricobius nigrinus, nicknamed Lari, may be the one to bring the adelgid down. Lari’s habitat includes parts of British Columbia and Colorado, with elevations and temperatures similar to the infected regions of the Southeast. In the lab, Lari feeds on the hemlock woolly adelgid eggs and lays her own eggs within the adelgids’ ovisacs—white cocoon-like structures housing the adelgid eggs. The hatching Lari larvae then devour the adelgid eggs, thereby potentially lowering the next generation adelgid population.

“The laricobius beetles are the safest biological control agent we’ll ever have. I can’t foresee them ever being a problem,” says Ashley Lamb, post-doctoral research associate at the University of Tennessee. According to Lamb, if the laricobius beetle has no hemlock woolly adelgid, it dies. Even on the off chance that it would try to feed on anything other than the adelgid, the beetle is only active in the winter when no other insect is around to feed on.

“I would disagree with others researchers that say that Lari only feeds on the hemlock woolly adelgid,” says Michael Montgomery, scientist in the Ecology and Management of Invasive Species and Forest Ecosystems in the USDA Forest Service, “The thing is, species adapt, and we are not sure yet how Lari will adapt to the Southeast. It might not be as specific as we think.” Montgomery was on one of the original teams to introduce the China lady beetle, a beetle he admits eats other insects in the U.S.

Entomologists continue fervent research into introducing Lari into the Southeast. In fact, Scott Salom, professor of entomology at Virginia Tech University currently works with a team to bring Lari’s cousin laricobius osakensis—the hemlock woolly adelgid’s natural predator in Japan—onto the control team. The Forest Health Protection backs Salom’s research—with some restrictions.

“We were hoping to introduce osakensis this fall; however, we’ve been held up on that. Now, it looks like next year’s our target date.” All the research team’s understanding of Japanese Lari’s potential behavior in the Southeast come from lab work and field assessments of the beetle in its native Japan habitat. If the Forest Health Service approves the move next fall, Japanese Lari will enter the fold of adelgid exterminators along with the other species already introduced into the Southeast.

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