Winter losses

Losing colonies over winter is part of beekeeping. But high losses almost always have an identifiable cause, usually set in motion well before the first frosts. A large European study showed that disease control, especially of varroa, and beekeeper training strongly influence colony survival¹.

This page explains how a colony gets through winter, why it may die, how to “read” a dead hive and how to prepare colonies for winter.

How many colonies die over winter?

The international COLOSS network runs a standardised survey of beekeepers every year. For winter 2018-2019 it covered 35 countries, nearly 28,600 beekeepers and over 738,000 colonies: the overall loss rate was 16.7%, ranging from 5.8% to 32% between countries. These losses broke down into dead colonies (10.7%), unsolvable queen problems (4.1%) and natural disasters (1.9%)³. For winter 2019-2020, across 37 countries, the overall rate was 18.1%, ranging from 7.4% to 36.5% between countries².

These averages hide large differences between operations. In the European EPILOBEE study (17 countries, nearly 5,800 apiaries, 2012-2014), small apiaries run by inexperienced hobbyists lost about twice as many colonies over winter as professional operations (14% against 8%)¹. Training, keeping an apiary record and coordinated varroa treatment were associated with better survival. The COLOSS surveys also find fewer losses in large operations and among beekeepers who winter a high share of young queens³.

How a colony gets through winter

Winter bees

In the active season a worker usually lives a few weeks. Bees born from late summer onwards are different: these are winter bees, able to live for several months and carry the colony through to spring⁶. Their longevity rests on a well-developed fat body and high levels of vitellogenin, a storage protein that also plays a role in protection against oxidative stress and in immunity. As long as these bees do not rear brood or forage, their vitellogenin stays high and they age slowly⁷.

The practical consequence: wintering success is decided when these bees are reared, in August and September in temperate climates. A Dutch study showed that winter bees from colonies not treated against varroa lived markedly shorter lives, and that colonies treated as early as July had the best individual survival and the largest spring population⁵.

The winter cluster

When the outside temperature drops, the bees gather in a cluster on the combs around the queen. Bees in the core produce heat by vibrating their flight muscles without moving their wings; those in the tightly packed outer layer form an insulating shell. Measurements show that the core stays much warmer than the shell and that heat production is highest at the centre and falls towards the surface: the cluster’s thermal stability depends both on the insulation of the outer layer and on the work of the bees in the centre⁸. When the colony resumes brood rearing in late winter, the brood area is kept much warmer, which increases the consumption of stores.

The cluster moves slowly, eating the honey it is in contact with. In prolonged hard frost it cannot reach another comb and may starve a few centimetres from its stores.

The main causes of losses

Varroa and viruses

This is the leading cause of winter mortality in Europe. In the German colony monitoring project, colonies that died over winter carried around 15 mites per 100 bees in October, against around 3 for survivors. Deformed wing virus and acute bee paralysis virus in autumn were also associated with losses⁴. Parasitised winter bees live shorter lives and the colony dwindles, often with stores intact. See Varroa and Deformed wing virus.

Starvation

Insufficient stores, badly placed honey, early resumption of laying or a cluster trapped by cold: starvation often strikes in late winter, when brood rearing resumes.

The queen

An ageing queen, a poorly mated one or one that dies during winter: the colony cannot renew its bees in spring. In the German monitoring, colonies headed by young queens survived better⁴; the COLOSS surveys point the same way³.

Colonies that are too weak

A small cluster loses more heat relative to its volume and covers less of the stores. In the German monitoring, colonies that died occupied markedly fewer seams of bees in October than survivors⁴.

Nosema and dysentery

The role of nosema in winter losses is debated: the German monitoring found no significant effect of Nosema⁴. Heavy faecal staining can also come from non-infectious dysentery (unsuitable food, a long period without cleansing flights).

Damp, rodents and poisoning

Condensation chills the cluster and encourages mould. A mouse destroys combs and disturbs the colony (see Mice and rodents). Late-season poisoning can weaken winter bees. Often these factors add up.

Reading a dead hive

Look before touching: cluster position and size, location of stores, brood, combs and floor.

A layer of dead bees on the floor is normal at the end of winter: what matters is the bees and stores that remain.

What to do straight away

1. Close the entrance to prevent robbing, which would spread mites and diseases.
2. Check any remaining brood. Brood suspected of American foulbrood must be reported to the veterinary services; nothing leaves the hive until the result is known.
3. Remove the hive and only reuse the combs after checking them; soiled or old combs are rendered down and the equipment disinfected (see Disinfecting equipment).

Regulations

In France, an ordinary winter loss is not a notifiable disease. However, suspected American foulbrood must be reported to the veterinary services, and abnormal or massive mortality may be reported. The Observatory of honey bee mortality and weakening (OMAA), being rolled out region by region, collects reports of mortality, depopulation and weakening through a telephone helpdesk, even when the cause is unknown¹⁰. Rules differ between countries: check with your local veterinary authorities.

Preparing for winter

Treat varroa early

This is the most effective measure: main treatment straight after the last harvest, a follow-up mite count, then a broodless treatment in early winter^{5, 6}. The FAO stresses regular monitoring and coordinated treatments⁹.

Secure the stores

Needs vary with climate, bee strain and the length of winter: no universal figure applies. Stores should be capped, in contact with the brood nest and above the future cluster. Syrup is given early enough to be capped before the cold; later on, only fondant is suitable.

Unite weak colonies

A colony that is too small or queenless in autumn has little chance of surviving winter: it is better to unite it with a healthy colony, for example using the newspaper method. The EPILOBEE study actually linked uniting and buying swarms to higher losses, probably because they are used to make up for colonies already in difficulty¹.

Site, ventilation and protection

• Site: sheltered from prevailing winds, away from damp hollows, hives raised off the ground.
• Ventilation: enough airflow removes moisture; an insulated roof limits condensation above the cluster; a slight forward tilt lets water run out.
• Mice: entrance reducer or mouse guard fitted before the first frosts.
• Hornets: autumn predation by the Asian hornet weakens colonies just when winter bees are being reared.

Monitoring colonies without opening them

In winter, hives are not opened: each opening chills and breaks up the cluster.

• Weighing. Hefting or weighing gives a consumption trend, to be read with caution: the weight includes wax and pollen, and does not say where the stores are in relation to the cluster.
• The entrance. In mild weather, cleansing flights signal a live colony. Crawling bees, deformed wings or heavy faecal staining on the front are warning signs, as is a silent entrance while neighbouring hives are flying.
• Inspection boards. Under a mesh floor, the board shows the position and width of the cluster (strips of cappings debris), mite drop after a winter treatment, and sometimes signs of mice.

Coming out of winter

On the first warm enough day, a quick inspection checks the brood, stores and colony strength. Light colonies are fed, queenless or drone-laying ones united, and dead hives closed and examined. The apiary review is used to correct the following autumn’s preparation.

The most common mistakes

• Treating varroa too late, in September or October: the winter bees are already parasitised.
• Wintering colonies that are too weak instead of uniting them.
• Relying on hive weight alone without looking at where the stores are.
• Feeding syrup too late, when bees can no longer cap it.
• Leaving a dead hive open or reusing its combs without looking for the cause.
• Keeping old queens without replacing them.

Frequently asked questions

What winter loss rate is “normal”? There is no official threshold. The COLOSS surveys recorded 16.7% then 18.1% losses for winters 2018-2019 and 2019-2020, with very large differences between countries and between operations^{2, 3}. Losses markedly higher than in neighbouring apiaries call for a search for the cause.

My hive died with plenty of honey: why? This is the typical varroa-and-virus picture: winter bees parasitised during development die too early and the colony empties. More rarely, a cluster trapped by cold dies next to its stores.

Why do colonies die in March, after the worst of the cold? The resumption of laying sharply increases the consumption of stores and the workload of the old winter bees. A colony already short of stores or bees may then collapse from starvation or exhaustion.

References

1. Jacques A., Laurent M., EPILOBEE Consortium, Ribière-Chabert M., Saussac M., Bougeard S., Budge G.E., Hendrikx P., Chauzat M.-P. (2017). A pan-European epidemiological study reveals honey bee colony survival depends on beekeeper education and disease control. PLoS ONE 12(3): e0172591. doi.org
2. Gray A. et al. (2023). Honey bee colony loss rates in 37 countries using the COLOSS survey for winter 2019–2020: the combined effects of operation size, migration and queen replacement. Journal of Apicultural Research 62(2): 204-210. doi.org
3. Gray A. et al. (2020). Honey bee colony winter loss rates for 35 countries participating in the COLOSS survey for winter 2018–2019, and the effects of a new queen on the risk of colony winter loss. Journal of Apicultural Research 59(5): 744-751. doi.org
4. 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(3): 332-352. doi.org
5. van Dooremalen C., Gerritsen L., Cornelissen B., van der Steen J.J.M., van Langevelde F., Blacquière T. (2012). Winter survival of individual honey bees and honey bee colonies depends on level of Varroa destructor infestation. PLoS ONE 7(4): e36285. doi.org
6. Döke M.A., Frazier M., Grozinger C.M. (2015). Overwintering honey bees: biology and management. Current Opinion in Insect Science 10: 185-193. doi.org
7. Amdam G.V., Omholt S.W. (2002). The regulatory anatomy of honeybee lifespan. Journal of Theoretical Biology 216: 209-228. doi.org
8. Stabentheiner A. et al. (2003). Endothermic heat production in honeybee winter clusters. Journal of Experimental Biology 206: 353-358. doi.org
9. FAO, IZSLT, Apimondia, CAAS (2021). Good beekeeping practices for sustainable apiculture. FAO Animal Production and Health Guidelines No. 25.
10. Faure B., FRGTV PACA (2024). L'Observatoire des mortalités et des affaiblissements de l'abeille mellifère (OMAA), un système de surveillance innovant. DRAAF PACA (in French). draaf.paca.agriculture.gouv.fr
11. USGS Bee Inventory and Monitoring Lab. Apis mellifera, light body, side, Beltsville, MD (public domain). usgs.gov
12. USDA ARS (2020). Helping honey bees make it through winter with early cold storage. Photo D4399-1, Jeff Pettis (ARS image gallery, public domain). ars.usda.gov

Updated : October 8, 2026
Written by : Apisanitas editorial team
Reviewed by : Review in progress