Introduction
Forests cover approximately one-third of the land area in the United States. These forests are comprised of approximately 500 species of trees plus thousands of identified species of terrestrial and aquatic animals and non-woody plants (USDA APHIS and Forest Service, 2000) and likely thousands of undescribed species (http://www.discoverlife.org). Representatives of almost every type of vegetation that occurs worldwide can be found within the United States or its protectorates (cf. USDA APHIS & Forest Service, 2000). Additionally, many exotic plant species are grown for horticulture, Christmas trees, and other uses. Approximately 4,000 exotic plants are established outside cultivation in the United States (Kartesz, 1999; United States Geological Survey, 1998). This combination of native and exotic species across the United States provides ample opportunities for imported pests to find suitable hosts (USDA APHIS and Forest Service, 2000; Niemala & Mattson, 1996). The estimated 4000 exotic insects and 200 pathogens that are permanently established in North America (USDA APHIS & Forest Service, 2000) demonstrate the vulnerability of our natural resources to exotic organisms.

Forest ecosystems vary in their susceptibility to exotic pests. Forest ecosystems comprised of relatively few tree species, e.g., forests dominated by Douglas-fir in the Pacific Northwest, are more affected by species-specific pests than forests which have more diversity. Conversely, the diverse eastern and Californian forests have greater opportunities for exotic pests to find suitable hosts. Damage to host species may range from negligible to potential extinction. The impact of some exotic pests is noticeable in a relatively short period after introduction, e.g., Asian longhorned beetle, or can be delayed as with Asiatic oak weevil (Triplehorn, 1955). Although some generalizations can be made, there is an uncertainty about how an exotic species will react in a new environment, what impact it will have on host species, and when it will be recognized as a problem species (USDA APHIS & Forest Service, 2000).

Sources for introduction
Kartesz, J.T. 1999. A Synonimized Checklist and Atlas with Biological Attributes for the Vascular Flora of the United States, Canada, and Greenland. First Edition. In: Kartesz, J.T. and C.A. Meacham, Synthesis of the North American Flora, Version 1.0, North Carolina Botanical Garden, Chapel Hill.

Niemela P. and W.J. Mattson. 1996. Invasion of North American Forests by European Phytophagous Insects. Bioscience Vol. 46, No. 10 (November 1996), pp. 741-753).

Triplehorn, C. A. 1955. The Asiatic oak weevil in Delaware. J. Econ. Entomol. 48: 289 293.

United States Department of Agriculture Animal and Plant Health Inspection Service and Forest Service, 2000. (Pasek, J.E., H.H. Burdsall, J.F. Cavey, A. Eglitis, R.A. Haack, D.A. Haugen, M.I. Haverty, C.S. Hodges, D.R. Kucera, J.D. Lattin, W.J. Mattson, D.J. Nowak, J.G. O'Brien, R.L. Orr, R.A. Sequeira, E.B. Smalley, B.M. Tkacz, W.W. Wallner) Pest Risk Assessment for Importation of Solid Wood Packing Materials into the United States.

United States Department of Interior. United States Geological Survey 1998. Status and Trends of the Nation's Biological Resources. Washington, D.C.

TNC Regions

For organizational (and not scientific) reasons, The Nature Conservancy has divided the world into a number of geographic regions. On this page we have present the pests in the Gallery of Pests regionally, using the TNC geographic regions. If you are uncertain of your geographic region, look at this pdf map for clarification.