Periodicity of western spruce budworm in Southern British Columbia, Canada

Abstract

The western spruce budworm (WSB), ChoristoneuraoccidentalisFreeman), a defoliator of conifers in western North America, causes severe timber losses to forests. In British Columbia, Canada, where the main species damaged is Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco, outbreaks of C. occidentalis have been recorded since 1909. However, there is little information on the frequency of outbreaks of this defoliator for previous centuries. This information is needed to establish baselines defining the historic range of variability of this disturbance, to calculate potential depletions in timber supply from defoliation, and to refine forest management plans. Also, precise estimates of budworm recurrence are needed to assess potential ecosystem changes and possible departures from the historic range of this disturbance due to global warming. We used dendrochronology and time series analysis to determine past frequency of spruce budworm outbreaks in southern BC and found that, since the 1500s, outbreaks have been periodic, with a mean return interval of 28years (95% Confidence Interval 21–35years). No data was available before the 1500s. We found the number of outbreaks per century, since the 1800s, was fairly constant, with 3–4 outbreaks per century.