Brood nest temperature is of extreme importance to the colony and is controlled with utmost precision. Honey bees maintain the temperature of the brood nest between 32°C and optimally 35°C so that the brood develops normally. When the temperature in the nest is too high the bees ventilate by fanning the hot air out of the nest or use evaporative cooling mechanisms. When the temperature is too low bees generate metabolic heat by contracting and relaxing their flight muscles (having uncoupled the wings from them). The resulting vibration generates heat in those muscles. Many insects heat up their flight muscles before taking off, but bees have exploited this function to thermo-regulate their environment (Winston 1987, Tautz, 2008).
Recent studies (Jones et al., 2004) have demonstrated that brood temperature tends to be more stable in more genetically diverse colonies (many patrilines) than in genetically uniform colonies (single or few patrilines). Different patrilines have varying response thresholds thus thermoregulation occurs in a series of graded responses.
Research has shown that even small deviations (more than 0.5°C) from the optimal brood temperatures have significant influence on the development of the brood and health of the resulting adult bees. Bees raised at sub-optimal temperatures are more susceptible to certain pesticides as adults (Medrzycki ,2009). Interestingly, pupal developmental temperature affects the probability of the task allocation in the resulting adult bees (Matthias 2009).
Tight thermo-control of the nest climate and specifically the brood area, which is highly sensitive to the temperature fluctuations, is achieved in the following ways. Capped brood cells are heated by bees which press their thoraces firmly down on the caps of the cells and transfer the heat to the pupae beneath the cell cap. In this way only one cell is heated and a heater bee can hold this position for up to 30 minutes while its thorax is at around 43°C. In order to minimise the dissipation of heat produced by these heater bees the other bees are thickly packed on the comb around it. The other, even more efficient way of heating the brood is by means of heater bees that occupy strategically positioned empty cells within the brood area. The capped brood area in the comb normally contains 5-10% empty cells. The percentage of empty cells varies depending on the outside climate. More than 20% empty cells in the brood of all stages can be a sign of an unusual situation in the colony. These empty cells are populated by the heater bees that insert themselves into the cell head first with their abdomens pulsating. They also have an average thorax temperature of around 43°C, while other non-heater bees’ body temperature is that of the ambient. Therefore, these heater bees are acting to heat the brood and in order to achieve this they expend tremendous amounts of energy in the form of highly concentrated honey, brought to them by other bees from the stores. Occasionally they will use the nectar from the brood area but this fuel is not as high-quality as is mature honey that is transferred from mouth to mouth (Tautz, 2008).
Just as they have to heat the brood, bees need to occasionally cool it, although in northern and central Europe cooling is required a lot less than heating. Nonetheless, even short periods of too much heat can damage the brood. Bees achieve this by the means of fanning, evaporating water and even partially evacuating the nest.
Having considered the lengths to which a colony goes to maintain the stable brood temperature we can use this physical factor as an important indication of the state of the colony. Therefore, variation in the brood temperature signals one of the following scenarios:
1) Broodless state, which could be due to seasonal influences (winter or a dearth)
2) Non-laying queen
4) Preparations for swarming
Arnia hive monitors enable beekeepers to closely track brood temperature and easily identify if it becomes unstable. The screen shot below shows a brood temperature graph from a hive fitted with a brood temperature sensor. The graph clearly shows that brood temperature is initially stable at around 34°C but then drops and becomes unstable. In this instance it was a late season dearth leading to the queen stopping laying.
Similarly, stabilisation of brood temperature from an unstable state can be a very reliable indication that the queen has started laying as the bees have started to regulate brood temperature. This is shown on the graph below, also from a hive fitted with an Arnia hive monitor. This shows brood temperature rising and stabilising at 34°C in early March as new season brood rearing begins. It is therefore possible to identify that the queen has started laying without opening the hive.
The example below shows a hive following introduction of a new mated queen. Brood temperature rises to 34°C after 6 days. Thus without disturbing the bees it is possible to see that the new queen is accepted and has started laying.
As already mentioned, bees maintain an exceptional control over the nest environment which is due to the combination of worker activities (individual and on a colony level) and the nest design. What a beekeeper can do to help thermoregulation of the nest will greatly depend on the type of the hive used. In nature the nest of honeybees in temperate areas are most often found in the holes of old trees whose walls are several fold thicker than any hive in use. The thickness and the composition of the nest walls will act as insulation and buffer for the internal environment of the nest. To illustrate, a top bar hive in Scotland will probably benefit from some kind of winter insulation more than, for example, a double walled WBC in the South of England. Also it is important to note that insulating the hive may affect the humidity levels, importance of which will be covered in our next article.
Aston D (2013) personal communication
Jones JC, Myerscough MR, Graham S, Oldroyd BP (2004) Honey Bee Nest Thermoregulation: Diversity Promotes Stability Science 305 (5682): 402-404
Medrzycki P, Sgolastra F, Bortolotti L, Bogo G, Tosi S, et al. (2009) Influence of brood rearing temperature on honey bee development and susceptibility to poisoning by pesticides. J Apic Res 49: 52–60.
Matthias AB, Holger S, Robin FAM (2009) Pupal developmental temperature and behavioral specialization of honeybee workers (Apis mellifera L.). Journal of Comparative Physiology A 195: 673–679
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.