Dynamics of a nonoutbreak population of the Douglas-fir Tussock moth (Lepidoptera: Lymantriidae) in southern Oregon

A low-density population of the Douglas-fir tussock moth, Orgyia pseudotsugata (McDunnough), was studied for 13 yr in a mixed-conifer forest in south central Oregon. The densities of tussock moth and associated mortalities in each life stage were estimated over seven consecutive generations by sampling larvae and artificially stocking egg masses and pupae. A single census of early larvae was made annually for an additional 6 yr. Mortalities were summarized in life tables, converted to k -values, and analyzed to detect key factors, compensatory interactions, and density dependence. The largest losses in each generation were from the disappearance of larvae and the parasitization of eggs. A large, unexplained error was also present in most generations. None of the estimated mortality factors qualified alone as a key factor; however, the combined parasitization of eggs, larvae, and pupae was more closely related to changes in generation mortality than was predation of these stages. Parasitization was also delayed density-dependent, but predation was independent of density over the 7 yr that life-table data were collected. The major mortality factors interacted in a compensatory way, so that their combined effects for the full 13 yr of the study were delayed density-dependent and regulatory. This parasite/predator-prey system with a time lag apparently explains the low-density oscillation of tussock moth abundance.