Regional Variation in Susceptibility of Eastern North American Forests to Budworm Attack Based on History of Outbreaks

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Abstract
The history of spruce budworm [Choristoneura fumiferana] outbreaks based on radial growth studies of host and non-host trees from 5 main regions and 8 subregions in eastern North America, and covering a period of 2 centuries for most regions, throws some light on the epidemiology of this insect. Past outbreaks, instead of embracing the whole area suject to the most recent series of outbreaks, occurred in separate parts of the area. Epidemics of this insect did not recur regularly within regions, the time of occurrence of future outbreaks therefore cannot be predicted with accuracy on the basis of the periodic -ity of past ones. Pulpwood stands in the Atlantic sector were more prone to attack than those in Ontario. Optimum forest conditions (abundance of balsam fir [Abies balsamea]) for budworm outbreaks are found in the Atlantic sector, while optimum weather conditions (dry summers) occur in Ontario, but forest conditions in Ontario are a more limiting factor than climate in the Atlantic sector. Outbreaks have occurred most frequently and have been most severe in the southern Quebec -Main-New Brunswick region. Although fir reaches greatest abundance in central Quebec and in the Gaspe Peninsula, these regions afford marginal weather conditions for the development of outbreaks due to the prevailing cool wet climate. Past outbreaks were almost completely the result of natural factors. These continue to be important, but cutting practices, forest fires caused by man, and chemical treatment of budworm-infested areas are altering natural forest succession; this will probably affect the course of future outbreaks.