Ventilation Nation

December 1, 2016

Ask ten beekeepers how to ventilate the hive in winter, get one answer. Something’s not right. Does adding ventilation holes at the top of the hive overcompensate for moisture? Having evolved in the warm, wet hollows of a tree cavity equipped with a single, small entrance at the bottom (Seeley 1976), apis mellifera is likely not a stranger to mold. Its presence in the hive is well tolerated, and does not cause the colony to dwindle in winter or produce less honey in spring (Fries 1982). And although the threat of condensed water icing the cluster is real, such a phenomenon is reserved for extreme circumstances (Gates 1914).


Ventilating the top of the hive allows warm, wet air to escape, and we justify this sacrificial loss of heat on the popular myth that cold doesn’t kill honey bees. In actuality, it is the colony’s remarkable ability to stay warm that it can survive winter at all. Condensed water is a problem because it’s cold water. It chills the bees. Randy Oliver, a widely respected researcher and commercial beekeeper, identifies chilling as one of four primary drivers of colony collapse. Cold absolutely kills honey bees, and any hive modification that draws heat away from the winter cluster should be met with a healthy dose of hesitation.   


The efforts of thermoregulation are intended to heat the cluster, not the hive interior (Stabentheiner 2003). Heat does escape, however, and joins the circulation of air inside the hive (Sudarsan 2012).  As fresh, outside air flows in through a narrow, inch wide bottom entrance, it mixes with escaped heat in the space below the cluster.  It isn’t the nature of cold air to rise. It has to be heated first. This is why an open screened bottom can work in winter, and why hive inspections in fall often reveal a lack of activity in the bottom brood box. The coldest air is trapped at the bottom. Provided the hive is protected from the wind and free of upper vent holes, air at the bottom only rises when its temperature increases above that of the surrounding air (Sudarsan 2012).   

With the lower hive area acting as a heat exchanger, preheated air penetrates the cluster and displaces warmer, humid air within. Moisture is a byproduct of the bees’ metabolism, and is held in warm air as vapor. When the flow reaches the hive ceiling, it mushrooms outward and downward into the cooler air below. Its heat is then lost to the fresh incoming air, further reducing its temperature and causing water to condensate in the lower hive area. Finally, a portion of the circulated air exhanges with fresh air at the entrance (Sudarsan 2012). 



All this happens without any direct effort by the colony or the beekeeper to ventilate the hive. It’s simply the nature of cold air to displace warm air, combined with the effects of gravity, that causes air to flow. As days get warmer, and there’s less variation in air temperature, flow decreases. By the time summer rolls around, and it’s 95 degrees everywhere, the air stops moving altogether. Bees must fan their wings to ventilate the hive. In winter, however, air circulates passively.


Ventilating the top of the hive replaces the circular flow of air with a wind tunnel effect, subjecting the cluster to a continuous one-way stream of cold, outside air. This undoubtedly keeps the hive dry, but also increases cold stress, forcing the cluster to work harder to stay warm. Colony metabolism must then increase, fueled by an increase in honey consumption, which results in greater amounts of water in the hive.  Other problematic consequences include reduced humidity in the brood nest (eggs less likely to hatch, higher mite reproduction) and dilution of pheromones (communication interference) (Erickson 1990).


Without upper ventilation, 98% of condensation takes place below the cluster (Toomemaa 2013). That’s because air has to cool before it can release water, and the coolest air is on the bottom. Adjusting your hive stand to incorporate a gentle forward slope allows condensed water to drain. If water freezes before leaving the hive, nothing changes. Ice is the result of cold air, not the cause of it. If water is condensing on the hive ceiling, something up there is cooling the air below its saturation point. This could be the cold underside of the hive cover (corrected with insulation), or even the vent holes meant to prevent condensation. If your quilt is collecting a large amount of water, try closing the vent holes. The colony has spent the better part of autumn hauling propolis to the hive to seal every crack and crevice. Why work against them? We humbly claim that the bees know best, but do we really believe it?


That’s a trick question. If the bees always knew best, they wouldn’t ball a fertile queen in favor of a laying worker. Or maybe they would. The greatest overall challenge to the beekeeper is knowing when to intervene. Less is more. Too little is too late. Every situation is different. There may be a case for ventilating the upper hive. For example, convection may be hampered by insufficient airflow through the entrance.  Or an abundance of space between the cluster and the hive ceiling may allow for air to cool prematurely. Maybe you are overwintering your hive indoors. If your colony would benefit from a top entrance, provide a very minimal opening. Upper ventilation is likely doing more harm than good.  


That’s my opinion. In the end, several thousand healthy bees with a plentiful supply of honey and minimal protection from the elements will survive winter, with or without the beekeeper. But they will struggle. Their mere survival should not be indicative of an ideal winter setup. That cold doesn’t kill honey bees is a tribute to their unique ability to regulate their own body temperature, not a literal statement of fact. Take all the information you gather, including what you read in this blog, with a measure of skepticism.  The bombardmant of conflicting opinions we encounter in beekeeping underscores the need to reach our own conclusions.   



“…ventilation draws moisture from the cluster, stressing the bees by causing them to step up the metabolism of honey to maintain both temperature and humidity… Several studies have shown that honey bees can compensate for and survive temperature extremes. However, what such studies have not considered is the drain on the physiological resources of the colony. The effects of this stress may well be significant in terms of reduced brood rearing or foraging and shortened worker bee life span.”         


- ERIC H. ERICKSON, Stress and Honey Bees





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