Nest Temperature Control

Based on the size of a honey bee colony, the homeostasis (steady internal conditions) of the nest is conditioned not only by temperature, but also by humidity levels, and by the balance of oxygen and CO2 levels, in relation to the conditions of the nests outside ambient environment. Thanks to the specialized receptors on its antennae, the bee is perfectly able to measure these parameters and can also trigger individual and collective action programs to keep them under control. Simply put, thermoregulation is the process by which the balance between production and loss of thermal energy is controlled in honey bees.

The Tree Nest

Tree nest design enables honey bees to adjust their environment by controlling the flow of two fluids – air and water vapor – plus something that acts like a fluid – heat. The honey bees select a tree hollow with an entrance at the bottom that makes rising hot air inside the nest less likely to escape. They then modify it by applying an antibacterial vapor-retarding sealant of tree resin over the inside walls and any small holes or cracks. This further prevents any warm air leaks and helps maintain the right level of water vapor. Inside the nest, the bees build a honeycomb containing thousands of cells, each of which provides an insulated microclimate for growing larvae (baby bees) or making honey.

Managed honey bees have been under extreme pressure. Beekeepers in the US have been losing and then replacing an average of 40-50% of their honey bee colonies every year since 2010, a rate that is probably unsustainable and would be unacceptable in other kinds of husbandry. The biggest contributor to this decline is viruses spread by a parasite, Varroa Destructor. But this isn’t a natural situation. The parasite is spread by beekeeping practices, including keeping the bees in conditions that are very different from their natural abode of tree hollows.

It’s been demonstrated that the heat losses in man-made honey bee hives are many times greater than those in natural tree hollows. Using engineering techniques more commonly found probing industrial problems, research has also shown that the current design of man-made hives also creates lower humidity levels that favor the Varroa parasite. Natural nests inside tree hollows create high humidity levels in which honey bees thrive and which prevent Varroa from breeding. So, if managed hives could be redesigned to recreate conditions found in tree hollows, it may be possible to reduce the parasites reproductive cycle and give honey bees a chance to recover.

Hive Design

Despite the importance of nests to honey bees, the hives we build them bear little resemblance and have few of the properties of the natural tree nests European honey bees evolved with. In the 21st century, we’re still using hives designed in the 1930s and 1940s, based on ideas from the 1850s. Natural nests were only scientifically surveyed as recently as 1974 and research into their physical properties only began in 2012.

In the absence of human intervention, the honeybee usually constructs its nest in a tree within a tall, narrow, thick-walled cavity high above the ground (the enclosure); however, most research and apiculture is conducted in the thin-walled, squat wooden enclosures we know as Langstroth hives. Resent research, using various hives and thermal models of trees, has found that the heat transfer rate is approximately four to seven times greater in the hives in common use, compared to a typical tree enclosure in winter configuration. This gives a ratio of colony mass to lumped enclosure thermal conductance (MCR) of less than 0.8 kgW−1 K for wooden hives and greater than 5 kgW−1 K for tree enclosures. The results for tree enclosures imply higher levels of humidity in the nest, increased survival of smaller colonies and lower Varroa destructor breeding success. Many honeybee behaviors previously thought to be intrinsic may only be a coping mechanism for human intervention; for example, at an MCR of above 2 kgW−1 K, clustering in a tree enclosure may be an optional, rare, heat conservation behavior for established colonies, rather than the compulsory, frequent, life-saving behavior that is in the hives in common use. The implied improved survival in hives with thermal properties of tree nests may help to solve some of the problems honeybees are currently facing in apiculture.

Hive Temperature

Temperature is an important consideration, especially when bees are subjected to harsh conditions as a result of the effort required to regulate them within their hives. The brood chamber should be kept between 93 and 95 °F (33 and 35 °C). Honeybees recognized temperature thresholds are 43 and 100 °F (6 and 38 °C); deviations above or below these values cause metabolic damage in honeybees and brood; the larger the difference, the greater the stress and negative impact on their post emergence physiological development. These eusocial insects have developed thermoregulating mechanisms in response to adverse environmental conditions; the colony compensates for this difference by either fanning their wings to create air circulation and causing evaporative cooling or raising the temperature by generating endothermic heat to raise the temperature of the breeding chamber.

Brood

As poikilothermic organisms, having a body temperature that varies depending on the outside temperature, insects lack the capacity to regulate their internal temperature, and their metabolic functions are influenced by abiotic factors.  The temperature range within brood chambers is even more narrowly defined, with minimal fluctuations between 94.1±2.7 °F (34.5±1.5 °C). Given that the developmental stages of larvae and pupae are stenothermal, capable of living or growing only within a limited range of temperature, their immediate surroundings must be maintained within this precise range. Humidity is another crucial factor, as it is necessary to ensure successful egg hatching. Optimal humidity conditions are 75 % for egg hatching and 90 to 95 % for larvae survival.

Honey and Comb

Human honey-harvesting processes and removal of the honey-filled comb (a source of thermal mass) have a detrimental impact on hive temperature that requires an increased investment of energy to rectify. This additional energy demand on the bees is a form of stress to the colony and diverts workers away from other essential tasks to that of environmental management. Honey provides thermal mass in the beehive, acting as a thermal buffer to external temperature change, which may mediate part of the thermal losses from the simplistic design of the Langstroth hive. Identification of these impacts in current apicultural practice and hive design allows for the improvement in the design of beehives and associated practices. These improvements may reduce stress to the bee colony, increasing colony efficiency for pollination and nectar foraging.

Langstroth Hives

Currently, conventional Langstroth chambers are the most popular among beekeepers due to their low cost and ease of operation. They are constructed with softwood from various timber species with a thickness of approximately ¾” (~23 mm). This design has a deficient isolating factor, which causes an unstable hive microclimate. This is critical in areas where temperatures reach extreme levels, resulting in declining colony populations, death, and the unexplainable disappearance of colonies. Beekeepers have traditionally suffered from winter losses, but in recent years, summer losses have increased, posing a new threat.  Studies of colony losses in the USA from 2020 to 2021 discovered losses of up to 32.2 % during the winter and 31.3 % during the summer.

The insulation capacity of Langstroth hives is rather poor when compared to those in wild-dwelling colonies both in the winter and summer. In areas where extreme conditions exist, man-made hive structures have resulted in thicker walls and smaller entries to reduce colony stress caused by temperature and humidity fluctuations. Hence, it is critical to develop hive designs that effectively reduce thermal stress while remaining inexpensive.

BTU Generation by Honey Bees

Average BTU Output

• A single honey bee colony can generate between 20 to 40 watts of energy.
• This translates to approximately 68 to 136 BTUs per hour for the entire colony.

Specific Colony Size

• For a small winter colony of about 10,000 bees, the output is around 33 BTUs.
• This is comparable to the heat output of a chicken.

Factors Influencing BTU Output

• The BTU output can vary based on the size of the colony and environmental conditions.
• Insulation and airflow within the hive significantly affect the heating efficiency. For example, better bottom insulation can reduce the BTUs needed to maintain warmth.

Practical Application

• Beekeepers can use BTU calculations to determine heating needs for hives in cold weather, ensuring the bees remain warm and healthy.

Clustering

To maintain a core thermal homeostasis in changing conditions, as the nest surface temperature rises above 50 °F (10 °C), the colony starts to come out of cluster. When nest surface temperatures fall below 68 °F (20 °C), the colony begins to cluster. Colonies in thin-walled hives cluster and uncluster with little hysteresis (minimal lag in reaction time). In contrast, colonies with thick-walled hives cluster and un-cluster with considerable hysteresis (lag behind the temperature change). The biological consequences are that for high thick-walled enclosures such as trees, honeybees will maintain mobility well into winter. However, should the honeybees be provoked to cluster by extreme weather or long periods of darkness, it will take significantly warmer weather outside the enclosure to break the cluster. This behavior is well suited to coping with long lasting extreme events by ensuring that the bees do not start to expend energy at higher rates until good weather has been well established. However, it should be recognized that the high heat capacity of the tree enclosures will also greatly increase hysteresis as the bees will be storing significant amounts of energy in the fabric of the tree.

Nest Humidity

In enclosures, with entrances only in the lower part, the buoyancy of water vapor in dry air and the generation of heat and water vapor from honeybee metabolism ensure that the nest humidity is limited by the temperature and vapor permeability of the enclosure walls. The honeybees coat the inside of the enclosure with propolis derived from tree resins which have very low water vapor permeability and form a vapor barrier. This implies an accumulation of water vapor in the top of the nest limited only by the enclosure wall temperature. For example, wall temperatures of 86 °F (30 °C) would enable a nest relative humidity (RH) of 90 % at 93 °F (34 °C). Previous work in this field have overlooked the dehumidification effect of the condensing, cool surface of the high conductance walls.

To overcome this, dehumidification effect requires continual expenditure of considerable energy (2.2 MJ kg−1 ) in evaporating the water to replace the vapor continually condensing on the walls and/or preventing air circulation close to the enclosure walls. This high energy cost may explain the weak humidity regulation observed by researchers. This is in contrast with the energy required to regulate humidity in a nest with low conductance walls where the wall temperature rises to 86 °F (30 °C) near the brood nest and is lower in other parts of the nest. As described above, the humidity in the air surrounding the nest will rise to circa 90 % RH. Regulation to lower humidity can then be achieved by circulation of the air into the parts of the nest where the walls are cooler. In this case, the latent heat released by condensation is contained within the nest. The net energy required is only that necessary to heat air from the required RH and dew point, back to 93 °F (34 °C), which is less than the latent heat released by condensation. Thick-walled enclosures, by reducing the energy expenditure in humidity control and enabling other humidity control mechanisms, may reveal more honeybee humidity control behaviors.

Varroa and High Humidity

Investigating the causes for lower varroa (V. destructor) breeding success in the tropics, researchers described that in three test series with a total of 127 brood cells kept at 79–85 % RH on average, only 2 % of the mites produced offspring, whereas with a total of 174 brood cells kept at 59–68 % RH on average, 53 % of the mites produced offspring. This demonstrated that high nest humidity results in very poor varroa breeding success. This suggests that If there are ways to artificially increase the hive RH to about 80 %, then the varroa mite population will never increase to a damaging level. In contrast, higher humidity has been shown to improve honeybee egg viability. It is then highly likely that honeybees, in temperate climates and in their natural home, with much smaller thermal conductance and entrance, can achieve higher humidities more easily and more frequently than in man-made hives. As a consequence, it is possible that varroa which loses fecundity at absolute humidities of 4.3 kPa (approx. 30 gm⁻³) and above, is impacted by the more frequent occurrence of higher humidities in these low conductance, small entrance nests.

Brood Humidity and Honey Desiccation

Honeybees appear on first inspection to have conflicting requirements of a high-temperature humid brood zone and dry air needed for nectar desiccation but if the humid air from the brood zone is heated it can desiccate nectar to low moisture levels. If air containing 4.3 kPa of water is then heated to 101 °F (312 K) then it will desiccate nectar to produce honey with only 20% water. This water content is low enough to prevent microbial growth in the honey and the vapor pressure is high enough to hinder the breeding of varroa. This fulfills both the need to have a long-term food supply and to reduce the impact of this parasite.

Referenced Materials

• Are man-made hives valid thermal surrogates for natural honey bee nests (Apis mellifera)?
• Hive Temperature Regulation By Honeybees In Response to Extreme Conditions
• To save honey bees we need to design them new hives
• Ratios of colony mass to thermal conductance of tree and man-made nest enclosures of Apis mellifera: implications for survival, clustering, humidity regulation and Varroa destructor
• Nectar, humidity, honey bees (Apis mellifera) and varroa in summer: a theoretical thermofluid analysis of the fate of water vapour from honey ripening and its implications on the control of Varroa destructor
• The nest of the honey bee (Apis mellifera L.)
• Thermal efficiency extends distance and variety for honeybee foragers: analysis of the energetics of nectar collection and desiccation by Apis mellifera
• Honey bee (Apis mellifera) size determines colony heat transfer when brood covering or distributed
• A review on thermoregulation techniques in honey bees’ (Apis mellifera) beehive microclimate and its similarities to the heating and cooling management in buildings
• Honeybee Colony Thermoregulation – Regulatory Mechanisms and Contribution of Individuals in Dependence on Age, Location and Thermal Stress
• Precision and accuracy of honeybee thermoregulation
• Thermoregulation in Honey Bees Part 1&2
• Comparison of colony performances of honeybee (Apis mellifera L.) housed in hives made of different materials
• Hygroregulation, a key ability for eusocial insects: Native Western European honeybees as a case study
• Thermal impacts of apicultural practice and products on the honey bee colony
• Bees Thrive in Heat: Uncovering Their Thermoregulatory Secrets
• Endothermic heat production in honeybee winter clusters