Honey Bee Thermoregulation
By Kat Kabanova, TTP Summer Student
Honey 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.
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!
Figure 1. Honey bees (Apis mellifera) fanning outside the hive entrance to bring cooler air in.
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.
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 2. Bee yard with water barrels.
When is it too hot or too cold for bees? The exact temperature range at which adult . Social insects, including bees, have not only a very high chromosomal recombination rate – the mechanism responsible for producing genetic diversity among relatives – but also tend to produce short-lived generations9. These attributes allow honey bees to evolve traits that reflect and are favoured by their changing environment at a faster pace than, say, mammals. For example, urban bees tend to have narrower temperature ranges (i.e. they cannot withstand as high or low of thermal extremes as their rural counterparts), because cities generally have more constant temperatures throughout the year4. On a larger scale, honey bee ecotypes from more stable climates (e.g. the tropics) also have narrower temperature ranges4, and can thus suffer more from extreme climatic events than the bees adapted to living in regions with more variable climates (e.g. in Canada).
Apis mellifera spp. are much more tolerant of the heat than the cold because they evolved in the tropics. Established nests of honey bees can be found in unfavourable locations exposed to high heat and low humidity, such as the oases of the Sahara and the Arizona deserts, and the only requirement for their survival is plentiful water10. In hot weather, honey bees’ body temperature increases, allowing them to suck up nectar from flowers faster, therefore reducing the time they spend foraging8.
Figure 3. Snow-capped hives, photo taken by Shelley Hoover.
Lower ambient temperatures, on the other hand, have been shown to increase brood mortality and decrease the longevity of emerged adults. If a large enough proportion of brood is chilled, the colony’s age distribution is skewed, resulting in a shortage of foragers11. If this happens, younger nurse bees may be forced to go out to collect nectar and pollen, increasing the risk of bringing diseases and pathogens into the hive and decreasing the overall honey production11. The good news is, it’s not likely that a strong colony’s brood temperatures will drop below those required for successful development, unless the colony’s population is greatly reduced after a pesticide kill11 or swarming12. Chilled brood may also arise during early spring, when there aren’t enough adult workers to keep all the brood warm during the chilly nights12. In overwintering colonies (Fig. 3), chilled brood is often found on the outside of the cluster, which is considered a normal occurrence12. When working on colonies in strong winds or at temperatures below 23 °C, it’s best to not keep the brood chambers open for too long, as this also presents an increased risk of chilling the brood5. Reducing the time a colony is left open is good practice in general, as disturbed colonies are not only less productive, but also more susceptible to diseases7.
Last but not least, honey bees can use their heat-generating abilities for defense. If a wasp or a hornet intrudes the colony, hundreds of workers can surround the trespasser in a tight ball and fry them alive. Temperatures within this death ball can reach up to 47 °C2, enough to kill the hornet but still tolerable for the honey bees, whose upper temperature tolerance limit is about 4 °C higher13.
Next time you look inside your colonies, take a minute to appreciate the beauty and complexity of your bees’ thermoregulatory behaviours. Are there bees fanning the outside of the entrance into the hive? Or is it cold outside and the workers are clustered together to retain heat? And remember – you can help bees with thermoregulation by managing them according to the season, but strong, healthy colonies have the best chance of surviving harsh winters and being maximally productive during hot summers!