Analysis of historic western spruce budworm defoliation in south central British Columbia

Abstract

The western spruce budworm Choristoneura occidentalis Freeman (Lepidoptera: Tortricidae) periodically defoliates interior Douglas-fir Pseudotsuga menziesii ((Mirb.) Franco) forests in British Columbia. Repeated budworm defoliation causes tree mortality, reduction in growth rates and reduced lumber quality. An overlay analysis using historic and recent annual aerial overview information was completed to describe population fluctuations of the western spruce budworm in the south central portion of British Columbia. The overlay analysis defined areas of chronic, intermittent and no budworm defoliation between 1916 and 2003 in potentially susceptible forest types. The majority of the defoliation records from south central BC during this time period occurred within the Kamloops Forest Region, where we identified five outbreak sub-regions that were geographically distinct. Stands within the Interior Douglas-fir biogeoclimatic zone were most susceptible and suffered the greatest amount of chronic, sustained budworm activity. Over 45% of the Interior Douglas-fir zone had recorded defoliation, with some outbreaks lasting over 9 years. Within the Interior Douglas-fir zone, the dry, cool subzone and very dry, hot subzone were most prone to sustained outbreak events. The moister ecosystems, such as the Interior Cedar Hemlock zone, had over 30% of its area defoliated. However, these outbreaks were less frequent and rarely lasted more than 3 years.