D3232: Shifts in the temperature dependence of metabolic rate in overwintering bumble bee queens

Many insects seasonally shift their metabolism, often in association with diapause. An overall reduction in metabolic rate at a range of temperatures allows overwintering insects to reduce energy expenditure, maximizing utility of energy stores. Queen bumble bees overwinter underground for 6-9 months and must survive on fixed energy reserves. To minimize energy expenditure, they may show metabolic depression across a range of temperatures, but data regarding bumble bee responses to temperature during both overwintering and active life stages are limited. We measured metabolic rates for both mated and unmated Bombus impatiens queens across a range of ambient temperatures (1-13 °C) during and after overwintering. As expected, metabolic rates increased exponentially up to about 8-10 °C. For all queens above ~8-10 °C, metabolic rates dropped considerably, suggesting that these heterotherms reached temperatures warm enough to allow regulation of respiration. Surprisingly, metabolic rates for overwintering queens were higher relative to those post-overwintering. These heterotherms may therefore have alternative strategies for managing overwinter energy usage.