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Patterns of Occurrence and New Sampling Implications for Instar IV Western Spruce Budworm (Lepidoptera: Tortricidae)

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This study examines relations of western spruce budworm (Choristoneura occidentalis Free.) larval distribution on terminal tips, whole branches, among crown strata (vertical crown thirds), and within and among whole trees. Study sites were in Washington, Oregon, Idaho, Montana, and New Mexico and represented 63 plot-yr of data. A range of densities from <1.0 to >600 larvae per m² of foliage on midcrown terminal tips were observed. We validated and expanded on earlier published descriptions of intratree and intertree distribution of larvae as they influenced sampling schemes. Larval density on nominal 45 cm terminal tips in the lower crown-third were slightly better predictors of whole-tree density than were midcrown tips. Therefore, we propose the use of a two-tip sample from the middle of the lower crown-third of each sample tree. Equations are given to determine the number of trees to sample for desired precisions and confidence levels. We provide equations describing relations between larval densities on branch tips and whole branches in vertical crown-thirds, and their relation to whole-tree density. For. Sci. 39(3):573-593.

Keywords: C. occidentalis; Choristoneura

Document Type: Journal Article

Affiliations: Formerly Forestry Technician, USDA Forest Service, Forestry and Range Sciences Laboratory, 1401 Gekeler Lane, La Grande, OR 97850

Publication date: August 1, 1993

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