How Bees Survive Winter

In the cold season, when snow lies outside at higher elevations, the question often arises of how bees survive winter.

Wild bees, which live primarily as solitary insects — meaning they build their nests independently and raise their brood without the support of conspecifics — spend the winter months in a developmental stage as larvae or pupae. Interestingly, they only hatch approximately one year after the female lays the eggs, depending on the species, between March and October.

Colony-forming wild bees such as bumblebees behave differently. Their entire colony, including the queen, dies with the onset of cold. Before this point, however, these colonies rear young queens who take over the following year. These young queens seek out sheltered spots before winter or burrow into the ground to survive.

In contrast, honeybees — also colony-forming insects — survive the winter as an entire colony. A honeybee colony can comprise up to 10,000 individuals and manages, thanks to its social structure and mutual support, to endure the cold season.

Early Preparation and the Dynamics of Bee Populations

Bees begin preparing for overwintering as early as summer. “The first winter bees already hatch from July onwards,” explains Sarah Grossenbacher from BienenSchweiz. Although they are indistinguishable in appearance from summer bees, winter bees live longer, are more passive, and do not participate in brood care or other tasks within the hive.

Winter bees have a significantly larger fat body than summer bees, and they store protein in their bodies that is crucial for the colony's survival during winter: “The body's own reserves are mobilized in February as soon as brood is being cared for again, and since winter bees live between 6 and 9 months, they survive the winter and rebuild the colony together with the queen,” Sarah Grossenbacher explains further.

In any case, the size of bee colonies changes throughout the year. “In May and June, the queen lays the most eggs — up to 1,200 per day,” Sarah Grossenbacher describes. However, from July onwards, egg production is reduced, leading to a decrease in colony size. And as soon as winter preparations begin, supplies are built up, and the summer foraging season draws to a close, drones — the male bees — are no longer tolerated and are driven out of the hive.

“The reason very likely lies in the fact that this allows them to overwinter as economically as possible — meaning they consume as little food as necessary while still being large enough to maintain warmth inside the hive,” Sarah Grossenbacher explains. Bees overwinter by forming a cluster inside their hive, in which they warm one another. The queen sits at the center of the cluster, with all the bees gathering around her. The bees on the outer edge of the cluster are regularly replaced by those sitting further toward the center. This ensures a consistently cozy temperature of 25 to 30 degrees within the cluster.

The Important Role of Beekeepers

Since only healthy and strong bee colonies are able to raise sufficient numbers of winter bees by the end of summer, beekeepers play a decisive role in ensuring the successful overwintering of bees. They must ensure that the bees have adequate nutrition in the form of carbohydrates such as honey or sugar syrup, as well as proteins in the form of pollen.

Replenishing supplies after the honey harvest is therefore essential. “A colony must never go hungry,” Sarah Grossenbacher emphasizes, “otherwise it becomes more susceptible to disease.” For winter bees in particular, pollen is of great importance, and it is crucial that the bees have an optimal location where plants are still in bloom in late summer, allowing sufficient pollen to be collected.

In addition, treatment against the Varroa mite is very important. A first treatment becomes necessary in late summer, as bees that were parasitized by the Varroa mite during their larval stage are weakened and have a shorter lifespan. "This means they cannot survive the winter and are also too weak to raise the new brood in February," explains Sarah Grossenbacher.

During the winter months, the focus is on monitoring bee colonies and preparing for the next season. This includes renewed treatment against the Varroa mite so that the colony can enter the season with the lowest possible mite burden, as well as regular checks of food reserves. Treatment against the Varroa mite has thus developed into a central aspect of bee management, says Sarah Grossenbacher: scientific concepts and methods have been developed to effectively control mite infestation and reduce winter losses.

Challenges posed by climate change

The effects of climate change are also being felt in beekeeping and can significantly influence the overwintering of bees. Mild winters can lead to premature brooding, which increases nutritional requirements. "In the brood nest, temperatures must then be around 34 degrees, which requires additional energy and correspondingly more food reserves — especially during renewed cold spells," explains Sarah Grossenbacher. In addition, the risk of an increased Varroa mite population also rises. For when bee colonies brood more and for longer periods, more Varroa mites are also raised.

Wild bees are particularly affected by the changing climatic conditions, however, as their life cycles are often closely linked to specific plant species. "With wild bees, the timing of hatching after winter is often synchronized with individual plants," explains Sarah Grossenbacher. Due to the warmer climate, it can happen that certain wild bee species hatch earlier, before the corresponding plants are ready, leaving them unable to find food.

A study examining the interplay between the pasque flower and its primary pollinator, the red mason bee, had also described the reverse scenario, according to Sarah Grossenbacher. In this case, the plant's flowering onset had responded more quickly to higher temperatures than the mason bee, meaning these species could miss each other.

"Honey bee colonies appear to be somewhat more adaptable than wild bees," says Sarah Grossenbacher, adding: "They are also under the care of beekeepers, who can help them in certain unfavorable situations." As such, the overwintering of bees involves comprehensive understanding and careful planning. Beekeepers play a crucial role in this process. They must not only take into account the physical needs of the bees, but also respond to threats such as disease and climatic changes. By adapting to these challenges, they make an essential contribution to the survival and health of bee colonies.