Humidity of the brood nest is important for the overall fitness of a honeybee colony. Numerous studies have demonstrated that either high or low levels of humidity affect the health of the brood and adult bees, either directly, for example at levels below 50% relative humidity in the brood cells no eggs hatch  (Doull 1976), this being particularly relevant for small nuclei, or indirectly by favouring the development of pathologies. For instance raising the humidity from 68% to 87% increases the percentage of brood mummification caused by the chalk brood by 8%. Varoa destructor reproductive rate falls with increasing humidity. Thermoregulation and nectar concentration are also intricately linked with humidity levels in the hive (Ellis, 2008).

During broodless periods it is expected to see humidity fluctuations in the nest as the nest temperature fluctuates. This is due to the fact that relative humidity is the amount of water held in the air relative to the maximum amount of water that can be held in the air at a given temperature. The warmer the air the more water it can hold thus as the temperature fluctuates so does the relative humidity with it.

During the brood rearing period median levels of humidity in the nest of a healthy strong colony is between 50% and 60%.  It is rarely found to be below 40% and above 80%. On a daily basis this value is stable and not correlated to the ambient conditions in a strong colony, whereas in a weak colony it follows the pattern of ambient conditions. During the winter cluster the humidity and temperature patterns are closely correlated with the ambient conditions, with a 1-2 h lag caused by the hive inertia.

An important factor influencing the humidity level in the nest is the amount of uncapped brood.  It is directly proportional to the level of humidity in the hive which is explained by the fact that the eggs and larvae are highly sensitive to desiccation. As brood area needs high humidity levels and nectar stores need lower humidity levels for evaporation to take place, it is thought that the brood cells have a microclimate with a relative humidity significantly higher than the whole of the nest. This is achieved by the presence of royal jelly which has high water content as well as cocoons being hygroscopic and absorbing water (Human 2006). In addition the nurse bees which cover the brood frames limit the amount of water evaporating from the uncapped brood cells.

It is now thought that bee colony regulates humidity botch actively, by fanning, droplet extrusion and tongue lashing, and passively by having moisture sinks (nectar and cocoons) and sources (nectar) (Ellis, 2008).

In practical terms, levels of humidity registered by the monitor, depending on the season, are a very good measure of the state of the colony. To summarise, during brood rearing times in a strong colony the humidity levels should be stable. Broodless periods are marked by the fluctuations which follow the hive temperature pattern. Finally, winter cluster period is marked by the fluctuations which follow the ambient fluctuations with a 1-2 h lag. Thus, any deviation from these trends is a reason for concern.

Monitoring hive humidity can therefore provide very useful information for beekeepers. The graph below from a hive monitored with the Arnia system shows typical summer readings with humidity level stable at around 55% with stable brood temperature at around 34°C.

Summer stable humid

The above graph can be contrasted with that below which shows higher winter humidity readings at around 65%. It can also be seen that the humidity levels fluctuate more in response to ambient environment conditions, rises in temperature correlate to reductions in humidity.

Winter variable humid

Most experienced beekeepers will say that during the winter it is the damp rather than the cold that kills bees. So what can a beekeeper do to help with hive ventilation? The guidelines are varied and there are two main schools of thought. One advocates assisted hive ventilation by leaving ventilation holes either by inserting matchsticks in the corners below the crown board or by leaving the feed hole in the crown board open. This advice is usually given by beekeepers that use solid floors. On the other hand, if the hive is fitted with an open mesh Varroa floor, the thinking is that the top ventilation methods are unnecessary as the mesh floor acts as a bottom ventilation system. It is also thought that in these setups extra insulation in the form of quilting or substantial roofs is beneficial. Therefore the type of the hive and obviously the ambient climate will determine what if any ventilation/insulation system is required.

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Doul KM (1976) The effects of different humidities on the hatching of the eggs of honeybees. Apidologie 7 (1) 61-66

Ellis MB (2008) Homeostasis: Humidity and Water Relation in Honeybee Colony, Master Thesis (University of Pretoria)

Human HNicolson SW, Dietemann V (2006) Do honeybees, Apis mellifera scutellata, regulate humidity in their nest? Naturwissenschaften 93(8):397-401.

Tautz J (2008) The Buzz about Bees. Springer-Verlag, Berlin Heidelberg.

Winston ML (1987) The Biology of the Honey Bee. Harvard University Press, Cambridge Massachusetts.