How do bees survive the winter?

by Vrinda Schallnau, Museum of Natural History, Berlin

We are most aware of bees in the spring and summer, when they are most active. But we often do not know exactly how they spend the winter. How do they feed? How do they protect themselves from the cold? And where do they hibernate? You can find answers to these and other questions in this article.

Figure 1: A Western honey bee (Apis mellifera, top) and a Yellow-legged mining bee (Andrena flavipes, bottom) searching for pollen. The Yellow-legged mining bee is a wild bee.

Of solitary and social bees

The best-known type of bee is the Western honeybee (Apis mellifera), whose way of life as a large colony that raises its young in hives and produces honey is familiar to many. However, it is less well known that there are almost 600 species of wild bees in Germany with very different lifestyles. The majority of wild bees are solitary, meaning that the females build their nests alone and take care of their offspring. Bumblebees (Bombus) and a few species of sweat bees (Lasioglossum) are the only wild bees that live socially in states, building their nests together and feeding their offspring together. However, their states are much smaller than those of honeybees. These different ways of living also have a decisive influence on how they overwinter.

Figure 2: Sweat bees (Lasioglossum) nest on open ground without vegetation. They build a small earth wall at the entrance to their nest.

Many wild bees overwinter as larvae

Most wild bee species overwinter as larvae. This is an important step in their development into full-grown, flying bees. Wild-bee females build several small chambers, called "brood cells", in their nests from various materials such as leaf parts, flower leaves, resin or wax. They place these in protected cavities such as plant stalks, dead wood or in the soil. They then collect pollen and nektar, which they store in the cells as food for their offspring. They then lay an egg in each cell on top of the food and close the cell.

Figure 3 & 4: The Brown-footed Leafcutter Bee (Megachile versicolor) builds its nests from leaf parts that it cuts off in a rounded shape.
Figure 5: The Large-headed armoured-resin bee (Heriades truncorum) closes its nest with resin and small stones. The larvae overwinter in the nest and emerge as adults in the summer of the following year.

Nesting female wild bees usually never come into contact with their offspring as they live only 5-11 weeks and die before their offspring hatch. They also do not survive the winter. The larva, on the other hand, emerges from the egg after a few days and eats its way through the food supply in 2-4 weeks. It then wraps itself in a silky cocoon and begins a long period of dormancy during which it will survive the winter. In this time, its metabolism is greatly reduced and its development is halted. Only in the spring does it resume its development from pupa to adult.

Figure 6: Opened bee hotel tubes with nests of Red mason bee (Osmia bicornis) in different stages of development. The younger stages, the eggs, are at the bottom of the picture and the older larval stages are further up. Larvae in cocoons are visible on the top right.

Wild bees that survive the winter as adults

Some wild bees, such as the Violet carpenter bee (Xylocopa violacea), survive the winter as adult animals. Both females and males spend the cold season in tree holes or cavities in masonry and clay walls. During hibernation, their metabolism slows down. As a result, they require very little food during this period and lose 10-15% of their body weight.

Figure 7: The Violet carpenter bee (Xylocopa violacea) nibbles its nest tunnels including the brood chambers with its upper jaw into the dead wood. It separates the brood cells by intermediate walls made of spittle-glued wood splinters and mark particles. The nest corridors are sometimes used as shelter by adult animals in winter.

Only the bumblebee queen overwinters

Bumblebees (Bombus) form one-year states in which only the fertilized queen hibernates. In autumn she eats to stock up on calories and then burrows into the ground to survive the winter. Meanwhile, her remaining brood dies off. At low temperatures, the queen produces her own anti-freeze to protect herself from the cold. It provides protection down to -15°C. She emerges from hibernation in the spring to found a new colony.

Figure 8: The above-ground nests of the Common carder bumble bee (Bombus pascuorum) consist of waxy brood cells, kokons and pollen stores.

Honey bees and winter cluster

The situation is different with the highly social honey bees (Apis mellifera), which form stable colonies that last for several years. They survive the cold temperatures of winter in the so-called "winter cluster". Their bodies form a tight cluster around the queen and they warm each other through movement and body contact. In this way, temperatures of 20-25°C can be maintained in the winter clusters. The heat is generated by the movement of the wings as well as by trembling. However, not all the bees are responsible for producing heat at the same time. As 'warming up' takes a lot of energy, the bees take turns about every 10 minutes. Bees on the outside of the cluster are replaced by bees from the inside of the cluster. They are also fed with the honey collected by the worker bees in the summer.

 

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