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1. Four groups of spruce budworm larvae, Choristoneura fumiferana, of the same physiological stage (at the beginning of diapause) were exposed to natural temperatures, starting in July, August, September and October. Post-diapause emergence and certain metabolites were monitored throughout the overwintering period.

2. Larval exposure to high temperatures for long periods before winter had a significant effect on temperature requirements for subsequent diapause development and apparently caused a certain amount of mortality, especially for those larvae that entered diapause earlier in the season.

3. Substantial loss of lipid and glycogen reserves was related to intensity of high temperature exposure during the pre-winter period, which in turn was associated with the timing of diapause initiation.

4. Patterns of glycerol accumulation during the winter were also influenced by temperature conditions in the early phase of diapause development.

5. Among several possible mortality factors, an extended cool period in spring was suggested as a potential cause of overwintering mortality, which was closely associated with a deficiency in energy reserves caused by high temperature exposure before winter.

6. These results stress the significant impact of pre-winter conditions on the overwintering process and highlight the importance of an ecophysiological approach to insect overwintering biology.