1. It has recently been suggested that expected increases in temperature might lead to an additional generation per season in bark beetles. Thus, populations would grow more rapidly. However, an additional but not fully developed generation could lead to high winter mortality in the pre-imaginal stages.
2. Winter survivability and post-diapause fitness as a function of the overwintering developmental stage of the spruce bark beetle Ips typographus L. were studied along an altitudinal cline. Fitness was tested in terms of weight, lipid content and flight capacity.
3. Mortality was significantly lower and emergence per brood system significantly higher for fully developed adults that entered overwintering than for larvae, pupae or callow adults.
4. Post-diapause fitness in terms of dry weight and flight capacity was significantly higher in individuals that completed development before winter, and lipid contents also showed a trend for being higher in those individuals.
5. In conclusion, in a scenario where effective temperature sums are not adequate for the complete development of an additional generation, models may overestimate population growth by neglecting increased mortality and reduced post-diapause fitness. The results highlight the importance of considering life-history traits and indirect effects in addition to abiotic factors such as temperature when modelling population dynamics.