Winter intensity shapes overwintering energy gain and use in bark beetles under range expansion.

The mountain pine beetle (Dendroctonus ponderosae) is an eruptive bark beetle that overwinters as a freeze-avoidant larvae under the bark of pine hosts. In recent years, D. ponderosae has undergone a climate change-driven range expansion into previously unsuitable habitats with historically more severe winter conditions. Dendroctonus ponderosae overwinters in a non-feeding dormant phase, and energy use is important to post-overwintering fitness. Little is known about how D. ponderosae balances energy supply and demand during overwintering. We quantified shifts in energy reserve (supply) and Complex I activity (as an index of demand) in D. ponderosae during natural overwintering and simulated early winter onset. We collected D. ponderosae larvae from infested lodgepole pine in the autumn (October), winter (January) and spring (April), and sampled a portion of these animals. During autumn and winter, another set of larvae were subjected to either mild overwintering conditions at 6°C or an experimental cold stress of stepwise decreases in temperature to test how an early onset of cold conditions influences the energetic status of overwintering individuals. Dendroctonus ponderosae larvae exposed to natural winter conditions accumulated lipids and proteins early in overwintering, which were then available for later use. Early exposure to cold stress in the autumn before full winter acclimatization, however, depleted energy reserves. These findings suggest that the timing and regulation of seasonal acclimatization in D. ponderosae have important implications for energy use that can influence subsequent fitness, and thus warming of the overwintering period may facilitate early winter feeding and enhance energy gain of D. ponderosae larvae, which could further exacerbate the spread and impact of this pest.