STREET GAP, CHEROKEE NATIONAL FOREST — Paul Merten has spent nearly a decade chasing down a killer in the Southern Appalachians, armed with no more than a pocket knife and measuring tape.
But recently the entomologist with the U.S. Forest Service in Asheville, N.C., has been homing in on the tiny, yet lethal pest with what he hopes is a secret weapon — parasitoids, also known as wasps.
Merten and Haywood County Community College forestry student Caroline McGough were deep in the woods on the Appalachian Trail slicing across the North Carolina-Tennessee border last week, unleashing parasitoids in a science fiction-like attack on the emerald ash borer.
The beetle is the latest in a series of invasive pests from Asia that are swiftly destroying ash trees across the country, causing billions in damage and wreaking havoc on biodiversity.
In a patch of ash trees in the rich cove forest, Merten released hundreds of the tiny wasps that feed only on the emerald ash borer in a “last Hail Mary” attempt to control the invasive insects.
The emerald ash borer is a half-inch-long metallic green beetle first discovered near Detroit in 2002. It arrived from its native Asia, likely transported by solid wood packing material in cargo ships or airplanes, Merten said.
After arriving in Michigan, the beetle spread to the Midwest, having been found in Ohio in 2003 and northern Indiana in 2004. It continued its swirling path of destruction, arriving in New England in 2012.
The emerald ash borer made its first appearance in Western North Carolina in 2013 in the Great Smoky Mountains National Park.
The news is "devastating," Merten said.
“I’ve been chasing this insect for eight years. Emerald ash borer is a very fit species for what it does. It seems to locate its host species with great skill. It will probably generally infest North Carolina in the next couple of years,” he said.
Emerald ash borer damage is actually caused by the larvae.
The beetles lay their eggs in between layers of an ash tree's bark. Larvae hatch in about a week and bore into the tree, feeding on the inner bark and phloem, the vascular tissue that transports nutrients to branches and leaves.
Their feeding creates “S” shaped tunnels, known as galleries. The larvae go through four feeding stages, which cuts off water supply and the flow of nutrients within the tree. They then go through the pupal stage in late spring. Adults begin to emerge in “D” shaped holes in May and June. The adults then go on to mate and feed on ash tree leaves, Merten said.
“They are exceedingly difficult to control,” Merten said. “They’re protected under the bark they’re eating. The window of opportunity to save ash trees is very short. They don’t have the vascular health to uptake the chemicals very well once they are infected.”
“We’ve been keeping a pretty good eye on ash trees,” said Kristine Johnson, supervisory forester with the Great Smoky Mountains National Park, where the beetles were first spotted on the Tennessee side of the half-million-acre park. “We haven’t seen mortality on North Carolina side yet.”
Merten gets parasitoids, a natural enemy of the emerald ash borer native to Asia. The female adult parasite wasp senses where the the beetle larvae are, then pierces her ovipositor, a long tube that resembles a stinger, into the ash tree bark, Merten said.
The parasites lay eggs in and on the eggs and larvae of the beetle, which hatch and kill the beetle's eggs.
“It’s pretty crazy,” McGough said.
The two began releasing three species of parasitoids — oobius agrili, tetrastichus planipennis and spathius agrili — this summer in the Cherokee National Forest in Tennessee.
The forests in North Carolina did not choose to use biocontrol. But Merten said the wasps don’t read state lines, and is hopeful they will establish a population in North Carolina.
There are three ways of releasing them onto ash trees — as eggs in a small plastic cup, as adults in jars and in small diameter ash bolts also infected with emerald ash borer larvae.
Deborah McCullough, professor of entomology and forestry at Michigan State University, said that in 2002, no one in the United States had studied the beetle before. Entomologists found two pages about the beetle in a Chinese textbook that had to be translated to English.
“We didn’t know the biology; we didn’t know if it would attack other trees. We didn’t know how to stop it,” she said. “We knew it was going to be a big deal, but no one anticipated the amount of destruction, the dead trees, the economic costs and the ecological impacts as well.”
At that time there were few systemic insecticides available, and the ones they tried had inconsistent results. The course of action was to cut down trees within a half-mile radius of infested trees because beetle can be present in an area before a tree starts to show damage, she said.
Demise costs billions
There are 16 species of ash in the United States. The emerald ash borer infects the white and green ash trees found in WNC. They do not affect mountain ash.
The loss of ash trees is disturbing because of its splendor — they are canopy trees that can grow to more than 100 feet high, provide yellow to orange foliage in the fall, and are a backbone of landscaping. They are also an important source for lumber, furniture, firewood and paper, and are the preferred wood for baseball bats.
Green ash is important for riparian areas — it holds soil to prevent erosion, and provides shade for streams, keeping temperatures cool for fish and aquatic life.
Loss of the trees would also remove a carbon sink. These all-important forested areas remove carbon dioxide from the atmosphere, which is essential in slowing climate change.
“When ash trees die, we’re losing diversity, we’re losing part of the ecosystem,” Merten said.
A report by APHIS, the Agricultural Research Service and the U.S. Forest Service estimate federal and state resource managers spend $29 billion a year to manage emerald ash borer populations. They estimate the value of the 8 billion ash trees in the U.S. potentially infested with the beetle to be $282 billion.
The beetles are good at flying, but they are also unwittingly being helped by humans, Merten said. One of the biggest culprits is firewood. People cut up dead ash trees infested with emerald ash borer, then bring it on camping, hunting and fishing trips, releasing the beetles in new areas.
“Beetles can travel like a Trojan horse, undetected by anyone. They can arrive in a new place and voila, you have a new infestation,” Merten said.
He said people should only use locally bought firewood and use heat-treated firewood, which is stamped with a USDA symbol.
Follow Karen Chávez on Twitter: @KarenChavezACT