THE ROLE OF CLIMATE AND DISPERSAL IN THE INITIATION OF OUTBREAKS OF THE SPRUCE BUDWORM IN NEW BRUNSWICK: I. THE ROLE OF CLIMATE

Abstract

New Brunswick outbreaks of the spruce budworm which began in 1912 and 1949 are considered in relation to the theory of climatic release. Studies on a natural population show that larval development is more rapid in dry and sunny weather than in humid and cloudy weather. Polar air masses bring the favorable conditions, and tropical air masses and cyclones the unfavorable. June precipitation and temperature records, analyzed in conjunction with weather maps, show that climatic changes took place in regions where the outbreaks developed. The outbreaks were preceded by dry and sunny summers during four or five consecutive years. Although direct mortality of the budworm due to adverse weather conditions has not been observed, favorable climatic conditions may have indirectly promoted population increase from the endemic to the outbreak level in the following ways. Flower production became more frequent in periods of dry years and larvae which fed on staminate flowers developed more rapidly. Larvae that developed early in the season gave rise to more fecund females than larvae that developed late. The age of current foliage consumed is also related to fecundity. The rate of development of balsam fir relative to the budworm varies from year to year. It is postulated that the average fecundity of the budworm increases in the pre-outbreak years as the result of favorable climatic conditions and greater than usual flower production. Larval mortality is greater in years with a prolonged developmental period although the increase is not statistically significant. The theory that outbreaks in New Brunswick resulted from the spread of populations from outbreak areas to the west will be considered in the second part of this two-part paper.