Life Tables for a Declining Population of the Douglas-fir Tussock Moth1 in Northeastern Oregon3

Abstract

Changes in population density and their causes were quantified on 22 sample plots representing 4 different defoliation intensities in a Douglas-fir tussock moth outbreak. Density of larvae was extraordinarily high at the beginning of feeding in June 1973. Thereafter, populations either declined sharply or completely collapsed on all of the plots studied. Measurable larval populations of the subsequent generation were found on only 6 plots in June 1974, and these had virtually disappeared by midsummer. Rate of decline was variable under different conditions but appeared to be predictable from the level of past defoliation. In general, the decline rate was faster on heavily or moderately defoliated plots than on lightly or proviously undefoliated plots. Population collapse was the result of a multitude of natural factors operating in compensating ways against the high density outbreak. It was characterized by an increase in the effect of numerous mortality factors including virus disease, insect parasites, predators, starvation, and larval dispersion, as well as changes in population quality and structure resulting in lower fecundity, egg quality and a change in sex ratio. The largest proportion of mortality occurred early in the larval cycle and was probably due to losses from natural dispersion and predation; however, other factors may have had an equal or greater effect on ultimate collapse of the population although they accounted for less total mortality.