Humidity in the Hive
To maintain a stable environment in the hive, bees need to control temperature, CO₂ and humidity; however the focus of attention is usually brood temperature. There has been limited work on CO₂ regulation, but surprisingly there is also very little literature about hive humidity; indeed you can probably count the number of relevant research papers over the past century in single digits.
When we first conceived our hive monitoring system, almost ten years ago now, including a sensor for humidity was an obvious choice as it is an environmental parameter that affects vital processes within the hive, such as brood development, nectar processing and disease evolution. Yet beyond sending beekeepers an alert to ventilate their hives in the winter, we knew very little about its significance. Most research at the time indicated that honey bees do not actively regulate humidity and that from a beekeeper’s perspective high levels of humidity should present concern.
Since then, not only have we managed to collect and analyse a large amount of humidity data, more research in this area has emerged. Huang et al (2016) describes active regulation of humidity and a fascinating publication by Sachs and Tautz (2017) very clearly demonstrates (once you get your head round it) how bees regulate humidity levels in the brood area using ventilation. Also Frank Linton presented an excellent review of beehive ventilation in Bee Culture ( 2018).
What is humidity and how is it measured?
While humidity is not a complex concept it is quite difficult to understand, unlike temperature which is far more intuitive. Our bedside clock measures both temperature and relative humidity, I have always been slightly confused why the humidity reading drops a few degrees when I open the window when it is damp, foggy or even raining outside. Confused? Me to! So let’s go back to basics.
Absolute humidity is the actual amount of water vapour present in air and it is measured in g/m3. However, the maximum amount of water vapour that can be present in air before it condenses (the saturation point) depends almost entirely upon the temperature of the air. (Saturation is also dependent on pressure but at atmospheric values this effect is not significant, thus for our purposes it will not be considered further).
Relative humidity (RH), used by most instruments and meteorologists, helps put absolute humidity into context. Relative humidity is the percent of water vapour present in the air relative to the maximum amount that can be contained at a given temperature. To illustrate, 5 g/m3 of water vapour at 10 ºC, gives a RH of 53%, so the air is just over half full of water vapour before it starts to condense. The same amount of water vapour in the same volume of air at 30 ºC air gives a RH of only 16%, quite a difference!
Back to the bedroom, the air outside on a cold rainy day is incapable of holding much water vapour but when it enters our bedroom, and warms up, it has the capacity to hold more water leading to a net decrease in relative humidity once inside the room (the temperature in the room goes down a little but the change in humidity is greater than the change in temperature). You can see this effect in reverse during winter, as warmer room air hits the bedroom window it causes condensation on the cold glass surface as air cannot hold as much water vapour at the lower temperature, as it also does on the inside wall of the hive! Another interesting and non-intuitive fact about humidity is that moist air is lighter than dry air, as water vapour molecules actually weigh less than diatomic nitrogen and oxogen (main constituents of air). This is why moist air rises contributing to cloud formation.
Honey bees and humidity
Bees have evolved adaptations to deal with the environmental fluctuations, including humidity. In addition to ambient humidity, bees create moisture via living processes such as bringing in water, moist nectar and even respiration. While it is important to keep the brood above a certain humidity level it is important that wet air does not condense on the inner hive walls or even the comb itself. So how is the balance in the hive maintained?
In temperate climates during the summer, humidity in the hive does not appear to be an issue due to the fact that the outside air is warmer and dryer hence the passive circulation of air is more effective and helped by larger entrances and mesh floors. In addition, bees’ behavioural adaptations such as fanning and bearding present an active way of regulating humidity .
In contrast, during winter and spring months in temperate climates ambient humidity tends to be high due to precipitation and lower air temperatures. Interestingly, the majority of moisture that is found in the hive during winter is the direct result of bees’ metabolism; respiration (or breathing) produces CO2 and H2O. Frank Linton did the math in an earlier Bee Culture article (2015): if a colony consumes 18kg of honey stores over winter it will produce almost 12l of water via respiration!
So what is the optimum humidity range inside the beehive? How do nectar flow and in hive treatments affect humidity levels? What does all this mean to the beekeeper? We will address these questions by analysing the data from our monitors.
Hive humidity range
Knowing when the relative humidity of the hive is so high that there is a risk of condensation generates a useful alert for beekeepers to increase hive ventilation. Graph 1 below clearly demonstrates this, where during the first week of February relative humidity in one hive reached levels of 90% and even 100% (red line). This means condensation is accumulating inside the hive which could drip onto the cluster. While bees can deal with cold quite well, not so if they are wet! The hive next door (pink line), shows same pattern but without the excursion to saturation levels. Besides the high humidity event the levels are otherwise quite uniform and representative of what we normally observe in the healthy queen right colonies: 40-60%.
Graph 2A below shows the humidity of two neighbouring hives in the south of France, one which contained a healthy colony and the other was empty as the colony had failed earlier that winter; all measurements were taken hourly. This example clearly demonstrates that the thriving colony is managing to minimise fluctuations in ambient humidity. The empty hive is a direct reflection of ambient humidity (see graph 2B), the only difference is the ambient humidity range spans 35-100% and inside the hive the range is between 35-55%. The box acts as a buffer as it maintains higher temperature than the surrounding air. The bee colony inside the hive on the other hand shows very little variation, 65-70%. This is a bit higher than the most other data examples most likely due to sensor placement.
In another part of Europe, early spring humidity levels in the hive are in a familiar range, 40-60% (pink line). At the beginning of April there is a sharp decrease in humidity, down to around 30% which is correlated with a strong nectar flow, as clearly visible in the overlaid weight data (blue line), indeed there is a 22kg net weight increase over a period of 14 days! Incoming nectar has high water moisture content which needs evaporating; this is actively performed by bees fanning which results in relative humidity dropping quite markedly.
Strength of colony
In a graph 4 below we observe two queen right colonies of differing strengths (see the difference in weight). The strong colony manages to minimise the humidity fluctuations much better than the weaker colony.
When compared to the ambient temperature (green line) in the graph 5A below it becomes clear that the weaker colony humidity readings (blue line) are the exact inverse, indicating very little if any active/effective regulation of the nest microclimate.
The strong colony on the other hand shows much less overall fluctuation in humidity and its pattern is unrelated to the ambient temperature, therefore indicating a significant degree of regulation (See Graph 5B below).
Even though the data in graph 6 is not taken at this time of the year it is a very interesting example which may be relevant for those beekeepers that treat for Varroa in spring. It demonstrates how humidity in the hive was altered drastically by bees’ action. Unlike the involvement of a few ventilating bees maintaining the humidity of the cluster as demonstrated by Sach and Tautz study (2017) here we see fanning by a greater number of bees for prolonged period, a few days. The reason for this behaviour was thymol based Varroa treatment, applied on 15th of August, which triggered the ventilation which resulted in significant reduction of humidity.
Finally, while on the subject of Varroa, two studies demonstrate that Varroa reproductive success is dramatically reduced at high humidity levels (Huang 2012 and Kraus and Velthuis, 1997). What this means in practical terms remains to be discovered but it gives a possible explanation why Varroa is unsuccessful in tropical climates where humidity is generally high but has spread like a bush fire through temperate regions. Until further notice, for us that keep bees in any kind of temperate climate, research suggests that high level of humidity (over 75%) is detrimental to our bees as it favours certain pathological conditions such as chalkbrood, development of mold and dysentery.
- Humidity in the hive is a complex environmental variable, result of exogenous (ambient meteorological conditions) and endogenous (bees’ metabolism and behaviour) factors.
- Latest research indicates that colonies actively regulate the nest humidity.
- From our data relative humidity in healthy colonies tends to lie within 40-60% range, however it is the relative stability of the humidity rather than a precise value that emerges as a possible colony fitness indicator.
- Arnia Ltd (2014); Hive humidity. http://www.arnia.co.uk/hive-humidity/
- Frisch H. R. (1921) Humidity in the Hive.—I, Bee World, 3:3, 72-73; Humidity in the Hive.—II, Bee World, 3:4, 97-98; Humidity in the Hive.—III, Bee World, 3:5, 120-120
- Huang Z. (2012) Varroa mite reproductive biology. http://articles.extension.org/pages/65450/varroa-mite-reproductive-biology
- Kraus H. and Velthius H. H. W. (1997); High Humidity in the Honey Bee (Apis mellifera L.) Brood Nest Limits Reproduction of the Parasitic Mite Varroa jacobsoni Oud.
- Li Z. et al (2016); Drone and Worker Brood Microclimates Are Regulated Differentially in Honey Bees, Apis mellifera. http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0148740
- Linton F. (2015); Wait, How much water? http://www.beeculture.com/wait-much-water/
- Linton F. (2018) Beehive Ventilation: We Need to Know More and Do Better. http://colonymonitoring.com/cmwp/wp-content/uploads/2018/03/Hive-Ventilation-Linton.pdf
- Sachs R., and Tautz J. (2017); How Bees (Apis Mellifera) Reduce Humidity in the Beehive by Means of Active Ventilation. https://www.researchgate.net/profile/Roland_Sachs/publication/315083892_How_Bees_Apis_Mellifera_Reduce_Humidity_in_the_Beehive_by_Means_of_Active_Ventilation/links/58c9b578aca27286b3af9f83/How-Bees-Apis-Mellifera-Reduce-Humidity-in-the-Beehive-by-Means-of-Active-Ventilation.pdf