Woody plant grazing systems: North American outbreak folivores and their host plants

In North America, about 85 species of free feeding and leafmining folivorous insects in the orders Lepidoptera and Hymenoptera periodically cause serious and widespread defoliation of forest trees (Appendix 1). We call these insects expansive outbreak folivores based on the following criteria: 1) population eruptions occur two or more times per 100 years, 2) severe host defoliation (> 50 percent) occurs for 2 or more years per eruption, and 3) the area of each individual outbreak exceeds 1,000 contiguous ha. There are about 20 other insects whose populations meet criteria one and two, but not criterion three. These we term local outbreak folivores and do not deal with them in this paper because they may operate on an entirely different scale than the expansive species.