Winter colony loss
What causes it and why is it getting worse?

In the lifecycle of the honeybee colony, the winter period is when populations experience the most dramatic losses. During this period, the colony stops foraging and the queen stops rearing brood. ‘Winter’ worker bees have a longer life span than the summer bee, to endure the entire season. Though colony growth and decline are a natural part of the cycle, the past decades have seen beekeepers experiencing far greater losses, often claiming ‘a bad winter’, or ‘weather’ as the reason. The reality, however, is more complex.

Döke et al. stated in  the 2018 study that ‘overwintering success is influenced by the weight and population size the colonies reached prior to winter’. A strong colony is better equipped to handle the platitude of winter stressors, but even large colonies experience significant population falls. Weather, parasites, pathogens and forage all play a role in the mortality of the colony. In the dynamic hive environment, changes to just one variable often have compounding effects on the others.

Weather conditions, foraging and food stores

Weather conditions effect the availability of forage, the ability of bees to thermoregulate, and the time when the colony can begin brood rearing for the coming season.

According to the study ‘Summer weather conditions influence winter survival of honey bees’, from Penn State University, winter survival of honey bee colonies is ‘strongly influenced by both the summer temperatures and precipitation in the prior year’. The analysis looked at 36 weather, topographic and landscape variables that influence the availability of floral resources, as well as pesticide exposure risk. The study concluded that of all the variables, the most accurate predictor was ‘degree days in prior summer’. Researchers believe this may be related to the correlation between temperatures and the availability of foraging resources.

It is not only the pre-winter weather that determines the likelihood of survival. During the winter months, outside temperatures affect the ability of the colony to thermoregulate. In temperatures below 10°C, bees bunch together and together to form a single mass called a ‘thermoregulating cluster’. In this structure, bees activate their flight muscles to control the internal hive temperature. It stands to reason then that warmer winters would require less energy and thus, be less stressful on bees. However, this is not always the case.  In the 2017 study ‘Modelling seasonal effects of temperature and precipitation on honeybee winter mortality in a temperate climate’, researchers analysed data from Austrian beekeepers about wintering bee mortality rates from 2009 to 2014. They concluded that ‘warmer and drier regions often accompanied higher mortality rates’.

It is also the comparably much older ‘winter bees’ that then must initiate brood rearing in preparation for the coming spring. Without adequate food stores, many colonies are not able to grow to pollination strength, sending the colony into a death spiral.

Feeding is a common practice undertaken by beekeepers to compliment natural food stores and give colonies the best chance of winter survival. However, beekeepers that use incorrect feeds can do more harm than good. Imdorf et al. stated in a 2006 article that honey dew or forest honey is not suitable for overwintering and can lead to diarrhoea and large colony losses, whereas liquid feeds such as sugar water solutions made from cane sugar, beet sugar or maize starch show no evidence of bee intolerance.

Parasites and pathogens

The parasite with the most devastating impact on the European honeybee is the varroa mite, and the largest losses attributed to varroa, occur over winter. It is believed that the damage caused by varroa infestation is increasing globally. As the winter-ready worker bees develop in the Autumn, for the beekeeper who has not pro-actively treated against varroa, a substantial population can be present. Varroa mite infestation during the pupal stage of winter worker bee development can severely impact the ability of the bees to make it through to the spring.

‘Bees infested by V. destructor as pupae show reduction in body weight, hemolymph volume, hemolymph protein content, and abdominal carbohydrate levels at emergence (De Jong et al. 1982, Weinberg and Madel 1985, Bowen-Walker and Gunn 2001).’

The weakened compositions of the winter-worker bees, in turn results in weakened immune systems. This makes them susceptible to the pathogens and viral infections spread by varroa.

Additionally, the use of phoretic treatments (treatments that kill only the mites on adult bees such as oxalic acid), have limited effectiveness over the pupal stage, as the varroa can still multiply under the capped wax cells.

In the long term 2010 study titled ‘German Bee Monitoring Project’, researchers demonstrated that factors significantly related to winter colony loses were: high varroa infestation level, infection with deformed wing virus (DWV) and acute bee paralysis virus (ABPV) in autumn, queen age, and weakness of the colonies in autumn.

Scientists have coined the term ‘the goldilocks effect’, to describe the ‘just right’ circumstances required for successful winter colonies. However, the interplay between each factor is not so clearly understood and changes to one stressor, can magnify the others. While long, abundant summers make foraging easier, they are also favourable for varroa population growth. And what if the food stores have been collected from pesticide rich crops? As humans, we may prefer a warm winter, but what about the natural behaviour and physiology of the winter bees? Climate change is making seasonal change more dramatic. What beekeepers are seeing is that the contributing factors are becoming increasingly volatile. While there is still a lot to learn about winter colony losses, there is hope. Beekeepers are more connected than ever before, and the scientific community, as well as governments and global citizen science projects are enabling us to discover more all the time. Let’s just hope we are not too late.


Colony Size, Rather Than Geographic Origin of Stocks, Predicts Overwintering Success in Honey Bees (Hymenoptera: Apidae) in the Northeastern United States

Mögliche Ursachen für die Völkerverluste der letzten Jahre

Summer weather conditions influence winter survival of honey bees
Penn State

Altered Physiology in Worker Honey Bees (Hymenoptera: Apidae) Infested with the Mite Varroa destructor (Acari: Varroidae): A Factor in Colony Loss During Overwintering?

The German bee monitoring project: a long term study to understand periodically high winter losses of honey bee colonies
Elke Genersch, Werner von der Ohe, Hannes Kaatz, Annette Schroeder, Christoph Otten, Ralph Büchler, Stefan Berg, Wolfgang Ritter, Werner Mühlen, Sebastian Gisder, Marina Meixner, Gerhard Liebig & Peter Rosenkranz

Modelling seasonal effects of temperature and precipitation on honey bee winter mortality in a temperate climate
Matthew Switanek, Karl Crailsheim, Heimo Truhetz, Robert Brodschneider

Vatorex AG, Grant Morgan 25 February, 2021
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