Relationships of Douglas-fir Tussock Moth Defoliation to Site and Stand Characteristics in Northern Idaho

Relationships between intensity of defoliation caused by Douglas-fir tussock moth, Orgyia pseudotsugata McDunnough, and descriptive characteristics of forest sites and stands in an outbreak area of northern Idaho were quantified. Defoliation hazard predictive models were developed from inventory data collected in 70 stands covering a range of successional stages and site and stand conditions within the grand fir-western red cedar ecosystem. Two models are presented, each accounting for approximately 50 percent of the variation in defoliation intensity. Defoliation was heavier on upper slope and ridgetop sites, negatively correlated with depth of volcanic ash mantle, and positively correlated with host tree age, proportion of grand fir in the stand, and the ratio of stand density or biomass to site index. A hypothesis that tussock moth outbreaks develop in response to changes in host foliage quality resulting from stresses is presented