From the Washington Post:

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.

From 1860 to 2006, at least 455 tree-loving insect species arrived on American shores, as did 16 damaging tree diseases ... 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.

See Brian Vastag's Post article for more.