Numerical Analysis of the Causes of Population Collapse in a Severe Outbreak of the Douglas-fir Tussock Moth

Abstract

Natural populations of the Douglas-fir tussock moth, Orgyia pseudotsugata (Mc-Dunnough), were studied on 22 sample plots in 1973–74 during the decline of an outbreak in the Blue Mountains of northeastern Oregon. Causes of the decline were evaluated by comparing rates of mortalities during the generation that decline began. Factors having the greatest effect on density variation in the subsequent generation were also identified by determining the major sources of variation in a model of population change. The largest source of loss was the disappearance of young larvae from the foliage because of dispersal, predation, and starvation. Virus disease and insect parasitization accounted for a relatively small proportion of the total loss. Survival of instars III and IV and the production and survival of overwintering eggs had the most influence on density differences in the next generation. Most of the mortality in these stages and the reduction in fecundity were directly or indirectly a result of intraspecific competition, which caused food stress and starvation in the outbreak. Although insect parasites and virus disease caused significant mortality in the later stages of tussock moth development, they did not account for dissimilarities in the downward trend of numbers on different plots. The failure of natural controls, especially a viral disease, to develop earlier in response to rapidly increasing numbers of tussock moth caused the outbreak to be especially severe and, ultimately, to be limited primarily by the depletion of suitable food.