Should Beekeepers Help with Hive Temperature Control?

“Can I Leave the Supers on in Mild Winters?”

There are beekeepers who ignore advice. There are beekeepers who live in environments with mild winters, where they leave the honey supers on. There are also beekeepers who live in mild winters, keep their supers on, ignore advice and then ask why their hive died. Yours truly was one of those – I kept the supers on during our winters (average temperature range 8-16°C) and got lucky by only having a weak population for spring.

I know, I know – there are internal hive temperature requirements and the ideal “winter cluster size”, and yet, ignored advice – adding a tarp atop my hive boxes to protect them from the rain (and also reflect the sunlight… well done me). Colonies can produce heat by clustering and contracting their wing muscles. If it’s too hot, excess workers will head outside and beard, whilst others fan cool air to ventilate. Should beekeepers help hives with temperature regulations?

There are two shirs that with this article – designed & pressed by Yours Truly.

Cold Temperature and Honey Bees

Modelling work by Becsi and colleagues showed that the duration of extreme cold accounts for 20% of winter colony deaths in Austria (Becsi et al., 2021). The most impactful were the periods of low temperatures in the mid- and late-winter, which meant a larger drain on colony resources, and less time to restock in preparation for the spring surge.

Ventilation & Insulation Help Bees Survive the Cold

Beekeepers can assist bees with managing cold climates by:

• Removing excess (i.e. all) honey supers. Even in mild climates the bees will not be ripening to make honey. Reduce them to the brood box and let the colony control temperatures (“thermoregulate“).
• Covering & insulating the hives – in snowy regions, let the snow build up and act as an insulating blanket.
  • Better yet: add a layer of corrugated polypropylene boards followed by foam insulation. St Clair & colleagues found this method to increase overwintering survival, reduce food consumption and maintain a higher cluster temperature compared to controls (St. Clair et al., 2022).
    • Caveat: the covers did not appear to have an effect during the coldest times in the winter. Only when the ambient temperatures exceeded 0°C and colony buildup commenced that a difference appeared in covered vs uncovered colonies.
• Ventilation. All but one of my hives have a meshed bottom board, and these allows for airflow. I can’t comment on solid bottom boards in snowy winters, as I do not live there – I wonder if they have issues with airflow on account of being a solid slab?
• Placing hives in locations protected from strong winds and with adequate sun exposure during the day. Long ago, the advice was to “put them indoors”, or keep them in a trench to minimise cold exposure.
• Placing candied sugar or fondant on the top bars near where the middle frames to provide the bees with a food source that won’t disrupt the cluster’s formation.
  • Candied sugar or fondant are more favourable than sugar syrup on account of their low water content; bees do not process watery compounds in cold conditions.
• Treating for Varroa mites, which are known to reduce overwintering success.
  • Inspect the hive in autumn to make sure they’ve got sufficient stores/populations to survive the winter months. Consider combining colonies or adding extra feed to strengthen weaker colonies.

Too Cold to Fly: Winter Cluster

A winter cluster is when bees group together (IN a “rugby ball shape”) to maintain warmth. The bees on the outer layer act as insulation, preserving the heat generated by those inner bees. Heat generation occurs by contraction of their flight muscles, termed “shivering thermogenesis” (Stabentheiner et al., 2003). Most heat-generating bees were located in the core (~15% of all bees in the hive), and their population decreases towards the surface; a reduction in insulating bees means a faster rate of heat loss.

Recall that convection currents transfer heat from one place to the other. These air currents carry the carbon dioxide waste released by the bees out of the cluster. Previous studies showed that when temperatures drop, colony oxygen consumption increases – slight between +10°C and-5°C and extreme between -5°C to -10°C (Southwick & Heldmaier, 1987). Further, the smaller the colony cluster¹, the greater the oxygen demands, as a greater number of bees are required to contribute to its survival (Southwick 1985).

Worker Rotation and Avoiding the Chill Coma of <~10°C

The “chill coma” occurs when the bee’s thorax cools below 9-11°C, rendering her unable to contract its flight muscles for heat generation (Watmough & Camazine, 1995). This bee falls off the cluster and dies. To avoid this, the authors assumed that the other cluster bees would generate heat, but as stated above, it would be an inefficient use of worker energy. Only in emergencies do the outer bees start shivering- For example, when a layer of bees finds itself on the outermost comb, away from the remaining populations.

A speaker at a beekeeping workshop, noted how bees rotate between the inner and outer cluster. This gives the outer bees a break to warm up, eat and rest, while those inner, cozy bees rotate to the mantle and serve as insulators. If you want a reference for this, then quote my memory.

Cold, But “Not Too Cold”, Climates

What about mild winters? I live in an temperate area and daytime winter temperatures are ~12-15°C. Bees can (and do) fly in small numbers. The remaining bees are inside, keeping the hive warm with a less-intensive cluster. This means the hives don’t get a “brood break” – queens continue to lay eggs, and brood rearing proceeds slowly. A few flowers species like camellias, daphnes and violas bloom for nectar and pollen. Worse comes worse, the bees can use their honey stores for food.

These bees still shiver to raise their own body temperature, not for the sake of the cluster. Through this, they can increase their temperature from ~6°C (on a cold morning with the sunrise) to 37°C in less than 20 minutes (reviewed in Vallejo-Marin, 2022).

Protecting Bees from Excess Heat

The first visible sign of heat stress is bearding at the hive entrance. Many bees cluster together to reduce the number of bodies inside the hive box in an attempt to reduce temperature. At this stage, some of the returning foragers have crops full of water rather than nectar, used to cool the hive and quench colony thirst. Further worker bees are at the hive entrance, fanning their wings to circulate fresh, cool air inside. How can beekeepers help hives keep cool, and how do bees survive hot conditions?

Beekeeping Interventions During Hot Weather

The idea here is to use the inverse of cold climate management- more water when there is none, shade instead of sunlight:

• Place hive under trees with the entrance facing southwards – they’ll get the morning sun and avoid the afternoon sun. Elevate the hives – add more bricks to the hive legs or invest in hive stands. The higher off the ground they are, the better the ventilation.
  • If that is not possible, consider placing shadecloth around the hive, one that is reflective of heat. Foam also helps, on account of it being able to withstand temperature changes.
• Use a screened bottom board – it’ll help with ventilation. Some beekeepers say to use a ventilation insert that keeps the lid slightly ajar- I’m not a fan of this method as it could further attract pests, and cause issues in rainy days.
• Provide a water source. Even if you don’t, the bees would have found a water source in the form of a swimming pool, birdbath or a animal trough.
• Add a super to reduce overcrowding and give bees more space. It’ll reduce the heat generated from densely-packed bees.
  • Warning: if the colony is weak, an extra super will end up attracting small hive beetles or wax moths… Then again, if your hive is weak, you wouldn’t see as much bearding or fanning as their populations are lower.
• If necessary, feed the bees with sugar syrup. The sugar and water will provide hydration and energy to the hive.
  • If you do this, be sure to replace the contents and clean it often. Hot environments + water + sugar = rapid microbial growth, which translates into possible infections.

Bees and Heat

The temperatures at which bees first begin foraging varies. Tan and colleagues showed that the first signs of foraging occurred at an average air temperature of 6.6°C, peaking at 20°C (Tan et al. 2012). In another study, Woyke and colleagues (Woyke et al., 2003) noted that the starting temperature was 10°C. Granted, both studies were done in Asia (China & Nepal respectively), yet both authors found that the number of foragers increased by a factor of ten when temperature rose to 12°C.

In contrast, foraging is at its lowest when ambient temperatures are 43°C (Blazyte-Cereskiene et al. 2010). Any returning foragers would be gathering water, as plants don’t produce nectar in hot conditions.

Bearding of Bees Outside the Hive Entrance

Bearding is the “external cluster”. It is a response to the warm weather, and workers will clump outside the hive to reduce the number of bees inside. This lowers the heat generated by their activities – cleaning, feeding, nectar processing. The increased hive space improves ventilation, as more air can pass through. Pair this with fanning, and the bees can effectively reduce the humidity and heat, maintaining the ideal internal environment.

Fanning to Circulate Air

Extracted from “Why do bees buzz“

When temperatures reach higher than 24-25°C, beekeepers see honey bees rapidly flutter their wings while remaining stationary at hive entrance (Terenzi et al., 2020). It produces a soft buzzing that’s less intense than the flight buzz, as their wing strokes are less forceful. Bees fan for three main reasons:

• To circulating air within the hive and cool it during hot conditions,
  • Also reduces carbon dioxide levels within the hive
• The speed up the evaporation of the water in nectar in the process of making honey (after all, honey is ~20% water),
• Dispersal of pheromones, as in after swarming, to help the disoriented bees find the hive.

Water Gathering & Evaporative Cooling

A honey bee colony does not need to store water, because unlike nectar or pollen, water is generally available. During times of thirst or high water demands, the colony has a system for rapid activation and deactivation of water collectors according to their needs. When bees experience a hot day and the demand for water increases, water-collectors readily offload to receiving bees. They may even do waggle dances to recruit additional bees to water sources.

The collected water cools the hive through evaporation as the workers spread droplets on the combs. In one study, evaporative cooling of the brood started at environmental temperatures higher than 29-30°C (Stabenthiener et al., 2021). The authors note that these hives were not exposed to the sun (heat came from an electric heater), and it was not until the air temperature reached the aforementioned range that they begun collection water. Were these colonies placed in direct sunlight, water foraging would have commenced before the air temperature reached threshold on account of the sun’s heat.

Heat Shielding

In a colony, there are always “idle” bees- those waiting in line for their next task. When heat rises, these bees become employed as heat shields- placing themselves on the hotter regions of the hive wall (i.e. where the sun is hitting) to absorb excess heat (Starks & Gilley 1999). The reasoning behind is thought to be because workers can survive temperatures up to 50°C, whereas the brood is negatively impacted when the temperature rises above 36°C.

This shielding was more evident in the brood box rather than the honey super, as the former contains a living thing, and the latter a store of honey. In their natural environment, heat shielders serve as a mobile insulation unit for the thin tree-nest cavity walls exposed to harsh sunlight. It adds another layer of heat adaptions, along with fanning and evaporative cooling.

Heat Shielding is Useless?

Stabenthiener & colleagues suggest that heat shielding is a faulty adaptation, as it only delays heat flow into the colony, and increases the passive heat generated by those shielding bees as they attempt to cool it (Stabenthiener et al., 2021). The hotter the bee, the higher the heat generation from her metabolism; as she cools off, the heat is released back to the hive. Bearding is suggested as superior alternative, as the absorbed heat is released into the environment.

Perhaps like the winter cluster, the heat-shielding bees should rotate between the hive walls and the hive entrance beard to release her excess heat?

Drones Help Keep the Hive Warm?

The old wives’ (beekeepers’?) tale of “drones are only good for sex” is being refuted. Again. Kovac and colleagues showed that drones participated in colony thermoregulation during cold weather by heating their thoraxes (Kovac et al. 2009). This occurred in drones over two days of age, who had a higher metabolic rate and thus heat generation.

That being said, Fahrenholz and colleagues suggest that even younger drones (less than 2 days old) can contribute to colony warming due to their large numbers (you see more younger than older drones inside hives; Fahrenholz 1992). The big benefit to beekeepers from drone as insulators is that they will not cross the queen excluder and head into the supers, thus contributing to brood box temperature management.

If you’ve read this and smell bull faeces, you’re correct. This is a passive process with little to no real world impact. If a colony is relying on drones for warmth, it’s got bigger problems than thermoregulation. Drone biology is about reproduction, the other material is clutching at straws (albeit successfully).

Measuring Hive Temperature

There are commercial and scientific monitors for hive temperatures. One commercial example is the Broodminder- strips placed on the top bars of a hive to monitor the box’s temperature. Useful for super or brood box monitoring, as well as temperature spikes during swarming.

An example of a more advanced approach is the LoRaWAN technology, a remote monitoring system that collects data on hive temperature and humidity in real time (Gabitov et al., 2022). This device is more suitable for wild colonies without established comb, as the authors note its significance for wild beekeepers (“Bashkir beekeepers”) in Russia.

Subspecies-Dependent Temperature Control

The functional differences between subspecies deserves its own article(s).

Different subspecies have different responses to the their environment. For example. it is not surprising to know that Apis mellifera jemenitica is better adapted to the Arabian deserts than the gentle Austrian A. m. carnica.

In one study, Mitchell modelled the responses of the savanna bee A.m. scutellata with the northern European bees (likely hybrids of A.m. ligustica, carnica & mellifera; Mitchell, 2022). He found that the broader European bees have better heat-retaining abilities and brood covering area compared to the slender savanna bee. Viewed in light of their environment, the (relatively) broader European bee evolved a greater girth in response to the cold weather, whereas the slender frames of the A.m. scutellata is an adaption to the constant heat of Africa. Further:

• Kovac and colleagues showed a difference in metabolism and heat tolerance between A.m. carnica and ligustica, with the latter having a higher tolerance as a response to their environment – Southern vs Central Europe respectively (Kovac et al., 2014).
• A higher number of heat shock proteins (and thus increased heat tolerance) in A. m. jemenitica compared to A.m. carnica – again, a response to their environment (Alattal & Alghamdi, 2023)
• Even in Africa, A.m. adansonii can withstand higher temperatures than the A.m. capensis – suggestive of adaption to the African deserts compared to the windy regions of the Western Cape of South Africa (Worswick, 1987)

Beekeepers Should Help Weaker Colonies

I conclude by stating that yes, weaker colonies – those impacted by pests and disease or with low populations – should be helped by the beekeeper. In cold climates, consider adding extra insulation via foam, or leaving the snowcaps on. In warm conditions, give them a meshed bottom board and position them in a place that receives morning sun and shelter from the afternoon sun- trees are great for this.

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Footnotes

1. This also extends to swarms, which are “small clusters of bees” themselves (Heinrich 1981). However unlike the winter bees, swarms happen in warmer conditions, so the bees have a lower heat production requirement than winter clusters. ↩︎