D3361: Mortality skyrockets after a simulated heat wave: Are bumble bees toast?

A key characteristic of ongoing climate change is an increase in the frequency and intensity of extreme events, including heat waves. Although heat waves are understudied (likely because they are rare and unpredictable), they can have striking effects on organisms. Bumble bees are critical pollinators, in decline in part due to ongoing climate change. They can tolerate acute exposure to high temperatures (CTmax >50°C), but it is largely unknown whether exposure to high temperatures below these critical limits harms bumble bees. Based on field microclimate temperature measurements taken during a heat wave in the Pacific Northwest USA, we exposed bumble bee (Bombus vosnesenskii) workers and males to 45°C for durations of 10, 20, and 30 minutes, and then measured survival for the next 48 hours. Only 10 minutes at 45° C significantly decreased 48-hour survival of female (worker) bumble bees relative to controls and most workers died within minutes of a 30 min exposure to 45°C. Conversely, male bumble bee survival was largely unaffected by heat treatments. These findings suggest that heat waves may profoundly affect bumble bee populations, even when maximum temperatures are well below their upper critical thermal limits.