Susceptibility of northern British Columbia forests to spruce budworm defoliation

Stand susceptibility to defoliation by spruce budworm, Choristoneura fumife rana (Clem.), was examined in the: Fort Nelson area of the Prince George Fo rest Region of British Columbia. In a retrospective study, defoliation maps of the study area were overlaid onto British Columbia Ministry of Forests cover type maps using a geographic information system. Analysis of the comb ined data identified forest characteristics associated with increased susce ptibility to defoliation by spruce budworm. These were stands where the lea ding species was white spruce (Picea glauca (Moench) Voss). or where spruce was associated with aspen (Populus tremuloides Michx. and P. balsamifera L .) in mixed stands. Susceptibility to defoliation also was related to site quality, level of crown closure and stand age. Spruce stands on medium qual ity sites (site index 15 to 25 m, at reference breast height age 50 years) were more susceptible than stands on both poor- and high-quality sites. Whe n spruce was mixed with aspen, stands on higher quality sites were more sus ceptible to budworm attack than poor sites. open stands, where crown closur e was <50%, were more susceptible to attack by spruce budworm than closed c anopy stands. Older stands (120-199 years) were more susceptible to budworm attack than younger stands (40-110 years). In defoliated plots monitored f or 6 years, tree mortality and top-kill reached a maximum of 30.4 and 47.2% , respectively. The losses varied with level of defoliation and were reduce d by applications of the biological pesticide Bacillus thuringiensis. Crown Copyright <(c)> 2001 Published by Elsevier Science B.V. All rights reserve d.