Synchronous Patterns in an Outbreak of the Douglas-fir Tussock Moth 12

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Abstract
Population density was estimated annually from 1970–76 at 3 independent locations in the Blue Mountains of northeastern Oregon. Characteristic outbreak patterns occurred simultaneously at each location, although one of the 3 populations peaked at a density below the level of visible tree defoliation. Rates of population increase did not differ significantly among the 3 sites during the increasing phases of the outbreak. (1970–72). The observed differences in peak population densities were apparently the result of differences in larval densities at the inception of the outbreak. Synchronous patterns in the outbreak, and the potential use of population growth rates for prediction, accentuate the value of a system for annual monitoring of larvae to evaluate the status of tussock moth populations over a wide area.