Oecologia (1993) 93:25-30 Oecologia 9 Springer-Verlag 1993 Large-scale forest fragmentation increases the duration of tent caterpillar outbreak Jens Roland Forestry Canada, Ontario Region, 1219 Queen St. East Sault Ste. Marie, Ontario, Canada, P6A 5M7 and Department of Zoology, University of Alberta, Edmonton, Alberta, Canada, T6G 2E9 Received: 11 May 1992 / Accepted: 19 August 1992 Abstract. I examined historical data (1950-1984) on the duration of outbreaks of the forest tent caterpillar (Malacosoma disstria) in northern Ontario, Canada. Outbreak duration was compared to host tree species dominance and forest structure over large areas of boreal forest partially cleared for agriculture. Abundance of the principal host tree species Populus tremuloides had no consistent effect on duration of outbreak within forest districts, and was negatively correlated with duration of outbreaks among the eight forest districts examined. The amount of forest edge per km 2 was the best, and most consistent, predictor of the duration of tent caterpillar outbreaks both within individual forest districts and among forest districts. Because forest tent caterpillar populations are driven largely by the impact of parasi- toids and pathogens, results here suggest that large-scale increase in forest fragmentation affects the interaction between these natural enemies and forest tent caterpillar. Increased clearing and fragmentation of boreal forests, by agriculture and forestry, may be exacerbating out- breaks of this forest defoliator. Key words: Insects - Edge effects - Population dynamics - Habitat heterogeneity - Landscape ecology The dynamics of insect predator-prey systems and the severity of insect pest outbreaks may be strongly affected by the spatial heterogeneity of natural environments (Taylor 1991). Spatial heterogeneity may alter relative dispersal rates of insect hosts and their natural enemies allowing the host to "escape" from predation, parasitism, or disease. Support for this view comes largely from the analysis of mathematical models of insect host-predator interaction (Reeve 1988; Hastings 1990; Hassell et al. 1991) and from small-scale empirical studies (Huffaker 1958 ; Kareiva 1987). There is virtually no evidence, how- ever, that large-scale dynamics of insect predators and their prey are affected by habitat heterogeneity (Taylor 1990, 1991). Here I document that outbreaks of forest tent caterpillar, in boreal mixedwood forests in Ontario, Canada, are of longer duration in areas that have high structural heterogeneity (expressed here as the ratio of forest edge per km 2) resulting from forest clearing and fragmentation. The forest tent caterpillar is one of the most dramatic examples of North American forest pest insects that exhibit cyclic population outbreak and decline. Out- breaks occur predominantly in forests with at least some trembling aspen, the principal host plant. In 1991, 190,000 km 2 of forest suffered moderate to severe defo- liation by this insect in the province of Ontario (Anony- mous 1991a), up from a low of 306 km 2 in 1983. The cyclic pattern of population outbreak and decline has a mean period of about 10 years (Hodson 1941; Sippell 1962; Daniel 1990). The decline from peak density of this pest is associated with high mortality caused by a nuclear polyhedrosis virus (Clark 1958; Stairs 1966) and by the parasitic fly Sarcophaga aldrichi (Hodson 1939, 1941, 1977; Sippell 1962; Witter and Kulman 1979); the latter is also implicated as a transmission vector of the virus (Stairs 1966). Despite a relatively consistent period of the population cycles over a wide geographical range (Hod- son 1941; Daniel 1990), the duration of the high-density outbreak phase is as short as 2 years in some areas and as long as 9 years in others (Sippell 1962). Because de- cline of tent caterpillar is strongly associated with in- creased impact by its natural enemies, the variation in the duration of outbreak may result, in part, from the effect of habitat structure on the host-parasitoid or host-patho- gen interaction. Methods I examined historical data on the spatial extent of the past three complete caterpillar outbreaks (rise, peak and decline) in the province of Ontario, over the interval 1950through 1984.Variation in duration of these outbreakswas comparedto the spatialstructure of forested and nonforestedland over 26,623 km 2 in 261 townships in 8 forest districts (Fig. 1). For the analysis, I selected forest