Introduction

In 2000, annual forest losses and control costs in the United States due to nonindigenous forest insects and pathogens were estimated at $4.3 billion (Pimentel and others 2000), and that figure is likely to rise due to the increasing transport of species beyond their native habitats (Brockerhoff and others 2006b). In ecological terms, these pests alter forest composition, structure, and productivity; in some cases, pests basically remove host tree species from forest ecosystems (Brockerhoff and others 2006b, Levine and D’Antonio 2003). A case in point is the once-dominant American chestnut (Castanea dentata), which was virtually eliminated from Eastern forests by chestnut blight (Cryphonectria parasitica) within 50 years of the pathogen’s introduction from Asia (Liebhold and others 1995). Over a similar timeframe, the hemlock woolly adelgid (Adelges tsugae) has caused extensive mortality throughout the Northeastern United States and has recently spread into the Southern Appalachian Mountains, raising fears that it will decimate both eastern hemlock (Tsuga canadensis) and Carolina hemlock (T. caroliniana) populations in the region. Notably, the adelgid was considered a mere nuisance pest of ornamental hemlocks for a few decades after its accidental introduction around 1953, until it began to spread to natural forest stands during the mid-1980s (Souto and others 1996). A related pest, the balsam woolly adelgid (Adelges piceae), was also accidentally introduced in the latter part of the 20th century, and has removed more than 90 percent of Fraser fir (Abies fraseri) from already sparsely distributed spruce-fir communities in the Southern Appalachians (Pimentel and others 2000). In a recent example, the emerald ash borer (Agrilus plannipennis) now infests an estimated 40,000 km² in the Great Lakes region and Ontario, where it has killed at least 15 million ash (Fraxinus spp.) trees and has regularly jumped quarantine boundaries. It is likely that the insect was introduced to the United States 10 or more years before it was first recognized in 2002, during which time populations were able to establish and spread undeterred (US-GAO 2006).