Overwintering mortality of forest tent caterpillar [Malacosoma disstria (Hübner)] eggs was estimated over a 360 km2 grid of 83 plots in north-central Alberta over the period 1992–1996 during a local outbreak. Egg mortality in the trembling aspen (Populus tremuloides Michaux) canopy was generally low; however, 20% of the eggs laid in the summer of 1995 failed to hatch in the spring of 1996. In the shrub layer, 70% failed to hatch. In both the canopy and shrub layers, the spatial pattern of mortality was density-independent, with high mortality occurring in low-lying areas. Daily temperature records suggested that the proximal cause of death was freezing during mid-winter. Caterpillar populations peaked in 1995, before perturbation, and collapsed during the summer of 1996, largely as a result of larval parasitism. The timing of this perturbation-assisted population collapse coincided loosely with the penetration of larvae down into the shrub layer. We illustrate how winter temperature, albeit a density-independent factor, probably acts in a partially density-dependent manner through interactions with density-dependent behavioral and physiological processes that influence spatial variation in vulnerability and susceptibility to winter cold. We argue that cold winter temperatures are an important factor influencing the long-term dynamics of forest tent caterpillar populations in northern climates.
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