- Ives, W.G.H.
Canadian Forestry Service
Weather records from 48 climatological stations across Canada were examined in relation to reported outbreaks of the spruce budworm, Choristoneura fumiferana. Heat units above 40ºF and below 0ºF during the overwintering period and above 50ºF in the 6-week period following peak third install were related to population change. Paired comparisons showed that years with increasing populations tended to have lower values for the first two variables and higher values for the last than did years with decreasing populations. Continuous records over a 40-year period for non climatological stations showed that most expansions in areas of infestation occurred during relatively dry years, and that weather tended to be more favorable during outbreak periods than during non-outbreak periods. Extended periods of favorable weather, plus suitable host stands, appeared to be prerequisites for outbreaks.