Outbreaks of the Forest Tent Caterpillar, Malacosoma disstria Hbn., a Periodic Defoliator of Broad-leaved Trees in Ontario

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Abstract
Severe defoliation of aspen stands by the forest tent caterpillar, Malacosoma disstria Hbn., became apparent in 1948 in three widely separated regions in Ontario, and by 1949 it was clear that another general outbreak of this defoliator had begun. In 1950 the Forest Insect Survey (cf. McGugan, 1958) initiated an intensive program of observation, collecting, and rearing to record the progress of the outbreak and study the parasite complex of the insect. This paper compares the outbreak in the early ‘fifties’ with earlier ones, and attempts to describe the pattern of build-up and decline of forest tent caterpillar populations within extensive forested areas.