EVIDENCE OF SYNCHRONIZED CYCLES IN OUTBREAK PATTERNS OF DOUGLAS-FIR TUSSOCK MOTH, ORGYIA PSEUDOTSUGATA (McDUNNOUGH) (LEPIDOPTERA: LYMANTRIIDAE)

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Abstract
Outbreak patterns of Douglas-fir tussock moth, Orgyia pseudotsugata (McDunnough), over western North America historically appear to be synchronous, particularly in British Columbia, Washington, Oregon, and northern Idaho. Populations of the insect increase to outbreak and collapse in a variable cycle, averaging 9 years between peaks. A review of all outbreaks suggests repeated, widespread, nucleopolyhedrosis viral epizootics are responsible for the collapse of the population and, hence, the cycle. The virus appears to survive in the soil between outbreaks and to be carried incidentally to foliage where it is occasionally consumed by larvae. Ingestion of a single particle is probably sufficient to cause infection. Populations of the moth increase until density reaches the point where larvae to larvae infection is established. The viral inoculum builds rapidly following that point and spreads widely so that distant populations at all densities become infected, and collapse in the same year. The epizootic continues for another year. Then foliage contamination disappears, and populations reach their lowest densities before starting the cycle again.