In late summer, preparations for overwintering honey bee colonies should begin. It is important to ensure that colonies go into winter in a healthy state, with an adequate supply of healthy bees and stores. Colonies should be queen-right, disease free and secured in weatherproofed hives. Colonies will need to be regularly checked, throughout winter and especially in early spring when the risk of starvation is high. This fact sheet provides guidance on how to successfully overwinter honey bee colonies.

Timing... Winter preparations should begin after the removal of the last honey crop of the year. Often, preparations will begin with application of a varroacide, which can be administered after the summer honey crop has been removed. This date will vary by apiary location but is normally from early to mid-August. However, with late flows from crops such as heather and Himalayan balsam, it may be later.

During winter checks, assess each colony to ensure:

1. It has sufficient food stores for the winter
2. It has a sufficient number of adult bees
3. It is queen-right
4. It is disease free
5. It is pest free
6. That the hive is in sound condition

1. Ensure the colony has ample food... One of the most important parameters that predicts the overwintering success of a colony is the quantity of food stores it has going into winter. Many colonies are lost every year due to starvation.

As a general rule, each colony should have approximately 20 kg of honey in autumn to provide sufficient stores to feed the colony during the winter1. Any less than this will reduce the likelihood that the colony will survive the winter, unless supplementary feed is provided. In reality, the colony may consume more or less, as the consumption rate depends on a number of different factors, therefore, checking the level of stores throughout this period is very important.

To estimate the level of stores present in a hive, it is possible to either count the frames that contain capped honey or to heft the hives; although judging levels of stores from hefting will take some practice. In the beginning, it is recommended to count frames and learn how heavy different levels of stores feel. With practice, it will be possible to determine the approximate levels of stores by hefting. When full, a British Standard brood frame that fits into a National hive contains about 2.3 kg of honey, while a super frame holds approximately 1.2 kg of honey; a full super should contain at least 12 kg of honey. Therefore, a full super and 3 brood frames of honey should be sufficient on its own. Note that larger brood frames such as the Commercial or Langstroth brood frames can hold up to 3 kg of honey.

However, if there are less than 20 kg of stores, the outstanding balance can be fed back to bees as sugar syrup. Count up the existing colony stores and calculate how much the bees need and feed the required balance back to them using winter strength sugar syrup; 1 kg of white granulated sugar per 630 ml of water (or 2 pounds per pint) as a supplement to the honey stores. Watch out for robbing bees during supplementary feeding, this can be a problem, particularly in late summer. Feeding in the evening and quickly cleaning up spills will help prevent robbing.

Hefting the hive...

At the end of autumn, after the colonies have received their supplementary feed, it may be helpful to heft each hive to gauge the ideal weight. This is the weight that should be maintained throughout the winter and, if the colony is becoming particularly light, then it is likely that the bees will need to be fed with fondant. It is advisable to heft hives every other week during winter. When hefting, be sure to heft one side of the box, and then the other, in case the stores are unevenly distributed. Please be careful when hefting hives. Hives can be heavier than expected, so please use a safe lifting technique, and ask for help if necessary.

Fondant and pollen...

Depending on the locality, weather conditions and the colony size, it is possible that bees may be unable to process syrup to store it properly beyond mid-October. If there is insufficient honey stored on combs by the end of October, then they can be fed fondant. Sugar syrup should not be fed to colonies during winter as it will be too cold for them to use it and leaving syrup in feeders over long periods may encourage growth of mould. Colonies also require ample pollen to overwinter successfully, especially to rear brood. Ensure that overwintering bees have access to pollen at the end of the season and early in the following season. If there are few pollen stores in the frames, consider feeding them pollen supplement or pollen substitute.

Isolation starvation...

Even when a colony has plenty of food, the bees may become isolated from it and end up starving. This happens when the bee cluster moves towards one end of the brood box and consumes all the stores. Remote from the rest of the stores and unable to move in very cold conditions because they are in a tight winter cluster, they will starve very quickly. Sometimes a short period of warm weather leads to the winter bee cluster breaking up and when it re-forms as temperatures drop, it might be away from the stored honey. Guard against this by checking inside the hive every other week if weather permits. Either re-distribute combs with honey so that they are adjacent to one side of the nest or give them fondant placed onto the brood frames directly over the cluster, within an eke.

2. Ensure the colony is populous... Alongside food stores, the autumn bee population size is one of the most important factors in determining overwintering success2. During the winter, the queen will stop laying, and resume again in the spring. This means that there are no replacement bees for those that die over the winter. Therefore, it is important to ensure a colony has a sufficient number of adult bees to see the colony through winter.

A minimum number of honey bees is required to ensure that the colony can efficiently thermoregulate. As the temperature drops in winter, the bees will form a cluster and generate heat to keep themselves warm. Energy consumption is more efficient in larger clusters as a higher number of bees allows for more efficient heat generation and insulation, so larger clusters consume less energy per individual bee than smaller clusters.

Smaller clusters have a greater surface area to volume ratio, so will lose more heat than a larger cluster and hence, consume more energy per bee.

Colonies with less than 9,000 bees have a poor chance of surviving winter (this is about 3 full BS brood frames of bees), with some sources recommending a bee population of 18 to 20, 000 bees (more than 6 frames) or more.

A good rule of thumb is to ensure a colony has a minimum of 5 standard brood frames or equivalent, covered in bees before going into winter, and that any colonies smaller than this should be combined with another colony, providing both colonies are disease free.

3. Ensure the colony is queen-right... Queenless colonies will have a low chance of surviving winter as they will form a less efficient cluster for thermoregulation and there will be no laying queen to replace the workers in spring. Queenless colonies should be combined with another colony, or requeened with a young, mated queen if it is not too late in the season.

It is better to overwinter a colony when it is headed by a young, healthy queen, as younger queens are less likely to die or become drone layers than old queens. The brood nest of a young healthy queen is likely to be bigger later in the season than that of an old queen, thus helping to ensure an adequate replacement of the older worker bees. If the queen is more than 2 to 3 years old, consider re-queening the colony with a young queen after the summer honey harvest, as they will keep laying eggs later into the season.

4. Ensure the colony is disease free... Check each colony for signs of brood and adult bee diseases. Remedial action or culling should be undertaken as appropriate. Culling can be used when a diseased colony is unlikely to recover from a serious disease, such as CBPV, of if they have become overly defensive and represent a serious stinging risk to others. However, beekeepers should not cull colonies with signs of EFB or AFB. If European foulbrood (EFB) or American foulbrood (AFB) is found or suspected, then the National Bee Unit must be informed immediately, as these diseases are statutory notifiable. If foulbrood is suspected, contact the NBU at: nbu@apha.gov.uk.

It is important to be able to recognise the signs of EFB and AFB, and to inspect colonies for foulbrood and other bee diseases throughout the season. You can find further information on how to spot the signs of brood diseases and how to inspect for them in the NBU leaflet ‘Foulbrood disease of honey bees and other common brood disorders’. It is recommended to perform regular brood inspections throughout the season.

If colonies are small, investigate the possible cause. If they are pest and disease free, they can be combined with another colony and re-queened. Care should be taken if combining a poorly performing colony with a stronger one, as there may be underlying disease in the weaker colony that is then passed on to a healthy one. The signs of adult bee diseases and how to recognise them are explained in the NBU leaflet ‘Common pests, diseases and disorders of the adult honey bee’. If a colony is diseased, it should not be combined with another colony until remedial action is taken. The aforementioned leaflets provide advice on how to deal with each disease.

5. Ensure the colony is pest free... Monitoring colonies for the presence of Varroa mites is an important year-round activity, but it is especially important to ensure mite levels are minimal going into winter as Varroa populations in the colony will usually peak in late summer. While the honey bee brood population in a colony reduces significantly during July and August, the Varroa population is doubling every three to four weeks (as illustrated in Figure 3). The winter bees are developing in late August and early September, but if they are heavily infested with Varroa during development, then their lifespan will be reduced, which may limit the overwintering ability of the colony.

 Varroa treatments that are applied too late will kill Varroa mites but will not be able undo the parasite damage already caused to the winter bees. However, Varroa treatments cannot be applied while honey supers are on the colony, Therefore the timing of removal of honey supers and application of varroacides should be appropriately judged. Regular monitoring of the mite levels in colonies will inform when varroacides need to be applied. If bees forage on late crops, then they may need a treatment between flows. Monitoring infestation levels is crucial so that treatments can be applied appropriately.

6. Ensure that the hive is in sound condition... Protect hives from the elements... Apiary sites should be carefully selected to ensure that they are in a location with good access in all weather and are on firm, well-drained ground. Colonies should be situated in a sunny location, protected from heavy winds and should not be situated in a frost pocket. Hives need good air circulation to regulate humidity, as it is the damp, rather than cold, that kills bees. Check hives, especially roofs, to ensure rain can not leak into the hives. It is best to ensure that your hives are off the ground on suitable stands. Honey bees create a lot of water vapour as a by-product of metabolism, which then condenses on the cool hive walls. Ensure the hives are set up to allow this water to run out the entrance, by having the hive slant very slightly forwards, or by having the floor inserts removed. If you are using open mesh floors, then the floor inserts can be taken out over winter, unless your apiary site is vulnerable to windy conditions. This also allows for good ventilation. Ventilation provided by the roof, entrance and floor is usually sufficient. Insulation may be used in areas that experience particularly harsh winters, provided good ventilation is maintained.

It is strongly recommended to ensure that hives are safely fastened. Using hive straps, nylon straps or ratchet straps to secure the hives to hive stands is a good option to prevent the roofs being blown off hives in windy weather. In Figure 4, two hives are securely fastened to hive stands with ratchets, without blocking the entrance. Be careful with polystyrene hives as overtightening ratchets can cause damage.

Protect hives from intruders... Ensure that mouse guards are fitted to prevent access by small rodents, which often nest in hives during the winter. Mice can kill colonies during winter. In some areas, green woodpeckers may cause damage to hives, so if this is a known problem in, or near, your apiary, place a simple cage of chicken wire around and over the hive. Place the chicken wire at least 300 mm from the hive walls to prevent damage whilst permitting bees to fly (Figure 5). If larger animals are liable to gain entry to the apiary, such as sheep or badgers, using ratchets to secure the hives to the stands will be helpful to prevent them from being knocked over.

Reconfigure the hives to prevent starvation and chilling... In winter, it may be required to re-organise the boxes. For example, smaller colonies that are usually kept in a double brood box may need to be reduced down to a single brood box, to allow the bees to more effectively control the temperature in the hive. If there is too much empty space relative to the size of the cluster, the bees may struggle to keep it warm. Otherwise, try to moderate the volume inside the hive by using dummy boards or similar to reflect the size of colony so that it is not struggling to warm up a large space. Remove the queen excluder to avoid a situation where a winter cluster goes through the excluder in the search for food stores, leaving the queen behind to perish from the cold and starvation.

Opening the hive in winter... Once you have taken every precaution to ensure that your colony is prepared for winter, it is important to remember to check them regularly during the cold months and into spring: when colonies start building back up again in spring, they can consume a great deal of stores over a short period and the risk of starvation is high, especially when spring weather is poor.

When opening the hive during winter, choose a day that is as warm and sunny as possible, it is best if some bees are flying as this means that they are not in a tight cluster. Open the colony to perform a quick assessment and check that there are enough food stores and that the cluster is close to the stores. Be careful not to disturb the bee cluster, as they will lose heat quickly and may struggle to reform.

This inspection should only take around 20 seconds, as the colony will be losing heat while the hive is open. It is better to perform a quick check for stores even when its cold, than to find a dead colony that has starved in the spring. The use of ‘glass quilts’ or polycarbonate crown boards instead of plywood ones help with checks of the colony without letting heat escape.