Patterns of occurrence and new sampling implications for instar IV western spruce budworm (Lepidoptera: Tortricidae)
1993
Torgersen, T.R. | Colbert, J.J. | Hosman, K.P.
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.
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