Honey bees are wonderful creatures. Although they prefer summer temperatures, they manage to survive cold and harsh winters. This remarkable feat shows the resilience and strength of a honey bee colony. Nevertheless, surviving winter remains a tall task and requires a strong and healthy colony. Therefore, for beekeepers, the preparation for winter starts already in late July and August. In this article we will explain what you need to know about winter bees and how you should prepare your colonies for winter.

Before we will discuss how to prepare your bees for winter, we will take a look into what happens in a bee hive during winter and discuss the:

• Nature of winter bees

• Heat management within the hive

• Feeding behavior

• Rearing of new brood

Nature of winter bees

Winter bees are no regular worker bees. Where a regular worker bee lives around 6 weeks, a winter bee might live up to 8 months[1]. During winter, the bees will feed on stored honey or sugar water, however a bee colony cannot survive on sugar alone. Therefore, winter bees have a larger abdomen compared to summer bees, which contains valuable proteins, carbohydrates and other substances to help them through the winter.

When exactly a colony starts rearing winter bees depends on various factors, but the availability of pollen is of large influence [1]. What is clear however is that, when fresh pollen becomes scarce, larvae will develop into winter bees. Honey bees store pollen as well, which is often called «bee bread». This bee bread is also an important source of food during winter, but has a different nutritional value then fresh pollen, which presumably leads to the development of winter bees. This implies that colonies which struggle to find pollen late in the season, for example because of a dry spell, will start rearing winter bees relatively early. On the other hand, colonies that experienced lots of flowering at the end of summer will start rearing winter bees in late September.

Heat management

In order to withstand freezing temperatures, winter bees huddle closely together and crawl into empty cells in the combs. Winter bees form a so-called «winter-cluster», within this cluster the bees stay warm and can survive outside temperatures of -20 °C [2]. Bees on the outside of this cluster form an insulation coat for the rest of the cluster. Here, the little hairs on their body come in handy as they capture and contain heat. The bees will swap positions so the warmed-up bees from the middle get some time on the chillier edges of the cluster.

Bees in the middle of the cluster also influence the temperature. By «shivering» (the flexing of their wing muscles) they can heat themselves up to 45°C for no less than 30 minutes [3].

Honey bees will die if they are exposed to temperatures ranging from -2 °C to -6 °C, however already from 10 °C bees can get in a «chill-coma» [5]. It is normal that this happens to some bees on the outside of the cluster. In a chill-coma, the bees are inactive but can survive for over a month. When during this month temperature rises over 10°C the bee might revitalize. A relatively hot winter day might thus revive lots of winter bees

The exact temperature of these winter clusters has been studied extensively and different estimates are mentioned by researchers. However, the research institute HOBOS provides everyone with an abundance of data on honey bee colonies. This data shows that bees do not keep the hive temperature at a constant level, instead they allow the temperature to fluctuate [4].

Inside a bee colony during winter with HOBOS

HOBOS or «HOneyBee Online Studies» is a research platform which monitors various bee-hives and all the collected data is published directly online and accessible for everyone around the world. Feel free to check it out yourself here!

By using data from HOBOS it is shown that the bees will heat up the hive to over 30 °C (as shown in the graph). This enables the colony to move to next feeding resources and will liquify the honey. Afterwards, they let the temperature drop again and start heating again at around 10 °C or lower. By using these heat-peaks, the bees are much more energy-efficient then if they would keep the hive at a constant temperature. In this way, the bees are able to make it through the winter.

Feeding

The winter-cluster will slowly move around the beehive to feed on the stored honey. To move from one comb to the other, the colony needs a certain quantity of bees. The placement of the honey combs, whether they are in line with the bee hive entrance or 90 degrees tilted, does not influence the movement of the bees. Furthermore, bees can transport the honey to each other, so only a part of the cluster has to be located on stored honey to feed the entire cluster.

The outside temperature determines the size of the cluster. The colder, the closer the bees will huddle together [1]. A long period of extreme cold might endanger the bees as they cover a relatively small area and need to move a relatively long way to the stored honey. Also, the bees on the outer edges of the cluster are in danger to get into a chill-coma or die, if they get separated from the cluster during relocation to other food sources.

Rearing

The queen can start laying eggs already in mid-winter. This usually occurs when temperatures are over 4 °C for an extended period of time. This brood will eventually inherit the hive from the winter bees. The brood is very vulnerable for cold temperatures and the bees will have to keep them at 35 °C constantly. Keeping the temperature at a constant level is much more energy intensive, so the colony will need a large supply of honey or sugar water to achieve this.

How to prepare a colony for winter

The stronger the colony before winter, the better the chances of its survival. Therefore it is said that the bee winter already starts in summer. A well-managed and healthy colony is of course better prepared than colonies that had to cope with stressors like diseases, excessive chemical treatments or limited nectar and pollen sources. Beekeepers should consider:

• Varroa control

• Size of a colony

• Feed

• When and how often they inspect their colonies

Varroa control

The Varroa mite is always a major threat for bees during winter and therefore it is vital to treat your bees either continuously during the year – our Varroa solution will help – or at the end of summer. A colony which goes into winter with a high presence of Varroa mites has little chance of survival.

Size of a colony

The stronger the colony before winter, the better the chances of its survival. Scientific research shows that a colony of 7500 bees or more is ideal. Which comes down to 5 or more brood combs fully covered with bees, if you are using «Schweizerkasten». For «Dadant» 4 completely covered combs can be considered as a minimum. If you have colonies which are too small in early September, it might be wise to merge them into one bigger colony.

It is natural that not all winter bees make it through, typically a third of the population will not make it to spring. So, the early spring population usually will be around 5000 bees. Bees will push dead bees out of the hives, so don’t worry if you see the outside of your hives littered with dead bees, this is perfectly normal.

Feed

The colony should have enough food to make it through winter. After all, a strong colony in winter is the base for a strong and healthy colony and sufficient pollination in spring. As a beekeeper, you can either leave plenty of honey in the brood compartment and/or replace some of the harvested honey with sugar water [9]. One should leave 3-5 kg of honey as a minimum. The precise amount of food differs per region and altitude. To learn more about the needs for your specific location, it makes sense to contact experienced beekeepers in the region, for example by joining a local beekeeping association. Colonies located in low-lying regions can forage for food already in March, while the winter for colonies on higher altitudes might last until May.

Checking on your colony

You should check on your colony multiple times during winter, however check without actually taking out the combs. This will lead to a drop in temperature which harms the bees and it will cost them lots of energy and food to re-heat the hive again. You can blow into the hive entrance and listen to the size of the bees. The amount of buzzing you hear can give you a rough indication of the colony’s strength. This is a skill which comes with experience and is one of the least intrusive ways to check on your colony. Another way to check on your hives is to control the bottom board to see how many combs they are sitting on.

To get an indication of the hive temperature, you can remove the top cover and feel the temperature by hand (a temperature sensor in the hive would however be a better and less-intrusive method). Also opening the hive should be done at relatively warm winter days only, if at all.

Additional feeding of pollen

Some beekeepers prefer to feed the bees extra pollen to increase the strength of the colony before winter. However, just like honey, pollen is stored in the combs and bees consume this bee bread during the winter. Supplementary feeding can give a wrong signal to the population and can mislead them. As mentioned, winter bees are reared when fresh pollen is scarce. When feeding pollen to your colony during fall, it will result in the rearing of summer worker bees[6]. These bees do not live as long as winter bees and will massively die after 6-7 weeks.

The supplementary feeding of pollen can however, be done in early spring, because at this moment, summer bees are reared. Now the additional pollen might help the bees to get through relatively cold periods. So, when temperatures drop to unusual low levels in early spring, you can consider supplementary feeding.

Learning by doing

In the end, helping your honey bees through winter is a matter of learning by doing. Many factors such as the colony’s size, the weather, the hive location and more, influence the colony’s chances of survival. Every experienced beekeeper has experienced winter losses, the trick is however to learn from these moments. Keep record of your colonies so that you can analyze in hindsight what might have caused the colony to collapse. Maybe you need to give your bees a bit more feed at the start of the winter, as the bees from your location might (for whatever reason) fly out a little later compared then other colonies in the region. Maybe diseases were not enough under control, or was the colony to small to begin with. Every colony can be different and therefore it is vital to develop insights on what your colonies at your particular location need.

Good luck with preparing your colonies for winter!

Literature

[1] Doeke, M. A., Frazier, M., & Grozinger, C. M. (2015). Overwintering honey bees: biology and management. Current Opinion in Insect Science, 10, 185-193.

[2] Owens, C. D. (1971). The thermology of wintering honey bees. Technical Bulletin of the U. S. Department of Agriculture 1429, 1-32.

[3] Sacher, C. (2012). Der Bienenwinter beginnt im Sommer. Schweizerische Bienenzeitung. Ausgabe 08/2012. p. 6-9.

[5] http://www.bee-careful.com/de/initiative/bienenvolk-im-winter/

[6] Stabentheiner, A., Pressl, H., Papst, T., Hrassnigg, N., & Crailsheim, K. (2003). Endothermic heat production in honeybee winter clusters. Journal of Experimental Biology, 206(2), 353-358.

[7] Mattila HR and GW Otis. 2007. Manipulating pollen supply in honey bee colonies during the fall does not affect the performance of winter bees. Canadian Entomologist. 139:554-563. [8] Sauer, B. (2012). Ideale Bedingungen für die Überwinterung schaffen –

Arbeiten im August. Schweizerische Bienenzeitung. Ausgabe 08/2012. p. 6-9.

[8] Genersch, E. et al. (2010). The German bee monitoring project: a long term study to understand periodically high winter losses of honey bee colonies. Apidologie 41, 332-352, doi:10.1051/apido/2010014 (2010).

[9] HOBOS. (2019) Daten-Archiv. Data from 11.11.2019-18.11.2019.

https://www.hobos.de/was-ist-hobos/live-daten/#/archive