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Sampling the Western Spruce Budworm: Fourth Instars, Pupae, and Egg Masses

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Abstract:

Foliage samples were drawn from the lower, mid, and upper crowns of host Douglas-fir and grand fir trees growing on sites in northcentral Washington, eastern Oregon, central Idaho, and both northwest and southwest Montana. Populations of the western spruce budworm, Choristoneura occidentalis Freeman, were sampled at three of its life stages: fourth instars; residual pupae (pupae remaining after predation) and pupal exuviae; and egg masses. Within trees, the western spruce budworm occurred in consistent vertical patterns. For a given plot, average fourth-instar and egg-mass density per square meter of foliage can be adequately estimated from midcrown, 45-cm terminal tips; average pupal density per square meter of foliage can be adequately estimated from lower crown, 45-cm terminal tips. None of these relations differed between Douglas-fir and grand fir. For each stage of the insect, among-tree variance is estimated as a quadratic function of the mean. A sampling scheme based on a predictive equation that links whole-tree density to density on 45-cm branch tips from a representative crown stratum is presented for each budworm stage. Forest Sci. 30:883-892.

Keywords: Choristoneura occidentalis; Douglas-fir; grand fir

Document Type: Journal Article

Affiliations: Research Entomologists, Pacific Northwest Forest and Range Experiment Station, Corvallis, OR 97331

Publication date: December 1, 1984

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