The European spruce bark beetle (Ips typographus) and the North American mountain pine beetle (Dendroctonus ponderosae) may kill millions of trees during outbreak periods. Both species have also experienced large outbreaks in recent years. But the magnitude of the outbreaks of D. ponderosae is much larger. In this review we compare the outbreak history of I. typographus in Sweden with D. ponderosae in British Columbia in Canada. We also discuss some possible explanations for the difference in outbreak magnitude between the two species. During the last fifty years (1960-2009), three outbreaks of I. typographus have occurred in Sweden which resulted in a volume of about 9 million m3 of killed Norway spruces (Picea abies). During the same period D. ponderosae has killed about 600 million m3 of lodgepole pine (Pinus contorta) in British Columbia. Based on a literature review we suggest two factors that may contribute to the much more severe outbreaks caused by D. ponderosae: (1) a lower colonisation density needed by D. ponderosae to overcome tree defences and (2) a higher reproductive success of D. ponderosae in killed trees. In addition, the proportion of old stands, susceptible to bark beetle attacks, is much higher in British Columbia than in Sweden
