Author : Campbell, R. Date : 2003. Title : Multicentury history of western spruce budworm outbreaks in Interior Douglas-fir forests near Kamloops, British Columbia. Publication : Unpublished M.Sc. thesis. University of Victoria, Victoria. Issue : Page(s) :
Western spruce budworm (Choristoneura occidentalis (Freeman)) is a native defoliator of the Interior Douglas-fir (Pseudotsuga menziesii var. glauca (Mirb.) Franco) forests of British Columbia, Canada. This thesis used dendrochronology and the software program OUTBREAK to reconstruct a defoliation history of Douglas-fir for nineteen forest sites near Kamloops in central British Columbia. By comparing the radial growth response of non-host ponderosa pine trees to Douglas-fir trees growing in nearby but separate stands, seven western spruce budworm outbreaks were distinguished over the past 300 years. Although there is considerable variation in the timing and duration of these western spruce budworm events at the stand level, synchronous outbreaks have occurred at approximately 43-year intervals. Climate variation appears to have been important to budworm outbreaks in the 20th century. Notable outbreaks tended to occur during years of early springs with average air temperature, following winters with lower than average precipitation. Based on this finding, it is proposed that with high overwintering survival, increased population growth rates, and a longer growing season, the extent of future outbreaks will shift northward and may increase in size.