Battle of the bugs:
Researchers using nature to save the hemlocks
By Michael Sutphin, College of Agriculture and Life Sciences
Invader kills native trees
A spiritual connection
The exact origin is still a mystery. In the early 1900s, either lumber or trees from Asia made their way to a North American port carrying a fungal disease now know as chestnut blight. Although chestnut trees in China and Japan had a natural immunity to the disease, their American cousins did not. By 1940, the once-widespread American chestnut tree was virtually extinct. Other than a few lonely stands, only small shoots from existing root bases remain today, and the blight quickly ravages even these.
Could this scenario repeat itself in American forests today? Or worse still – is it already doing so? Scott Salom, professor of forest entomology in the College of Agriculture and Life Sciences, is trying to save the eastern hemlock from the fate of the American chestnut tree. The tall, long-lived hemlock, which shelters river and streamside ecosystems throughout the eastern United States and Canada, is rapidly declining in number while the foreign-borne Adelges tsugae, or hemlock woolly adelgid (HWA), expands its domain. Salom and his colleagues have trekked around the world to find natural predators that will curb the spread of HWA.
The demise of the hemlocks
A white, woolly material near the base of hemlock needles may not seem like a dangerous plant feeder, but this sure sign of HWA infestation means death to eastern hemlocks and drastic change to the ecosystems they support. American scientists first noticed the tiny, aphid-like insect in the West as early as the 1920s, but it was not until the 1950s that they spotted HWA producing these cottony egg masses in ornamental plantings near Richmond, Va. This is when the real damage began. Hemlocks in the western part of the continent and in Asia, where most conifers co-evolved with an adelgid species, are resistant to HWA. But eastern hemlocks never developed a natural immunity to the insect.
Today, HWA infestations span from Georgia to southern Maine and as far west as West Virginia and Kentucky. In Virginia, the insect has killed more than 90 percent of hemlocks in the Shenandoah Valley. The statistics are morbid in the rest of the continent, too, where HWA covers more than 50 percent of the geographic range of eastern hemlocks. The adelgid will continue to spread at an average rate of 15 miles per year — or faster in warm, wet climates along the coast or other bodies of water. Cold, winter temperatures inhibit the insect’s ability to travel, slowing its expansion rate.
“Once the hemlock woolly adelgid appears in an eastern hemlock stand, it is not long before most or sometimes all of the trees succumb to it,” Salom says. “There are remnants of hemlocks that have survived in forests otherwise devastated by the hemlock woolly adelgid. We don’t entirely know why.”
Salom speculates that eastern hemlocks have a threshold for how large of an adelgid population they can tolerate and that sometimes the infestation does not exceed the upper bound, allowing some trees to survive. Whatever the reason, if left unchecked, HWA will continue to prey on American forests until few, if any, hemlocks remain.
Natural enemies
As they do with most pests, entomologists have more than one weapon in their arsenal against the adelgid. Pesticides can rid individual trees of HWA infestations, but this process is slow, expensive, and non-sustainable.
“Since 2001, we have been doing soil treatments and tree injections for 200 to 250 trees around Mountain Lake in Giles County, Virginia, but this is a drop in the bucket compared to all the trees out there,” Salom says.
To beat the adelgid, scientists need to think big. Pesticides are not a large-scale option. “Mass application of pesticides would not be effective,” Salom says. “Unlike the gypsy moth, which lives in tree canopies, you cannot spray pesticide over a forest in an aerial flight to kill the hemlock woolly adelgid, which lives at the base of newly formed needles. It simply won’t work.”
Instead, scientists are racing to find natural enemies of HWA. With the help of L.T. Kok, professor and entomology department head, Salom has put Virginia Tech in the lead.
In 1997, Salom and his colleagues at Virginia Tech imported Laricobius nigrinus, a tiny (2 mm long) beetle from British Columbia, to the Beneficial Insects Quarantine Laboratory at Virginia Tech to evaluate its potential and safety as a suitable biological control agent. They received permission to release the beetle in 2000, studied it in field cages until 2002, and then began open releases in 2003. In 2005, Clemson University and the University of Tennessee at Knoxville also began rearing the beetle using procedures developed at a Virginia Tech insectary. By the end of 2006, there were more than 22 research-based releases and 30 operational releases of L. nigrinus in the eastern United States.
“The number of beetles we have released exceeds 17,000, with an average of 300 beetles deployed at each release site,” Salom says. “What we are doing now is monitoring those release sites to determine whether the beetle we introduced from British Columbia can curtail the progress of the adelgid.”
Out of Japan
Meanwhile, a second natural enemy was located by discovering the infection source. For years, scientists were unsure whether A. tsugae came from China or Japan. Then Nathan Havill and colleagues from Yale University pinpointed the origin of the species.
“In 2006, they performed a series of DNA comparisons of hemlock woolly adelgid throughout its current geographic range, including China, Japan, western North America, and eastern North America,” Salom says. “They found an exact match between the hemlock woolly adelgid in eastern North America with the Osaka region of Japan. This answers once and for all any questions about the origin of the hemlock woolly adelgid.”
After Shigehiko Shiyake, a Japanese scientist at the Osaka Museum of Natural History, found a new Laricobius species in Japan, members of Salom’s lab visited the island country in March of 2006 to collect 300 adult insects and hundreds of larvae for quarantine evaluation. Virginia Tech entomologists included Ashley Lamb, a postdoctoral associate, and Tom McAvoy, a laboratory specialist. Salom and his group have high hopes for the Japanese Laricobius beetle. Because they spent the last decade studying L. nigrinus, they hope to apply what they learned to its Japanese cousin. This will dramatically accelerate the evaluation and mass rearing processes.
“From the time we found a potential match to today, we had spent nearly 10 years on just one predator,” Salom says. “But now that we have a closely related species, we only have to tweak the mass rearing process instead of starting from scratch. We hope to release the new predator in 2008 or 2009.”
Still, the research process is difficult and laborious. “We can’t just import a species from China or Japan and release it into the wild,” Salom explains. “Instead, we have to obtain permission to bring a biological agent into the United States, follow government guidelines to test the imported beetles in a quarantine laboratory, develop an effective procedure for mass rearing of the insect, and only then, after all of these tasks have been carried out, can we release the predator and study its effect.”
No risk
Releasing a foreign biological agent into the wild raises a number of ethical questions: Why would scientists want to introduce an insect into North American forests, when an accidental introduction caused the HWA problem in the first place? Could these releases cause more harm than good? To answer the latter, one must understand the difference between specialist and generalist predators. Specialists will eat only one food, their prey; generalists, on the other hand, will scour entire forests and munch on any suitable pest they find along the way. L. nigrinus and its Japanese cousin are specialists.
“If the beetles we introduce cannot feed or reproduce on other hosts, then the natural conclusion is that there is no risk,” Salom says.
The Asian lady beetle is a popular example of a generalist predator. Farmers used to introduce this spotted orange insect into their fields because the bug will feed on almost any aphid-like insect. But for homeowners who live near these fields today, the Asian lady beetle is the real pest. This bug can slip through vents and cracks under doors, invading a home in large numbers if the season and location are right. Luckily, Salom and his research crew have picked insects that will not often go near a home.
“These are mostly solitary insects that reside in forests,” Salom says. “In fact, they emerge at the same time as the adelgid and undergo a dormant period in the summer.”
Together, Virginia Tech and other schools researching the biological control of HWA have introduced multiple predator species. New studies on the competitive interactions among these species and already established predators in the wild aim to show how these predators relate to each other in terms of survival, feeding, and reproduction. Although a comprehensive approach with multiple predators might one day curb the explosion of HWA in American forests, Salom admits that complete eradication of the adelgid, or any pest for that matter, “only works when you have a new introduction of a species in a small, confined area. Because the hemlock woolly adelgid has been in the eastern United States for decades, we will never be able to completely wipe out the pest from our area.”
Support has come from the U.S. Forest Service’s Forest Health Protection Section, U.S. Department of Agriculture’s Animal and Plant Health Inspection Service, and Friends of the Blue Ridge Parkway. Virginia Tech has also provided infrastructure improvement and $100,000 for tuition for students working on HWA research.
