Several insects including the Asian longhorned beetle, gypsy moth and hemlock woolly adelgid are on that list. The emerald ash borer (EAB), one of the more recent species to invade the U.S., has already killed tens of millions of ash trees in Michigan and other states in the upper Midwest. McCullough, who has studied EAB since its discovery in 2002, says it is becoming the most costly forest insect to ever invade North America.

“Our analysis showed a new high-impact invasive forest insect is discovered about every two and a half years,” she says. “Some of those species are leaf eaters while others feed on sap. The third group of non-native insects is the wood-borers, which includes species like EAB that feed and develop beneath the bark. Since 1980, more than fifty percent of the new forest insects discovered in the US have been borers. The dramatic increase in new borers is a big concern. Several species of borers can kill their host trees, and because most of their life cycle is spent below the bark, they can be difficult to find and manage.”

The jump in the number of non-native borers since 1980 is likely a result of the widespread increase in containerized shipping. Wood-boring insects can be transported in wood pallets, wood crating and dunnage (unprocessed timbers) used to protect and support cargo in containers. Other exotic forest pests arrive on live plants imported for planting or propagation, while other insects simply hitchhike on imported cargo.

“Global trade has had tremendous benefits for Americans,” says the study’s lead author, Juliann Aukema from the National Center for Ecological Analysis and Synthesis in Santa Barbara. “Unfortunately, it also provides a means for the introduction of destructive insects and other organisms that threaten native ecosystems and the services they provide.”

“The people and companies importing the commodities that are bringing in the borers and other forest insects are not the ones paying the costs for the destruction,” McCullough notes. “It’s the municipalities, homeowners and regulatory agencies who foot the bill.”

The research was supported by the Nature Conservancy and the National Center for Ecological Analysis, which is funded by the National Science Foundation. The research team also included Andrew Liebhold, a research entomologist with the USDA Forest Service; Betsy Von Holle, an ecology professor at the University of Central Florida; and pathologists Kerry Briton and Susan J. Frankel with the USDA Forest Service.