D3023: Winter temperature variability alters cold tolerance of overwintering queen bumble bees

Increased variation in winter temperatures may profoundly affect insect winter dormancy. Queen bumble bees (genus Bombus) overwinter underground, where local conditions (e.g. snowpack and depth) may alter the temperature variation they experience. The limited available data suggest bumble bees do not survive freezing, which occurs at relatively warm temperatures: supercooling points (SCPs) were ~-8°C for two commercial species. SCP may therefore be an important determinant of overwintering survival for bumble bees, though it is unclear whether SCP shifts in response to thermal history. To determine whether temperature variation affects bumble bee SCP, we overwintered bumble bee queens for 6 weeks at three different daily temperature regimes (constant 4°C, 4°C ±2°C, 4°C ± 6°C) and then measured SCP. Temperature regimes simulated average highs, lows, and levels of variation estimated from ground temperature data. Increased variation in overwintering temperatures significantly increased SCPs, suggesting bumble bee queens experiencing increased levels of winter temperature variation will be at higher risk of freezing during winter.