Populations of western blackheaded budworm, Acleris gloverana (Walsingham), have fluctuated widely over the length of Vancouver Island with five outbreaks having occurred since 1941. The core of the severe defoliation occurred in the southern and northern portions of a central mountain chain running the length of the Island. Four monitoring sites were established in the central core area and five in peripheral sites during the 1970–1973 outbreak. At each site, samples were taken during five different life stages per generation, beginning at the peak of defoliation and ending when larval densities declined below detectable levels. The largest losses during the population collapse occurred during the larval period when large numbers abandoned the damaged trees by spinning off on silk threads. In addition, when age-specific survival ratios for each developmental stage were compared with the generation survival ratio, the closest relationship occurred between generation survival and the adult dispersal and oviposition period. Large emigration away from the central core area of defoliation to peripheral stands was detected, particularly during the first year of collapse. We suggest that, in response to damaged foliage, larvae and adults disperse from the stands and the outbreak collapses before the trees are killed. The dispersal response of defoliating populations appears to dampen the upward population trend. The trees survive, refoliate, and thus preserve the habitat for potential population increases during the next outbreak period. Host interrelationships during population gradations of the blackheaded budworm are compared with other conifer defoliators of British Columbia, particularly the western spruce budworm and the Douglas-fir tussock moth.