Call them America's most wanted critters: the emerald ash borer, the Asian long-horned beetle, the Asian gypsy moth. After arriving via wooden shipping pallets or crates, this insatiable trio has munched its way through millions of trees over the past 20 years, costing state, local and federal agencies tens of billions of dollars for eradication, quarantine, and tree removal and replacement.

Emerald ash borers - named for their habit of drilling through bark - have crawled into 15 states and two Canadian provinces since surfacing near Detroit in 2002, arriving in Tennessee this summer. In response, the U.S. Department of Agriculture and state natural resources departments have rolled out campaigns urging the public to look out for the bug and to use only local sources of firewood.

These high-profile offenders are among friends. From 1860 to 2006, at least 455 tree-loving insect species arrived on American shores, as did 16 damaging tree diseases, say the authors of a report in the December issue of the journal BioScience. Despite regulations designed to stymie the six-legged hoard, two to three new invasive insect species set up shop in the United States each year.

"They act as silent invaders, and nobody knows that they're trickling in until the beautiful hundred-year-old tree in your front yard is killed," says report author Julieann Aukema, a forest ecologist with the National Center for Ecological Analysis and Synthesis (NCEAS) in Santa Barbara, Calif. "They're very difficult to detect and once they are detected, the population is already established and they're virtually impossible to eradicate."

While most invasive insects do minimal damage, 16 percent of the species Aukema analyzed feasted their way onto her "high impact" list by killing trees and racking up containment costs. A single invasive species can decimate forests as well as urban trees if it reproduces quickly, finds trees it relishes, and lacks natural predators.

The emerald ash borer - "easily the worst insect we have now, a killer," Aukema says - fits that profile. The glittering half-inch beetle likely arrived in North America at least a decade earlier than its 2002 sighting, says Manuel Colunga-Garcia, an entomologist at Michigan State University in East Lansing.

Last year, the NCEAS, which is funded by the National Science Foundation, predicted that the beetle will cost government agencies $10 billion to $20 billion over the next decade.

The borer arrived in Prince George's County in 2003 on "bad nursery plant stock" transported from Michigan, says Steven Koehn, director of the Maryland Forest Service. Some of those plants were then moved to Fairfax County, sparking an infestation there.

After Maryland's initial eradication efforts appeared successful, the flying bug reappeared in the state in 2006, forcing a switch in tactics from eradication mode" to "slow-the-spread mode," Koehn says. State agents remove infected trees while leaving nearby healthy ash trees intact as "trap trees," to attract and collect more bugs. Both states monitor the range of the bug by widely deploying pheromone traps that attract the insects. Both states also advise residents not to move firewood and to contact their local agriculture extension office if they spot the shiny bug.

Likewise, the Asian long-horned beetle has caused consternation since its first appearance, in Brooklyn, in 1996. The beetle dines on maples, boxelders, and willows and has since been spotted in New Jersey, Massachusetts and near Chicago, in what was likely a second introduction via wooden shipping materials from Asia.

Aukema's analysis shows that while insect pests from earlier eras preferred to munch foliage, the latest wave of invaders tend to be wood borers that can silently lurk deep inside shipping containers.

The worldwide boom in container shipping presents an enormous hurdle to stopping invasive insects, says Frank Lowenstein of the Nature Conservancy, which funded the current study and is undertaking a broader economic analysis of tree pests. The Department of Homeland Security is tasked by the USDA to inspect imports for insects, but the huge influx of goods means "the entire U.S. military would not be able to stop the flow of these pests," Lowenstein says. "Inspection is not likely to be the answer."

Since 2005, USDA regulations have required wooden shipping materials to be heat-treated or fumigated before arrival in the United States, and 98 percent of imported materials are certified by the originating country as meeting that standard, says Alyn Kiel, a spokeswoman for the USDA's Animal and Plant Health Inspection Service. And yet, the bugs keep flowing. Lowenstein said the Nature Conservancy has encouraged the USDA to study why the regulations aren't working. "It's unclear whether the standard . . . doesn't kill enough bugs, or whether there are problems of implementing the standard, or whether intentional fraud is going on" in the country of origin, he says.

Imported live plants act as the second pest vector, Aukema and Lowenstein say. Some 2.5 billion decorative plants enter the country each year; a proposed USDA rule to strengthen plant importation standards has languished since its drafting in 2004. "The USDA has not successfully moved the rule forward," says Lowenstein. "There's no excuse for that."

Kiel says the agency is reviewing public comments on the proposed rule, which would make it easier for officials to restrict importation of high-risk plant species. She said that the agency has no timeline for finalizing the rule but that preventing the introduction of forest pests is a "high priority" for the USDA.

Aukema says that even if stricter import regulations are enacted, many "sleeper species" may already be on the continent, spreading slowly until "hitting the jackpot" by finding the perfect living conditions.

"We have to realize this is a byproduct of global trade," she says. "We don't want to get rid of that, but we do want to minimize the risk. There has to be a recognition of the seriousness of these pests."

- Special to The Washington Post