Water collection and storage
This study shows how two small experimental colonies coordinated ventilation, waste removal, and water collection when their brood nest was artificially heated. It also highlights temporary water storage in the crop of some workers and, to a lesser extent, in cells, without demonstrating the existence of a long-term reserve.
When a brood nest overheats, the colony does not respond with a single behaviour. It intensifies fanning, some of the bees temporarily leave the interior and the water foragers increase their activity. The water brought in can then be spread over the combs and cool the nest as it evaporates.
The study by Ostwald et al. (2016) made it possible to observe this coordination and to manipulate access to water directly. It also shows that a small quantity of liquid can be held temporarily in the crop of some workers or deposited in cells. This short-term buffer must not, however, be confused with a lasting store comparable to honey or pollen.
1. Aim and experimental set-up
The researchers studied two colonies in succession, housed in a greenhouse at Cornell University. Each colony comprised about 3’000 workers, a queen and one brood frame containing brood of all ages. It occupied a glazed two-frame observation hive. The bees had neither plants nor nectar available in the greenhouse: they lived on the honey stores present in the combs, were given pollen and used a single controlled water source placed one metre from the hive.
The brood nest was heated by means of a lamp. In most trials, the researchers aimed for a temperature of 40 ± 2 °C. In the main experiment, heating lasted 7.5 hours: the water remained accessible for two hours, was removed for 2.5 hours, and was then made available again for three hours.
The researchers monitored brood temperature, the number of bees fanning, the size of the bee beard at the entrance, the number of water foragers and the mass of water taken. “Colony thirst” was not measured directly within the bees’ bodies: it was estimated from the time needed to take up 0.2 ml of coloured water placed just inside the entrance. The crop contents of workers and the liquids found in some cells were then analysed by refractometry in order to estimate their sugar concentration.
2. Without water, fanning is no longer sufficient
At the start of heating, while water was available, one colony lowered the brood temperature after the initial rise and the other stabilised it around 40 °C. When the source was removed, the temperature immediately began to rise again in both colonies and reached 43.3 and 43.7 °C respectively. Fanning remained intense and the bee beard increased further: these two responses were therefore no longer sufficient to compensate for the imposed heating without evaporative cooling.
After the source was restored, collection increased sharply and the brood temperature fell, even though heating continued. This sequence provides the most robust result of the study: under these experimental conditions, access to water was necessary for the colonies to control overheating (Ostwald et al., 2016).
The response was not, however, entirely synchronous. Fanning and the partial evacuation of the hive increased almost immediately, whereas the thirst indicator, the number of water foragers and the quantity collected rose more slowly during the two hours of heating. The authors inferred from this that a small volume of liquid already present in the colony could cushion the onset of heat stress. This is an inference: this buffer was not measured directly at the moment when the temperature began to rise.
3. How are the water foragers activated?
The delay between the start of heating and the increase in collection suggests that the water foragers were not responding simply and immediately to brood temperature. They usually remained near the entrance, far from the heated area. This observation makes purely thermal control less likely in this set-up, without ruling it out in all situations.
When the need for water increased, workers that had been active as water foragers were more frequently solicited for liquid by bees from the nest. Shortly after these requests intensified, the former foragers resumed their activity. The sequence is consistent with a social activation signal, but it does not demonstrate that solicitation is the sole trigger.
A second explanation remains possible: after regurgitating the dilute liquid held in their crop, the foragers might themselves feel thirstier. The study does not make it possible to separate this physiological hypothesis from the effect of social interactions. The exact mechanism that triggers their departure for the source therefore remains undetermined.
Once collection is under way, unloading provides additional feedback. If the water is accepted quickly by receiver bees, the forager can set off again without delay. If she meets repeated refusals and is slow to empty her crop, she slows down or interrupts her trips. The colony thus adjusts collection on the basis of local interactions, without any command centre.
4. Where is water held temporarily?
4.1. The crop of some workers
After a day of overheating with free access to water, the water foragers more often had a very dilute liquid in their crop than the other bees sampled at the same time. After a more severe combination of heat and deprivation, many nest bees also showed dilute contents. Some bees with a distended crop still contained this liquid the following morning.
These results indicate that workers can serve temporarily as “living reservoirs”. The analyses concerned the sugar concentration of the crop contents, however: they did not directly measure the total volume of water held in the colony in this way. The phenomenon cannot therefore be converted into a storage capacity applicable at the apiary.
4.2. Open cells, in a single colony
In colony B, the researchers also found, in the evening, open cells containing water or very dilute sugar solutions in the unloading area near the entrance. These deposits were particularly numerous after a day combining overheating and temporary water deprivation. The following morning, no stored water was found in the cells: it had been used or had evaporated.
The experiment therefore demonstrates the possibility of transient deposition in the combs, but not its frequency in field colonies. This result does not justify speaking of a permanent water reserve. Water forms above all a flow that is rapidly collected, distributed, used or evaporated; the crop of some workers and open cells can buffer a small part of it for a limited time.
5. What the beekeeper can take from this
- Prevent interruptions: in hot weather, and when nectar provides little water, a nearby, stable and safe external source facilitates a rapid response by the colony.
- Do not interpret a single sign in isolation: intense fanning or a bee beard signals an effort at thermal regulation, but does not on its own prove a lack of water. Exposure of the hive to sunshine, temperature, colony strength, brood and resources must be considered together.
- Do not rely on an internal store: the buffer observed was limited and transient. It does not protect a colony lastingly if access to external water is interrupted.
- Do not look for a standard ration in this study: it does not make it possible to calculate how many litres a colony should receive per day.
- Do not open the hive to look for water in the cells: such deposits can be inconspicuous, short-lived and hard to distinguish from very dilute nectar. Looking for them offers no validated practical diagnosis.
The practical conclusion is therefore a sober one: during heat episodes, colonies must not be left dependent on a distant, hazardous source or one liable to disappear. The study does not, however, test watering point designs, the sanitary quality of the water, or the effectiveness of water supplied inside the hive.
See also:
- All About Water
- Water Collection in Honeybees: The Influence of Weather and Colony Size
- Which water for our bees?
- Beehive insulation put to the test of collective thermoregulation in honeybees
- Observations at the hive entrance
Reference
- Ostwald, M. M., Smith, M. L., & Seeley, T. D. (2016). The behavioral regulation of thirst, water collection and water storage in honey bee colonies. Journal of Experimental Biology, 219(14), 2156–2165. https://doi.org/10.1242/jeb.139824