The foreign insect pests that invade our nation’s forests may be small, and most of them get little attention.

But when they are bad, they are very, very bad, and we all pay the price. Trees are lost, forests are disrupted and millions of dollars are spent to deal with the problem.

According to a comprehensive study published in the December issue of BioScience, without better efforts to stop the transport of exotic forest insects into the United States and to control the devastating species that are already here, our forests, woodlands and urban trees are at serious risk, with economic losses projected to range in the billions of dollars.

Deborah McCullough, Michigan State University (MSU) professor of forest entomology, was involved in the study that included researchers from the University of California at Santa Barbara, the University of Central Florida (UCF) and the United States Department of Agriculture (USDA) Forest Service. They identified the number of non-native forest insect species that became established in the U.S. between 1860 and 2006. The scientists also looked at the extent and type of damage caused by different groups of pests. They also examined whether the kinds of pests coming into the country had changed over time. The comprehensive information compiled by the researchers should help state and federal regulators to strengthen regulations designed to exclude potentially damaging pests from entering the U.S. and to develop better methods to detect and manage the exotic forest insects that are already here.

“We found that more than 455 non-native species of tree-feeding insects and at least 16 pathogens that affect trees are now established in the continental United States,” McCullough says. “This isn’t a new thing – exotic forest pests go back to the 1800s, and at least one of them was here before 1700.”

McCullough notes that only about 14 percent of the 455 insect pests cause substantial economic or ecological damage.

“That may not seem like much, but it’s still pretty scary,” she says. “Some of these pests can be devastating when they become established in a new habitat.”

In past decades, most of the invasive forest insects in the U.S. were from Europe. Increases in global trade and travel, however, have provided more opportunities for forest insects from Asia and other world regions to enter the U.S.

Invasive pests entering U.S. from Asia

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.