Accuracy of Direct Visual Counts of Western Spruce Budworm Larvae (Lepidoptera: Tortricidae) on Douglas Fir and Grand Fir Branches

Abstract

Late-instar Choristoneura occidentalis Freeman on foliage samples can be counted accurately at the tree, but their density may be substantially underestimated if proper care and procedures are not used. In our study, we visually examined branches for larvae at the tree and then later checked for additional larvae by beating the branches on baffle screens. Visual examinations underestimated larval densities on five plots by 2.5–15.1%. Factors identified as affecting the accuracy of larval counts were experience of the examiners, procedures used in examining the foliage, temperature, stage of larval development, tree species, and amount of defoliation. Foliage examiners must be thoroughly trained and their work must be regularly and randomly checked to ensure accurate counts.