Surviving Weather Extreme Heat & Cold
Apis mellifera aims to maintains a stable colony temperature of ~36°C during environmental extremes. In hot weather, worker bees fan their wings, beard outside the hive and spread water over frames, or huddle into a ball and shiver to maintain warmth during a cold cluster. A consistent temperature is necessary for optimal brood rearing – too hot and they die, too cold and… they also die. Cold management is not-as-important outside of the brood nest, as the honeycomb withstands being frozen and thawed. Heat management matters more – no hive wants melting comb.
This article discusses temperature regulations at the two extremes of hot and cold, and describes what happens to drones, larva, workers and queens under such conditions. This is post is theory-heavy. For practical approaches, consider reading Hive Box Insulation for Bees & Beekeepers and Should Beekeepers Help with Hive Temperature Control?
There are two shirts that go with this article – designed & pressed by Yours Truly.
Thermoregulation is Performed by the Worker Bees
Worker bees can survive short exposures to ~50 °C, or in below-freezing temperatures so long as they remain in their cluster. When temperature extremes are lethal to an individual bee, the colony works as a single unit (“superorganism”) to buffer the climate and maintain preferred conditions. We start with the cold:
Clustering and Shivering to Keep a Warm Core
An individual worker starts to lose function as her body temperature decreases, becoming immobile at ~10°C. A colony in cold conditions generates heat by having individual bees shivering their thoracic (“flight”) muscles; they “detach” these muscles from the wings and rapidly contract them, warming their bodies and the surrounding comb. Any worker bee older than two days of age can participate (this is when their exoskeleton hardens and flight muscles are more developed). A new division of labour arises: Young adults (0–2 days old) who passively heat individual combs via normal metabolic heat release (minor contributors), and the remaining older workers shiver to heat the hive (Stabentheiner et al., 2003).
Thorax temperatures can reach into the mid-30°C regardless of ambient air temperature. Some workers press their bodies into the brood cells and transfer heat, while others huddle around them to preserve the generated heat (the cluster). The outer layer of workers insulate the heat generators and bees continually rotate between this cold outer layer and the warm core of the cluster; it keeps the outer bees alive and give the inner bees a rest.
Cluster formation depends on brood presence – when brood is present, bees cluster faster. Temperatures inside a cluster reach 35°C, keeping the queen and workers active for hive maintenance. The outer mantle drops to ~17°C, and if the temperatures go lower, or the bee rotation is slow, these workers will lose normal function… when the outer layer falls, the inner soon follows, leading to colony death via domino effect.
You know how they advise beekeepers to keep “sufficient honey” (or supplemental sugar) for the wintertime? Heating (and cooling) are metabolically costly– workers consume more of their honey stores to continue the muscle contractions, and if the stores are insufficient the colony may not survive. The common advice is to leave them with several frames of capped honey or several kilograms of icing sugar/fondue as a supplemental feed (NOT syrup – they do not need the excess water).
Tolerating Extreme Heat – Fanning, Evaporative Cooling
When hive temperatures begin rising above the optimal range (~36°C)1, workers ventilate the nest by fanning their wings at the entrance of the hive. Some fanning also occurs between frames, with the goal of circulating cool air into a colony. Fanning is coupled with evaporative cooling –some foragers are allocated to water gathering duties with the end goal of spreading the droplets over the comb. As the water evaporates, the temperature lowers. It’s easier to cool than heat a hive. Fanning affects large sections of comb, and water can be spread all over the brood comb, as opposed to clustering in one location.
Heat Shielding as a Mobile Temperature Regulator
An additional bee-haviour is heat shielding. This method is for managing a single area that’s overheating – think a patch of brood receiving direct sun on the hive wall. Some middle-aged workers will press their bodies against the hot area of comb to absorb excess heat and then release it elsewhere (Starks & Gilley 1999). Shielding is more prevalent in the brood box than the honey stores… Perhaps in the bees’ natural environment, heat shielders serve as a mobile insulation unit for the thin tree-nest cavity walls exposed to harsh sunlight?
However, 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 (what I don’t understand is why can’t that super-hot bee go outside to cool down?). Bearding is suggested as superior alternative, as the absorbed heat is released into the environment.
Lethal Heat: High Temperatures and Humidity Matter
We know from the work of Groh & colleagues (amongst others), that brood survival goes to zero once temperatures reach 38°C (Groh et al., 2004). Adults are more flexible; One study showed all workers dying within 24 hours at 45°C degrees in laboratory settings (Abou-Shaara et al., 2012). This same study reported long survivals at 35°C (unsurprising), however there was a subspecies-dependant advantage at 40°C – the Yemeni bees (A.m. jemenitica) had better outcomes than the Austrian bees (A.m. carnica) More on subspecies-dependant survival in part 2.
Temperature does not act independently; the environment’s relative humidity also matters. Bees survive better in higher humidities due to reductions in water loss2. Joshi & Joshi (2010) suggest that temperatures matter more than humidity for honey bee activity, whereas Li & colleagues found higher relative humidity to have a small effect on survival, to an extent (Li et al., 2019). To paraphrase: “At relatively high temperatures, high humidity can inhibit water loss in bees, and therefore impairs boy temperature reduction”.
Temperature Control is Stricter when Brood are Present
In a strong colony with brood, temperatures within the hive nest vary by only a couple degrees at 32-36°C. If brood are removed, bees relax their thermoregulation, and the hive temperature can (modestly) shift with ambient conditions.
Brood Rearing in Cold Conditions
In the absence of brood, a hive begins clustering around 20°C- when brood is present they cluster at warm threshold of ~20°C (Kronenberg & Heller, 1982). For heating, it is uniform – when the ambient temperature increases past 30°C, they begin with evaporative cooling and more-intensive fanning. Both strategies are used to maintain a brood nest temperature around 33–36°C.
Brood are temperature sensitive (Many studies; Tautz et al., 2003 used here). Even in this optimal range of 33-36°C, when brood are reared the higher end (~36 °C) they tend to develop slightly faster: emerger sooner, start foraging earlier, dance more vigorously, and show better short-term memory. On the hand brood reared at the lower end (~32–33 °C) take longer to develop, are slower waggle dancers and slower learners. In Tautz’s study, few of the low-temperature reared brood remained after two weeks; they speculate these bees left hive on foraging trips and failed to find their way back due to poor learning (Tautz et al., 2003)3.
Other studies show that lower temperatures are responsible for a decreased lifespan and general quality of life in the adult bees (e.g. Wang et al., 2016). Low-temperature reared brood are more susceptible to stress and have deformed body parts, like:
- Reduced wing length (flight is more difficult),
- Shorter proboscis (reduces access to floral nectaries and flower forage variety), and
- Abnormal abdominal tergums (less room for internal organs).
Exposure to 20 °C for a short period can kill most brood or reduce their eventual lifespan if they do survive (Wang et al., 2016). This is why you do not disturb the cluster- affected brood may die or worse, emerge deformed, a loss of the next generation of worker bees. Sealed pupae take priority over larvae as the former cannot move or generate heat to help themselves, nor have physical contact with a warmer worker.
Saving Brood from High Temperatures
The upper thermal limit for healthy brood development is close to the normal brood nest, with a 100% mortality rate for brood reared at 38°C (e.g. Groh et al., 2004). Constant 37°C showed insignificant development abnormalities, but the number of eclosing pupa (i.e. emerging adults) dropped to 36-42% (also Groh et al., 2004).
Bees do not decrease the brood quantity under chronic heat stress (Weinberg et al., 2023). Instead, they consume their nectar and honey stores (measured by a decrease in hive weight). This suggests that in response to extreme temperatures, A. mellifera would rather use their stored resources to maintain equilibrium rather than pause brood rearing. Increased temperatures and increased food usage would mean these bees are less likely to survive in the winter (Calovi et al., 2021). There is truth in this, as the colony stores honey for the sake of dearths and winter survival- in these extreme cases, beekeeper intervention through supplemental feeding may be necessary.
The Queen and Drones are NOT Involved in Temperature Regulation
Her Majesty and Her boytoys *AHEM* consorts are not involved in temperature regulation. Nerds can argue that “b-b-but they still release latent heat via their metabolism” and they are right, since that might contribute to some heating efforts in a winter cluster (e.g. Kovac et al., 2011), but how much of it is real-world significant? (if winter temperatures are cold enough for a cluster, the drones wouldn’t be alive anyway).
Queens are robust to temperature changes, however her sperm is sensitive, and can be damaged during postage, when Her Majesty is shipped by a queen breeder to another beekeeper for requeening hives.
Queen Temperature Tolerance Matters for Reproductive Health
In a winter cluster, the queen is kept warm at the core with her normal retinue of worker bees. In hot conditions, she her retinue follows her into the cooler parts of the hive. Her Majesty rarely experiences temperature stress under normal conditions. Where queens experience temperature stresses is outside her hive (e.g. during shipping or in weak colonies during extreme heat events – think heatwaves).
A healthy mated queen can survive brief exposure high temperatures (up to ~42°C recorded by McAfee et al., 2021), but the beekeeper’s real concern is not in immediate survival rather her reproductive health. After mating, the queen stores sperm in her spermatheca, and these gametes are sensitive to temperature, remaining near 100% viable between 15 °C and 38 °C (McAfee et al., 2020). Exposure beyond this range, even for a short while, greatly reduced the number of live sperm.
If you’re thinking that this leads to partially or wholly-infertile queen, you are correct, and it is (very likely) she will be superseded by the colony. If you were thinking this would be obvious due to an abnormal laying pattern… you are also correct, provided that queen continues “laying blanks” for several months. Why? Because McAfee and colleagues found that both temperature extremes did not impact laying pattern in the short term (2 weeks post exposure; McAfee et al., 2021)- probably because queens release multiple sperms to fertilize a single egg, and at this point, the sperm stores have not been substantially reduced.
A Brief on Mailing Queens to Beekeeper
When a queen is shipped through the mail, she’ll inevitably experience temperature spikes or chills that can lead to higher queen failure rates once in the hive. Consider queens transported without proper thermal insulation: they are more likely to be superseded or die early, even if the temperatures during transit were “only” in the mid-20°C, it is the rapid fluctuations that harms her- such fluctuations rarely happen in the hive (unless you’re frequent and careless in your inspections!).
Drone Contribution to Temperature Control is Passive
Drones do not actively help regulate nest temperature – they do not engage in fanning, water collection, or heat shielding. Their primary function is reproduction. That being said, Kovac and colleagues found that drones can generate heat and warm their surroundings, but since there are far fewer (older) drones reside in the brood area compared to workers, their overall contribution is small (Kovac et al., 2009). By chance, several drones might warm themselves and incidentally heat nearby cells, but the majority of brood incubation is done by the workers.
Implications for the Beekeeper
In short, leave clustering bees alone. Ensure they’re in a good hive with adequate sunlight, no matter how little your location may get it. For hot conditions, have a steady water supply nearby to reduce the distance foragers must travel. For further reading on proper hive box insulation, read Hive Box Insulation for Bees & Beekeepers. If you’re after practical approaches to temperature hive management there’s Should Beekeepers Help with Hive Temperature Control?
Part 2 compares thermoregulation in the A.mellifera subspecies, as well as to other Apis species
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Footnotes
- The rate at which fanning begins is genetically determined (Graham et al., 2006). Graham and colleagues found that “after controlling for other sources of variation (think hive size, moisture levels, honey stores etc..), our genetically diverse colonies had, on average, significantly less variation in brood temperature than the genetically uniform colonies”. There are also times when bees fan at low temperatures for ventilation of carbon dioxide (Seeley, 1974). Others fan because… the others are doing it – if an idle bee is surrounded by fanners, she too will start fanning (Kaspar et al., 2018). Peer pressure! ↩︎
- They can survive for (slightly) longer if they’ve been acclimatised to a higher temperature and the relative humidity is lower (Free & Spencer-Booth, 1962). Younger bees have better heat tolerance, presumably because they spend most of their time in the brood nest. Inversely, these bees had the lowest cold tolerance (“higher chill coma temperature”). Higher temperatures and lower humidity lead to better survival as the bees can cool themselves better by evaporation. In extreme temperatures (i.e. laboratory settings), the opposite is true, higher humidity made for worse survival because it was the evaporation of water (desiccation) that presented a risk factor. ↩︎
- This contrasts Szentghyorgyi’s finding who found that both workers and drones reared at the lower ranges had a larger mass and longer lifespan (Szentgyorgyi et al., 2018). They propose: “increasing body mass in lower temperatures is suggested to be an adaptive mechanism to increased heat conservation capacity in larger animals associated with their relatively small body surface area”. Whereas the increased survival could be due to bees being reared in laboratory conditions… much less hostile than a natural beehive. ↩︎
THis was so helpful thank you
You’re most welcome, anon