The damage is done: Delayed mortality after acute heat exposure reveals cryptic heat risk

Bumble bees are critical pollinators, in decline in part due to climate change. A key characteristic
of climate change is an increase in the frequency and intensity of extreme temperature events,
including heat waves, which can have striking effects on organisms. Temperatures during heat
waves rarely approach bumble bee critical thermal maxima (CTmax, ~ 50 C), but little is known
about effects of temperatures below CTmax. Bumble bees cease to fly when body temperature
exceeds ~42°C, so we asked whether bumble bees could tolerate short (30 min, about equal to
the duration of a foraging trip) exposure to air temperatures from 41 to 45°C. In accordance with
other work, we found minimal immediate effects of these temperatures. However, we discovered
dramatic increases in mortality 24-48 hours after exposure to 44 and 45°C, suggesting cryptic
damage incurred by exposure to seemingly benign conditions. Follow-up experiments with
varying durations (10, 20, 30 minutes) bolstered this finding. To relate our lab experiments to
field temperature data, we put out operative models in Laramie, WY in the summer of 2020. We
used lab mortality measurements across different time:temperature combinations to estimate wild
bee mortality throughout the summer season. Further, we used a spectral approach to simulate an
increase in mean temperature of 0.7°C, and show that even small increases in mean temperature
could significantly increase wild bumble bee mortality, suggesting the carryover effects of sub-
CTmax temperatures may, in part, be responsible for bumble bee declines and range shifts
worldwide.