PHYSICAL AND BIOLOGICAL INDICATORS OF THE DEVELOPMENT OF OUTBREAKS OF THE SPRUCE BUDWORM, CHORISTONEURA FUMIFERANA (CLEM.) (LEPIDOPTERA: TORTRICIDAE)

Abstract

Biological and meteorological records were examined for periods when outbreaks of the spruce budworm, Choristoneura fumiferana (Clem.), were known to have occurred in northeastern North America. The survey showed that the following significant events occurred during the period of three to four years preceding an outbreak. Decreasing annual numbers of low pressure centers passed over the area in which the outbreak later occurred. Therefore, the outbreak began at a time of decreased or minimal storminess. Drought occurred, chiefly in June and July, but also occasionally in spring and autumn. Annual increments of host trees on dry sites declined. Outbreaks of the forest tent caterpillar, Malacosoma disstria Hbn., occurred, particularly in Ontario. These events, in aggregate, occurred so consistently before spruce budworm outbreaks that they have future predictive value. In addition, they reinforce some suggestions made by earlier authors and suggest modifications of hypotheses concerning the behavior of developing populations of the spruce budworm.