Modeling the costs of overwintering bumblebee queens

Newly emerged bumble bee queens are the only members of the colony that overwinter and survive beyond the growing season. To do so, they must first accumulate enough energy reserves (e.g., lipids) during the growing season to last them through the winter. After building up their body reserves queens must find an overwintering refuge (e.g., underground crevice) that will both protect them from extreme cold and moderately warm temperatures. If the temperatures are too cold, overwintering queens may reach their supercooling point and die. If the temperatures are too warm, a queen will rapidly burn through her energy reserves which could result in starvation while overwintering, premature emergence, which could also result in starvation if food sources (e.g., flowers) have not also emerged, or a generally poor (i.e., nutrient low) body condition. A queen in a poor body condition likely requires initial emergence time for foraging rather than building a colony, which can reduce the number offspring produced and thus negatively affect fitness. To better understand the costs of overwintering bumble bee queens we developed a series of mechanistic models based on empirical studies of Bombus impatiens. Our models use measures of temperature-specific metabolic rates, total body lipids, and supercooling points to estimate the energetic costs of overwintering. Through these estimates, we predict the probability of surviving a variety of overwintering conditions, likely survival times for different overwintering conditions, and potential emergence times based on both changing body conditions and environmental temperatures.