Perched on a platform 50 feet above the ground in a big hemlock named Fern, Geoff Elliott points to an unwelcome Asian import: a little bug known as the hemlock woolly adelgid.
Small fuzzy white nymphs cling to the undersides of hemlock branches throughout the grove of trees. Both nymphs and adult adelgids can work quickly to destroy hemlocks 150 feet tall.
This tree is believed to be somewhere between 200 and 300 years in age and can be taken out by the adelgid in as little as two to four years,” says Mr. Elliott, a tour guide for Adventure West Virginia Resort LLC, which operates zip-line tours through the treetops. The company is trying to educate visitors about the dangers of the invasive insect as it diminishes the landscape the business relies on.
“Without any action we could lose the species,” said Mark Whitmore, a forest entomologist at Cornell University. He described the hemlock as a “keystone species,” because it provides shade that cools streams so fish can survive as well shelter for birds and animals. Losing it would be like “having all your front teeth fall out,” he said.
As global trade has mounted, more goods are coming in from overseas, sometimes bringing with them the accidental cargo of destructive bugs and plants. An estimated 500 million plants are imported to the U.S. each year, and shipments through one plant inspection station doubled to 52,540 between 2004 and 2006, according to the U.S. Department of Agriculture. Today, about 30 new invasive insects are discovered annually in the U.S., up sharply over the last decade, the USDA says.
The yearly economic impact of invasive species in the U.S. is estimated at $133.6 billion, according to a study in Agricultural and Resource Economics Review in 2006. That includes the cost of control and prevention such as pesticides, inspection programs at ports and damage to crops.
An estimated 50,000 plant, animal and insect species have been introduced into the U.S. throughout history. Many plants are initially introduced as food or ornamentals, while animals are occasionally introduced to control other pests. The English sparrow was brought over to control the canker worm on crops in 1853. But by 1900, it was considered a pest because it introduced diseases.
Among the most damaging are weeds that affect crops or destroy animal habitats. The Asian purple loosestrife, for example, was introduced as an ornamental plant in the early 19th century and now invades some 284,000 acres per year in the U.S., crowding out native plant species that help support duck, geese and muskrat.
More recently, invasive species can be directly traced to increased trade. The Asian longhorned beetle hitched a ride on shipping pallets to Brooklyn, N.Y. from China, while others like the zebra mussel have arrived in the Great Lakes in the ballast water of ships from Europe, having spread there from Russia.
Once invasive species take hold in regions where they have no natural predators, it is often impossible to eradicate them. The emerald ash borer, a shiny green beetle from Asia believed to have arrived on packing material, is attacking ash trees. In the northeast, the Asian longhorned beetle has killed thousands of maple trees and other species.
The U.S. also has exported some unwelcome organisms. The gray squirrel, native to the eastern U.S., is causing havoc in Britain and Italy, where it is larger and more aggressive than the red squirrels it is displacing. It is believed that the gray squirrel was accidentally released by the London Zoo near the turn of the 20th century. The Colorado potato beetle, which attacks crops, turned up in Bordeaux, France, during World War I and has spread throughout Europe.
There are U.S. laws to prevent the import of invasive species, but they haven’t been significantly revised since 1918. Last year, the USDA proposed new regulations that would ban imports of certain plants pending analysis ensuring they wouldn’t host pests. It cited a 20-fold increase in seed imports in the last decade.
“Inspection is approaching, or may have reached, the limits of its operational efficacy,” a USDA statement said.
The adelgid is thought to have first arrived in Richmond, Va., in the 1950s on nursery plants from Japan. It has since reproduced to a population that has devastated hemlock stands in the eastern U.S. They have destroyed an estimated 95% of the hemlocks in Virginia’s Shenandoah National Park.
A single insect produces 100 eggs twice a year. That means a single insect and its offspring can spawn a total of 10,000 insects within a year.
Experts say they can only prevent the death of select trees. Each of the hundreds of thousands of infested trees must be treated individually, with an insecticide that can be injected into the soil, into the tree or sprayed on the trunk. Several groups also are experimenting with releasing predator beetles.
“It’s labor intensive, but we are keeping them alive in those particular areas,” said John Perez, a biologist at the 70,000-acre national park called New River Gorge National River, near Fayetteville, West Va. In the park, 5% of trees are hemlocks.
A less direct approach is trying to educate the public who unwittingly transport invasive insects in firewood or nursery trees and can take steps to protect hemlocks on their property.
Will Blozan, an arborist in Asheville, N.C., surveyed and measured thousands of hemlocks between 2005 and 2007 throughout Southern Appalachia to document the species before it was killed off in vast areas. Some trees are 500 years old, and he identified the 15 tallest and 15 biggest in volume, all of which had become infested. In spite of their mass, only one survived, and that had been treated with an insecticide.
“I’ve gone from very, very rarely seeing adelgid in 2002 to utter destruction,” said Mr. Blozan, a former National Parks Service employee.
Hemlocks aren’t a major timber product outside of rustic fencing. But they are considered vital to tourism. And dead trees, which are hazardous, are costly to remove. Some scientists fear the loss of hemlocks could allow invasive species like the tree of heaven, native to China, to thrive. It could crowd out other species that are more vital to the state’s economy.
Forest products is second only to the coal industry in West Virginia, contributing about $4 billion to the state’s economy through taxes, revenue and roughly 30,000 jobs.
“The woolly adelgid is a flagship for a bigger problem,” said Rodney Bartgis, director of the Nature Conservancy in West Virginia.
Write to Kris Maher at firstname.lastname@example.org