Comparison of Binocular and Cut-branch Methods for Estimating Budworm Defoliation of Douglas-fir


  • T. L. Shore Canadian Forestry Service Pacific Forestry Centre 506 W. Burnside Rd. Victoria, BC V8A 1M5


Defoliation caused by the western spruce budworm , Choristneura occidentalis Freeman, was estimated on 91 Douglas-fir trees Pseudotsuga menziesii var. glauca (Beissn.) Franco, by both close examination of cut -branches and by observation with binoculars. For individual trees the accuracy attained with the binoculars was within 23% for current year's and 19% for foliage of all ages , with respect to the estimates made from cut branches. Inaccuracy was found to be mainly due to lack of precision as bias was minimal. When the trees were assigned. by each method , into the broad de foliation classes of light ( 1-25%). moderate (26-65%), and severe (66- 100%). as used in forest insect surveys in British Columbia. the results agreed in 89% of the trees studied for defoliation estimates of current foliage and 68% of the trees for defoliation of total foliage. Classification of the location averages into severity classes agreed for all 5 locations studied for damage to current and total foliage. We concluded that the binocular method is a quick and useful means of classifying stands into broad defoliation severity classes. but is not suitable if a high degree of accuracy and precision is needed.