Limits and mechanisms of honey bee colonial thermoregulation in the heat

Pollinators play critical roles in food sustainability and terrestrial ecosystems, yet we lack understanding of the limits and mechanisms of colonial thermoregulation during heat stress. Despite the extensive behavioral studies of thermoregulation in honey bees and the recognition that temperature extremes impact colony growth and survival, the upper limits of honey bee colony thermoregulation and survival under extreme heat have not been well studied. Textbooks and reviews state that honey bees can maintain internal conditions at air temperatures up to 60°C. Here, we provide the first measures of colonial gas exchange for standard honey bee colonies during an extreme but realistic heat stress, and provide the first quantitative assessment of the thermoregulatory capacities of a heat-stressed Apis mellifera hives. Air temperatures of 50°C for 4 h caused brood areas to reach damaging 38°C in most colonies. Heat stress reduced pollen foraging, induced fanning and bearding behavior, suppressed colonial metabolic rate and elevated evaporative water loss. Contrary to textbook paradigm, increased evaporative water loss was not the primary mechanism of colonial thermoregulation in the heat. Rates of heat buffering mechanisms during this heat challenge were passive heat storage in honey and wax (76%) and in bees (14%), metabolic suppression (7%) and increased evaporative heat loss (3%). Hive temperatures fall at night, allowing passive buffering to reduce diurnal heat stress. Social regulation of behavior, physiology and nest traits provide heat mitigation, but projected heat waves may exceed these capacities.