Solar radiation alters heat balance and thermoregulation in a flying desert bee

Solar radiation is an important environmental variable for terrestrial animals, but its impact on the heat balance of large flying insects has been poorly studied. Desert bees are critical to ecosystem function through their pollination services and are exposed to high radiant loads. We assessed the role of solar radiation in the heat balance of flying desert Centris pallida bees by calculating heat budgets for individuals in a respirometer in shaded versus sunny conditions from 16 to 37 ˚C air temperatures, comparing the large and small male morphs and females. Solar radiation was responsible for 43 to 54% of mean total heat gain. Bees flying in the sun had thorax temperatures 1.7 ˚C warmer than bees flying in the shade, storing a very small fraction of incident radiation in body tissues. In most cases, flight metabolic rate was not suppressed for bees flying in the sun, but evaporative water loss rates more than doubled. The most dramatic response to solar radiation was an increase in convection, mediated by a more than doubling of convective conductance, allowing thermoregulation while conserving body water. In large morph males and females, the increased convective conductance in the sun was mediated by increased heat transfer from the thorax to abdomen. Because convection is limited as body temperatures approach air temperatures, solar radiation combined with warming air temperatures may cause endothermic flying bees to reach a tipping point at which increases in non-sustainable evaporation are necessary for survival.