Invasion of North American Forests by European Phytophagous Insects

Over the last 500 years, nearly 2000 insects and 2000 weedy plants have invaded North America (Kim and McPheron 1993, Sailer 1983, Stuckey and Barkley 1993). Many of these inadvertent immigrants have become serious agricultural or forest pests (Liebhold et al. 1995, Wilson and Graham 1983). Kim and McPheron (1993) calculated that 40% of the major North American insect pests are of exotic origin even though as a group exotic insects constitute just 2% of the insect fauna. For example, Haack and Mattson (1993) discovered that alien, tree-feeding sawflies in North America are more prone to cause outbreaks than are the equivalent native species. Questions about managing these introduced insect species and concern about continued importation of new ones are timely because of the past severe and everspreading impacts of introduced insect pests on North American forest ecosystems (Campbell and Schlarbaum 1994, Liebhold et al. 1995). We have only begun to comprehend the manifold, often insidious effects of alien invaders on the