Pollinators are in decline, and the extent to which climate change plays a role is unclear. Heat waves are predicted to increase in frequency, but at what point will this limit pollination? Flying honey bees are known to use evaporative water loss to prevent overheating, but the limits on this strategy are unknown. I found that under hot conditions, bees can carry loads with 20% lower cost and heat production, helping them avoid overheating. In dry air, at thorax temperatures above 42°C, evaporative water loss increases exponentially, allowing bees to maintain thorax temperatures below their critical thermal maximum – even when carrying maximal nectar loads. However, the water demand for thermoregulation at such temperatures is tremendous, estimated to be 15% of body water content in a 30 min flight. At high humidity, evaporative water loss will be ineffective, and at an air temperature of 46°C, bees cannot carry a nectar load greater than 5% of body weight without overheating. Pollen foragers generally carry small amounts of nectar, and thus have minimal capacities to thermoregulate by evaporation. An 80 mg forager carrying a 20 mg pollen load, with 5 microliters to evaporate, will not be able to fly more than 6 min in dry air at 46°C. In desert areas, heat waves will restrict pollen foraging and colony growth, while in humid areas, heat waves will severely limit foraging and possibly colony survival. This research was partially supported by USDA 2017-68004-26322.
