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The Problem to the Solution of Winter Moisture

When you ask ten beekeepers and get one answer, something is wrong. Most seem to agree that winter condensation kills colonies. The warning survives on speculation and hearsay rather than direct observation. Water and heat are critical winter resources. Ventilating the hive to remove moisture may do more to reduce winter survival than promote it.

A Dry Bee is a Dead Bee

Honeybees originated in the tropical latitudes and are highly dependent upon humidity for their survival. Like most insects, their large surface to body ratio promotes water loss. Outside their native habitat, bees live their life engaged in a perpetual battle against desiccation. Inside the hive, they prefer 75% relative humidity (Ellis 2008). Honeybees breathe only twice per minute to retain water, which they cannot do when they are dead. Expect to find wet or moldy bees upon failure of the hive’s internal air and water pump that is the winter cluster.

Without moisture in the hive during winter, the colony would die of starvation. Bees need water to dilute honey for intake through the proboscis and all other phases of digestion. High humidity is also associated with a decline in mite reproduction (Kraus 1997). When the queen resumes laying in midwinter, water is critical for producing the brood food. Larvae do not hatch from cracked eggs but emerge from a membrane that dissolves in water. Maximum egg viability occurs at 95% relative humidity (Doull 1976) and serves as evolutionary evidence of the colony’s dependence on environmental moisture. Silk cocoons embedded in the wax transforms comb into a moisture sink that supports brood development (Ellis 2010). The higher water content of hygroscopic brood comb is the preferred clustering surface for overwintering bees. Still, rapid evaporation in the heated core threatens the interior bees with dehydration (Omholt 1987). Watch them fly straight to the ground on a warm winter day and perform low-level foraging flights in a desperate search for water.

“Beekeepers often go to great measure to avoid moisture buildup in the hive during winter. One should do so with caution…”

- R Oliver,

Moisture Management in the Tree Hive

Dumping heat and moisture would be uncharacteristic of an insect well-recognized for maximizing the utility of its resources. Honeybees have been evolving mostly in the humid confines of a well-insulated tree cavity having only a single point of entry. Workers fill cracks and seal the walls with a thin layer of propolis (Seeley 1976). Although the colony makes no attempt to heat the interior space, an insulated hive stays warmer than the outside air (Stabentheiner 2003).

Heat and moisture escape the cluster and rise toward the hive ceiling. Finding no exit or cold enough surface for water to condense, the current gets deflected outwards and back downwards into cooler air below. The movement from hot to cold is a matter of natural law that passively circulates the inside air during winter. As the flow reaches colder regions of the hive, water condenses along the walls or below the cluster before falling to the floor. Air is exchanged through the entrance. Condensation releases heat that remains inside the hive and helps warm the interior (Oliver 2016). This lowers cluster metabolism, expressed as a decline in food consumption and water production.

With good insulation and a low nest ceiling, no condensation would occur on the ceiling, but the moisture would descend as vapor and condense on the floor or leave the nest through the lower entrance.”

- K Toomemaa Determining amount of water condensed above… the winter cluster (2012)

… In the Managed Hive

The typical uninsulated and unmodified Langstroth hive is cold and wet. Adding upper ventilation allows some moisture to escape, resulting in a cold hive that is less wet. Wrapping insulation around a ventilated hive still allows heat to escape, so condensation in the upper hive may still be a problem. Enter the moisture quilt.

A permeable layer of woodchips above the cluster will shield the bees from any water that condenses on the ceiling and drips back down. It will also reveal the poor insulative value of woodchips, because rising humidity must pass through these layers to carry moisture to the top of the hive. Ventilation holes in the quilt box may keep the upper layer of chips cold enough for water to condense even before vapor reaches the ceiling.

Whether water condenses on the hive ceiling or the quilt itself, water is condensing inside the hive and above the cluster. This is a symptom of a larger problem: The hive is too cold. Thin pine walls and layers of woodchips do little to prevent the loss of heat (Mitchell 2016). Upper ventilation makes matters worse by encapsulating the bees in a continuous draft of cold, dry air. As the colony works harder to stay warm, the rise in metabolism results in higher levels of both food consumption and water production. The beekeeper’s solution for removing moisture from the hive has the unfortunate consequence of adding it.

Note that water production may increase if population declines, as each bee works harder to stay warm. If such a colony dies, it dies wet.

“Many of the arguments given to back up any recommendations for providing more and more top-ventilation are based on reasoned considerations or anthropomorphic thinking rather than on sharp-eyed observations of bee behavior.”

- B Möbus; Rethinking our ideas about the winter cluster (1998)

Conflict Resolution

To manage hive moisture, bees rely on insulation to conserve heat while the beekeeper relies on ventilation to remove heat. These two methods are incompatible and result in the colony and their keeper working against each other during the deadliest time of year. As beekeepers well know, trying to impose one's will on a colony of honeybees is a complete waste of time. In this case, it is also unnecessary. Bees have developed a superior system for managing moisture, because it prevents condensation above the cluster without removing heat. It is up to the beekeeper to recognize this and adopt the method so that teamwork is restored.

“The other important role of the insulation is the avoiding of excessive moisture inside the hive. It has been shown that under steady outside conditions condensation does not increase even when the ventilation is restricted, so long as the proportion of heat passing through the walls of the nest cavity is negligible.”

- K Toomemaa, Winter Mortality of Honey Bee Colonies: Reducing the… Factors (2016)

Wet Bees Happen Anyway

The irony in this debate is that the winter cluster gets wet from condensation as a matter of normal occurrence, and there isn’t anything a moisture quilt or upper ventilation can do to prevent it.

In the heated core, water evaporates rapidly and dehydrates the interior bees. Moving outward, temperature declines across a steep gradient causing relative humidity to sharply rise, such that conditions near the cluster surface are unfavorable for water to evaporate (Omholt 1987). Heater bees in the insulating shell prevent the temperature from dropping below 50F to avoid chill coma (Free 1960), but this may be low enough to cause the vapor they produce to condense directly on the bodies of bees (Oliver 2016).

Condensation in the vicinity of the cluster surface gives interior bees access to water and prepares honey for digestion by diluting nearby stores. Cold death temperature hovers around the freezing point for both a wet and dry bee (Free 1960). Drowning aside, a wet bee only dies if she gets too cold. But she will lose heat faster than a dry bee and is at greater risk of becoming immobilized in a chill coma. Torpid bees can remain comatose for nearly two days but must eventually be warmed back up, or fall from the comb and die (Stabentheiner 2003). The picture below (by Randy Oliver) reveals chilled bees in the periphery unable to fly away with the interior bees when smoke is puffed across the cluster.

“We’d expect much of the metabolic water from the core to condense upon the cool comb or upon the bodies of bees in the shell… Much of this water vapor will condense within the cool outer shell of bees...”

- Randy Oliver,

Reduce Moisture at the Source

To reduce moisture, reduce metabolism. Prevention is key, and that is where upper ventilation fails. The lowest metabolic rate for a honeybee occurs in a chill coma somewhere below 50F. Operators of indoor wintering facilities set the thermostat around 41F. At this temperature, every bee in the cluster can remain at rest and still avoid chill coma, provided the insulating shell maximizes its density (Heinrich 1981). CO2 levels rise and oxygen declines, inducing a quiet stillness that puts the colony into a state of semi-hibernation while broodless in early winter (Van Nerum 1997). It is a survival tactic characterized by the absolute minimum of both food consumption and water production. Any disturbance, including ventilation, must be avoided.

It is true the cluster insulates itself, but the closer the surrounding air is to 41F, the lower the bees’ metabolic rate. Although not as reliable as the indoor space, an insulated hive that is sealed at the top represents a practical alternative for the hobbyist beekeeper.

Note that ventilation of an indoor wintering facility circulates the air outside the hive, which obviously happens naturally outdoors. Also note that increasing hive ventilation in the above scenario is of little use to a cluster packed so tightly that CO2 barely escapes. It just creates a cold draft of outside air that cools the outer layers and forces those bees to engage in heat production. The result is more moisture in the hive, the very thing ventilation is meant to reduce.

“One of the major sources of mortality in honeybees is overwintering. Successful overwintering is achieved in large part by occupying a suitable nest.”

The mechanisms and energies of honeybee swarm temperature regulation. B Heinrich 1981


Honeybees have not evolved in a cold, dry, well-ventilated hive. Instead, nature has selected for colonies that nest in the humid confines of a poorly ventilated but well-insulated tree cavity. The advantages include a two-dimensional approach to moisture management that impacts both where and how much condensation occurs. Closing the vents and insulating the hive should not be considered in the context of colony comfort but as the catalyst for a multifaced and holistic impact on colony health that increases winter survival.

“It is widely known that the lower the metabolic rate of wintering bees and the associated food consumption, as well as the temperature of the winter cluster, the deeper is the bees' dormancy status, the smaller is the exhaustion of the organism and accordingly the more successful is the overwintering.”

- K Toomemaa, Winter Mortality of Honey Bee Colonies: Reducing the… Factors (2016)


Hygropreference and brood care in the honeybee. B Ellis 2008

High humidity in the honey bee brood nest limits reproduction of the varroa mite. B Kraus 1997

The effects of different humidities on the hatching of the eggs of honeybees. KM Doull 1976

Brood comb as a humidity buffer in honeybee nests. B Ellis 2010

Understanding Colony Buildup and Decline: Part 13a. R Oliver 2016

The Nest of the Honeybee. T Seeley 1976

Determining the amount of water condensed above and below the winter cluster. K Toomemaa 2013

Ratios of colony mass to thermal conductance of tree and man-made nest enclosures. D Mitchell 2016

Rethinking our ideas about the winter cluster. B Möbus 1998

Why honeybees rear brood in winter? S Omholt 1987

Chill‐Coma and Cold Death Temperatures of Apis Mellifera. J Free 1960

Endothermic heat production in honeybee winter clusters. A Stabentheiner 2003

The mechanisms and energies of honeybee swarm temperature regulation. B Heinrich 1981

Hypoxia-controlled winter metabolism in honeybees. K Van Nerum 1997


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