Alberta Bee News
August 2022
Honey Bee Thermoregulation
It’s important to know that not all bees thermoregulate the same. Both age and genetics3 can affect the extent to which workers contribute to keeping the colony warm. Older adults are much more tolerant to outside temperature shifts than newly emerged and unhatched bees. In addition, foragers can withstand lower temperatures than nurse bees, drones, and queens. Even so, when temperatures drop to 10 °C and below, foraging adults may start to lose neuromuscular function and enter chill-coma5. Bees older than 2 days old control the temperature of the brood comb to make up for the lack of thermoregulatory abilities in eggs, larvae and pupae1. Uncapped brood are kept between 33 and 36 °C, and pupae, which are the most cold-sensitive, are kept just around 35 °C6. To warm the brood, “heater bees” vibrate their thoracic flight muscles while on or near brood cells1 – an onerous but essential activity.
By Kat Kabanova, ABC Tech Transfer Program Summer Student
When outside temperatures rise, adult bees bring droplets of water into the hive. They then spread the droplets throughout the nest and fan their wings (Fig. 1) to evaporate the water, ultimately cooling the brood and reducing the hive temperature. This is also an essential activity, as both uncapped and capped brood will begin to die from overheating at 37 °C7. In contrast to nectar, water is not stored in the hive for later use. Therefore, having a close-by water source (such as in Fig. 2) is essential for bees to avoid spending too much time and energy on making water foraging trips in hot weather. Interestingly, bees are more attracted to water sources that have a smell to them – presumably, this indicates a higher nutritional value5. In practice, it is important to make sure that nearby water is not contaminated by insecticides or fungicides. The water source should also contain some sort of landing pad (rocks, straw, sand, etc.) to prevent the foragers from drowning during collection4.
Figure 1 Honey bees (Apis mellifera) fanning outside the hive entrance to bring cooler air in.
H
oney bees are a managed species, prized for their ability to make and store honey. Honey is bees’ main carbohydrate source and provides them with energy to perform all the tasks they need to continue thriving. In the tropics, honey serves as a food supply during the dry season, when flowers are scarce. In temperate climates, consumption of stored honey allows bees to continuously vibrate their thoracic and flight muscles to maintain cluster temperatures well above those outside the hive.
Figure 2 Bee yard with water barrels.
Why do bees need to thermoregulate, and how do they do it? Honey bees are heterothermic insects, meaning they can switch between producing heat (or cooling off) physiologically, and relying on external heat sources to meet their individual and collective thermoregulatory needs1. In colder conditions, bees actively “shiver” by quickly alternating contracting and relaxing their flight muscles to generate the heat they need2. As the weather warms up, bees will then rely on ectothermy gaining heat from external sources - and bask in the sun, their darker stripes helping them absorb more heat2. In the winter, bees cluster in a tight circle to retain heat. By constantly vibrating their wing muscles, they can keep the colony at a constant temperature of 15 °C to 30 °C, exact temperature depending on colony strength and brood presence5. This behaviour allows the colony to persist throughout the winter rather than dying off or entering dormancy like most insects5. Therefore, try to minimize winter inspections to avoid disrupting this finely tuned seasonal thermoregulation! 4
