Honey bee eggs, larvae, pupating juveniles, and pupae develop at slightly different temperatures and are all warmer than the brood nest

Apis mellifera (honey bees) use specific behaviors to maintain remarkably constant brood nest temperatures, from 33 °C to 35 °C, across a wide range of ambient temperatures. In addition to these colony-scale behaviors, a small subset of nurse bees behaves as “heater bees.” All nurse bees contract thoracic flight muscles to generate heat to warm the brood, but the heater bee thoraxes reach much higher temperatures (40 °C–46 °C) than those of other bees responsible for brood care. Whereas nurse bees maintain the elevated temperatures of the entire brood nest, heater bees focus their attention on incubating individual cells. We developed modified hives that allowed us to record the temperatures of undisturbed young in the brood nest area established by the colony and found that the temperature at which heater bees regulate developing worker cells is above the overall average temperature range of the brood nest. In an initial experiment, which measured larval and pupal temperature only, honey bees raised larvae at 36.38 ± 0.02 °C, substantially higher and with a narrower range than what has been reported for the brood nest, 33 °C–35 °C. Honey bees raised pupae at 35.18 ± 0.04 °C, which is also higher than the reported temperatures for the brood nest. In a follow-up experiment that measured brood temperatures throughout the entire 21-day developmental period, we found that eggs were maintained at 36.1 ± 0.03 °C, larvae at 36.2 ± 0.02 °C, pupating juveniles at 35.9 ± 0.03 °C, and pupae at 35.8 ± 0.03 °C. The temperatures of all stages differed significantly from one another, but importantly in this experiment, larvae were only 0.4 °C different from pupae. We then conducted another experiment with brood frames with the nurse bees removed and in incubators at 34.5 °C. Without nurse bees, the temperatures of eggs, larvae, and pupae were 34.4 ± 0.04 °C, 34.7 ± 0.05 °C, and 34.3 ± 0.04 °C, with larvae significantly different from all other stages and a 0.3 °C difference between larvae and pupae. However, the 0.4 °C difference between larvae and pupae in the second experiment vs. the 0.3 °C difference found with nurse bees removed suggests that the larvae themselves may be the major contributor to the temperature difference between the life stages. Either way, our results suggest that honey bees regulate temperature far more precisely during development than previously known.