In a colony, worker bees play many roles: providing food, taking care of the young and even managing the nest's heating and air conditioning systems. Bumblebees (Bombus impatiens) do this exceptionally well, generating their own body heat to keep the nest toasty, and ‘fanning’ the nest with their wings to cool it down. However, making sure the nest temperature is not too cold or too hot – but just right – takes quite a bit of energy. As climate change causes heatwaves to happen more often and for longer periods of time, more worker bees may need to help fan the nest, taking them away from vital jobs such as foraging for food. So, Tiffany Bretzlaff, Jeremy Kerr and Charles-A. Darveau from the University of Ottawa, Canada put bumblebees to the test, keeping colonies under high heat for 2 weeks to understand whether ‘fanners’ could cool down their nest and still maintain the other jobs required to keep their colony going.

Bretzlaff and the team tagged 43 worker bees from each colony and placed them with their queen in a ‘nest’ made from a Styrofoam cooler, a substitute above-ground nest with a convenient clear top for monitoring the colony. The researchers placed a total of 15 bumblebee colonies into three different environmental chambers, where five colonies were held at 25°C, 30°C or 35°C for 2 weeks. Each Styrofoam ‘nest’ was connected to a flight cage by a secret tunnel (clear tubing with a tag reader) that cleverly allowed the scientists to identify who was coming and going, and for how long. The flight cages allowed the bees to forage freely. A worker bee was considered a ‘forager’ if they logged 10 or more trips between 3.5 and 25 min long and considered a ‘fanner’ if, when in the nest, they stayed still and beat their wings continuously for at least 10 s.

Contrary to what they predicted, bees foraged at the same levels at 30°C and 35°C as they did at 25°C. However, the number of fanners each day was dramatically higher in colonies at 35°C (14 fanners) compared with colonies at 25°C (4 fanners), just to try to keep the nest cool. Unfortunately, these fanners weren't entirely successful. The scientists also measured the temperature inside the nest, and while colonies at 25°C and 30°C could keep their nests mostly within optimal nest temperatures (30–33°C), colonies incubated at 35°C remained at ∼35°C throughout the 2-week trial. And by the end of the 2 weeks, colonies at 35°C had produced less than one-quarter of the larvae and pupae found in the colonies at 25°C, suggesting that high heat may reduce the number of eggs laid or how well the eggs are developing.

On top of that, high heat caused nearly three-quarters of the bees to abandon their nest rather than stay in the heat. Although the bees couldn't fully escape from the heat, in these instances, they preferred remaining in the flight cage over their nests. Even at 30°C, Bretzlaff and the team found that 19 of the original 43 worker bees would rather abandon their nest than continue with the valiant effort of fanning. No one yet knows if these bees would have chosen to return to their nests if the ‘heatwave’ concluded, or whether they would move on to a new colony. While fanning didn't take away from other jobs such as foraging, high heat resulted in greater nest abandonment and fewer young, indicating bumblebees just can't cope with the cost of keeping cool.

J. T.
Handling heatwaves: balancing thermoregulation, foraging and bumblebee colony success.
Cons. Physiol.