Where Do Bees Go In Winter?

Do bees hibernate?  If so, where do bees hibernate? 

What happens to bees when the weather grows cold?   Where do bees live in winter?  Do bees hibernate in winter, and how do they survive the low temperatures and stormy weather? 

We may still see bees around in the autumn on dry days, feeding from the flowers on ivy - hedera helix, and pollinating autumn raspberries (and other late crops).   But what happens then?

It really depends on the species, but in summary:

  • Bumble bee queens hibernate in winter.
  • Most solitary bee species will overwinter in a birth cell, either as new, fully developed adults not yet emerged from their cells, or as pupae, waiting to complete their development.
  • Honey bees overwinter in their hive or nest, forming a winter cluster around the queen, with the colony itself much reduced in size.  They are less active though not entirely dormant, and the cluster 'shivers' to keep warm.

Given the many bee species in the world, there are of course,  exceptions to those scenarios described above.  One key factor is climate: in exotic locations, species may be active throughout the year.  In such circumstances, there remains sufficient abundance of the right kinds of foods to keep the bees active.

More about where bees go in winter

Here is a nice little quote that sums up the situation for many wild bees:

"The most common forms of overwintering in non-active bees involve either a prepupa or a fully developed, non-emerged adult within the  birth cell.  Eventually, rising temperature or some other physiological cue will trigger the bee to emerge.  Bumblebees  are different in that the new queens produced in late summer feed up on flowers and then depart their birth nest, find an old rodent hole or some other pre-existing cavity (including compost heaps) and hibernate in this." - Steven Falk - entomologist and author of Field Guide To Bees Of Great Britain And Ireland.

In cooler climates (rather than exotic locations) where there are far fewer flowers from which to feed in the winter, bees need to shelter from the weather and conserve their energy, until the flowers are blooming again. 

Where Do Bumblebees Go In Winter?

As autumn approaches, fewer species of bumblebee are seen foraging.  The natural lifecycle of bumblebees means that most of the colony will die, and only the new bumblebee queens will survive.  They will mate with the males, and importantly, feed to build up fat reserves ready for their winter snooze (hibernation).

Hibernation sites vary, but it seems most bumblebee species prefer soil banks (perhaps in an abandoned rodent hole) and slopes, and preferably in the shade or in a North West facing position (Goulson). 

However, I have read reports of bumblebee queens being disturbed during their hibernation, because they have been found hibernating in soft potting compost in a plant pot, or in a pile of wooden logs.  They may also seek winter refuge in a compost heap.

Are changes in climate affecting hibernating patterns of bumblebees - such that we even witness winter active bumblebees?

In milder climates – and even in milder weather zones of the UK for example, bumblebees have been known to remain active through the winter, foraging on mahonias and exotic species of plants found in park lands.

If winters are getting warmer, perhaps this should not surprise us.

In areas of South East Asia and South America, there are bumblebee species that have no annual life cycle (with winter hibernation) as such, and colonies can become rather large as a result, and even reach up to several thousand workers (Michener & Laberge 1954; Michener Amir 1977).  In Brazil, the species Bombus incarum can produce as many as 2500 new males and queens in a single nest!  (In the UK, colony size varies, but will perhaps reach 150 – 400 workers.  Read more about British Bumblebees or Bumblebees of North and Central America).


Where Do Honey Bees Go In Winter?

Honey bees become less active, but not entirely dormant  (inactive).  When the temperature falls below 10 °C the honey bee queen and workers huddle together in their brood chamber in a temperature regulating cluster - called a  thermoregulating cluster.  This is commonly referred to as a 'winter cluster'.  By this time, there will be no males (drones) in the hive or nest.

Older worker bees will form the outside of the cluster, with the younger bees toward the middle.  The clustering bees vibrate their wing muscles in a kind of shivering movement, which helps to generate heat.  The bees are also helped by their hairy bodies, because the fine hairs help to trap the heat, and this effect is magnified by the fact that there are lots of bees together, all with hairy bodies.

Amazing fact: 

whilst honey bees shiver to keep warm during the winter, they create conditions that would be toxic to humans!  Why? As the bees are shivering, they use oxygen and exhale (breathe out) carbon dioxide.  This is happening in an enclosed space.  In such an environment (carbon dioxide and restricted ventilation) humans could not survive, but the bees create such conditions intentionally, because it decreases the bees metabollic rate, and hence helps them to conserve their energy stores.  This higher level of carbon dioxide also helps to kill off some pests, such as the Varroa mite! (1)


The colder it gets, the more tightly the bees cluster together.  As younger bees in the middle of the cluster get warmer, they loosen the tightness of the cluster, allowing the warm air to circulate back to the older bees on the outer of the cluster, but the outer temperature of the cluster is usually maintained at around 12 °C, but no less than 6 °C (2).

The older bees that form the outer of the cluster were reared in autumn. However, when the temperature warms in spring, overwintered worker bees that were forming the outer cluster, get busy again, with foraging duties and so on.

In the wild, however, honey bees may be more protected from the elements within the trunks of trees or caves, than they are in a man-made bee hive.  In cooler climates, honey (winter food stores) is vital to help sustain the colony. In warmer climates, however, honey bees may be active all year, so that honey is essentially fuel for reproduction and swarming.

Is this winter behaviour in honey bees actually 'natural'?

Some researchers are questioning whether what goes on in a honey bee hive in winter, reflects natural behaviour, or is simply a  behaviour used by honey bees in order to cope with life in a man-made bee hive:

“Many honey bee behaviors previously thought to be intrinsic may only be a coping mechanism for human intervention; for example clustering in a tree enclosure may be an optional, rare, heat conservation behavior for established colonies, rather than the compulsory, frequent, life-saving behavior that is in the hives in common use. The implied improved survival in hives with thermal properties of tree nests may help to solve some of the problems honey bees are currently facing in apiculture.” - Researcher, Derek Mitchell (3).

Mitchell makes the point that in a natural bee cavity such as a tree trunk, insulation is provided by the surrounding mass of the tree.  (Indeed, I find it interesting that beekeepers of bygone years even used hollowed-out tree trunks in which to keep bees - see image below).

In a natural tree cavity, there is a large amount of natural insulation provided by the trunk itself, both above and below the colony,  and can be virtually any thickness, but more typically range from three to five inches....... which is about six times the insulation quality of a typical bee box provided by the type of conventional bee hive used today.

It should be noted that Mitchell used his background in physics to compare the heat loss from a tree to that of man-made bee hives.  He found that a man-made box from a conventional bee hive will lose four to seven times more heat than a typical tree colony, and that some behaviors of the colony may even be driven by that fact!

Where Do Solitary Bees Go In Winter?

In countries such as the UK, with its cool winters, solitary bees, depending on the species, may overwinter as mature adults, or as pupae.   Mining bees, such as the Ashy Mining Andrena cineraria, remain underground as adults in their natal cells. 

Some solitary bees are very tiny and may overwinter in hollow plant stems – something I am keen to remind gardeners of, as they begin to tidy up for the winter - especially if they are thinking of burning the plant stems!

I encourage gardeners to leave the hollow plant stems until the following year, or if necessary, leave the stems in a corner of the garden out of the way. 

Interestingly, according to the book The Bees In Your Backyard: not all the young of solitary species develop and emerge from the nest in the same year, and will remain in the nest through multiple winters:

"Not all Habroda larvae from the previous year emerge: instead, some individuals will stay in the nest for 2 years and maybe longer; some bee species will delay emergence for 7 or even 10 years.  Why all the bees from the previous year don't emerge together is not known, but it may have something to do with avoiding parasites, or even responding to rainfall cues."


References:

(1) The Potential of Bee-Generated Carbon Dioxide for Control of Varroa Mite (Mesostigmata: Varroidae) in Indoor Overwintering Honey bee (Hymenoptera: Apidae) Colonies Rassol Bahreini and Robert W. Currie Journal of Economic Entomology 2015 108(5), 2153-2167

(2) Phillips EF, Demuth GS: Temperature of the honeybee cluster in winter. Bull US Dept Agric 1914, 93:1-16.


(3) Ratios of colony mass to thermal conductance of tree and man-made nest enclosures of Apis mellifera: implications for survival, clustering, humidity regulation and Varroa destructor Derek Mitchell Published online 2015




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