HEAT UNITS AND OUTBREAKS OF THE FOREST TENT CATERPILLAR, MALACOSOMA DISSTRIA (LEPIDOPTERA: LASIOCAMPIDAE)

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Abstract
The numbers of heat units during a fixed overwintering period and a shifting early larval feeding period for the forest tent caterpillar, Malacosoma disstria Hübner, were calculated from daily temperature data and related to known infestations of this insect in the Prairie Provinces and Ontario. Paired comparisons showed that years with increasing populations had cooler overwintering periods and warmer early feeding periods than did those with decreasing populations. Continuous records for each of 10 climatological stations showed that all known infestations in these areas were preceded by a single year (2 to 4 years earlier) with a relatively cool winter and an unusually warm spring, and that most population collapses were accompanied by cool springs and some by warm winters. The same general pattern prevailed for infestations in southern Ontario, when compared with the numbers of heat units at Toronto for the period 1860 to 1969. Some of the assumptions made during the analyses and the possible use of the results in predicting when and where outbreaks are likely to occur are discussed.