The increasing threat of exotic pests

The introduction of exotic (nonnative) species are considered by many to be the single greatest threat to the integrity and viability of forest ecosystems. With increasing global trade the possibility of more frequent introductions increases. The mountains and oceans are no longer effective barriers to the movement of species. More than 4500 species of plants, animals, insects and fungi are known to be established in the United States. This includes about twenty organisms causing tree diseases and about 370 insects that feed on trees.

Many of these exotics blend in and cause no noticeable change to our forests. However, some cause significant forest health problems and ecosystem changes. Exotic species, for a number of reasons, are more likely to cause serious damage and changes than native species. Some introduced or exotic species like the fungus that causes chestnut blight cause catastrophic tree mortality and in this case virtually eliminated a once dominant tree species. American chestnut once made up 25% of the eastern hardwood forests.

Examples of some of the more important exotic forest pests and diseases in the eastern forests are gypsy moth, chestnut blight, white pine blister rust, Dutch elm disease, butternut canker, dogwood anthracnose, balsam wooly adelgid, beech bark disease and hemlock wooly adelgid.


Invasion by Exotic Forest Pests: A Threat to Forest Ecosystems. by A M Liebhold, WL MacDonald, D Bergdahl, VC Mastro. Forest Science Monograph 30.

Lost Species: Impact of Exotic Pests on the Eastern Forest. By Robert A Hack. North Central Forest Experiment Station East Lansing MI, USDA Forest Service Eastern Region Newsletter, May 1995.