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

The mountain pine beetle (MPB, Dendroctonus ponderosae) is an eruptive bark beetle that overwinters as a freeze-avoidant larvae under the bark of pine hosts. In recent years, MPB has undergone a climate-change-driven range expansion into previously unsuitable habitats with historically more severe winter conditions. MPB overwinters in a non-feeding dormant phase, and energy use is important to post-overwintering fitness. Little is known about how MPB balances energy supply and demand during overwintering. We quantified shifts in energy reserve (supply) and complex I activity (as an index of demand) in MPB during natural overwintering and simulated early winter onset. We collected MPB from infested lodgepole pine in fall (October), winter (January), and spring (April), and sampled a portion of these animals. Other larvae, during fall and winter, 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. MPB 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 fall before full winter acclimatization, however, depleted energy reserves. These findings suggest that timing and regulation of seasonal acclimatization in MPB 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 MPB larvae, which could further exacerbate the spread and impact of this pest.