Bumblebees active in winter in the UK
If you live in the southern half of the UK you may have seen bumblebees visiting winter-flowering shrubs on sunny winter days in recent years.
We all used to think that bumblebees hibernated during the winter. So what happened over recent decades? A few colonies even seem to have survived through the big freeze of the last two winters (2009/10 and 2010/11).
Where do bees normally go in winter?
Click here for summary
The Bumblebee Year
There are six species of bumblebee commonly found in UK gardens, the so-called 'Big Six'. Unlike honey bees, bumblebees have short-lived colonies and a nest typically lasts only a few months. At the end of the cycle young fertile females (queens) go into hibernation underground, and stay there for many months, to emerge the next spring when each will found a new colony.
Normally the last of the 'big six' to be seen in garden in the autumn is the ubiquitous 'Common Carder Bumblebee' (Bombus pascuorum), a relatively small species, gingery brown, which can be seen in the South until early November if the weather remains favourable. Then there should be a gap of at least three months until the earliest emergence of young bumblebee queens from hibernation the next spring. One of the consistently early species in the spring is unsurprisingly called the 'Early Bumblebee' (Bombus pratorum). This species is also relatively small, with two yellow bands and an orange tail. Several other species will also make an appearance by March or April.
These include the very common 'Buff-tailed Bumblebee' (Bombus terrestris ssp. audax). Queens of this sub-species tend to be large and wide, with a pale peachy-buff coloured tail. Workers have a white tail. Sub-species audax is confined to Britain and Ireland. If you cross the Channel you will find that in Northern France B. terrestris queens have a white tail.
Foraging from Winter-flowering garden plants
Evidence to date
Evidence from the last decade or so suggests that in some areas of the UK the Buff-tailed Bumblebee is able to maintain winter colonies in addition to its summer breeding cycle. It relies on autumn and winter-flowering plants growing in gardens as a forage source. Prime amongst these are Arbutus, and Clematis cirrhosa, from the Mediterranean area; Mahonia and winter-flowering honeysuckles such as Lonicera fragrantissima, from Eastern Asia; and Erica carnea, a winter-flowering heather from the European Alps. There are very few wild British plants that flower in the winter, and it seems that those that do (e.g. Gorse) do not provide sufficient or suitable forage to support winter bumblebee colonies. For this reason it is unlikely that winter colonies will survive in the open countryside away from gardens.
Favourite flowers of bumblebees in autumn and winter:
Left: Arbutus unedo (Strawberry Tree) in October
Left: Lonicera fragrantissima
(Winter Honeysuckle) in January
Left: Erica carnea (Winter flowering Heather) in February, with Bombus. terrestris
Left: Mahonia
hybrid in January.
Interestingly, there are many conspicuous winter-flowering plants that the bees do not seem to use as a regular forage source, or only visit as a last resort. These include Viburnum x bodnantense, Chimonanthus praecox, Jasminum nudicaule, Cornus mas, Iris unguicularis, and winter bedding plants such as pansies and polyanthus. The fact that they do not seem to visit these may be because these plants do not produce suitable nectar or pollen, or possibly it is inaccessible to the bees.
Ecology of winter bumblebees
Bombus terrestris in Southern Europe
The winter nesting behaviour of the Buff-tailed Bumblebee in the UK may not be as aberrant as it first seemed. Bombus terrestris in a broad sense is widely distributed in Europe, and a number of sub-species have been recognised. In the Mediterranean area, the local sub-species exhibit regular autumn and winter breeding behaviour, which fits in the with the local climate and the availability of forage plants such as Arbutus unedo, A. andrachne and other autumn and winter-flowering species.
Is this a reaction to Climate Change?
Many kinds of insects in the UK have expanded their distribution northwards in the last 20 years, and it is generally thought that this is due to the pattern of milder winters we generally experienced during these decades. The winter breeding behaviour of the Buff-tailed Bumblebee is particularly interesting because it appears to be a change in behaviour, rather than a change in distribution. It is still hard to go beyond this and make firm statements about this phenomenon without the collection of much more data, over a wide area, over a number of years. The first reference I have found to a winter bumblebee colony visiting the Strawberry tree, Arbutus unedo, dates in fact from 1957 when winters were still cold and what we consider to be recent climate change had not yet become apparent. So perhaps this phenomenon went on unrecorded for a long time simply because it was overlooked.
Should we all plant more winter flowering shrubs as food for bumblebees?
Not necessarily. B. terrestris is very common and is not a species of conservation concern in the UK at present. In fact if it can successfully breed in winter in the UK it might gain a competitive advantage over some of the other common bumblebee species, for example by monopolising potential nest sites early in the year, so that the other species might suffer. As yet we just don't know - we need more data, from more sites, collected over a number of years. This is where you can help.
The Cold winters of 2009-10 and 2010-11
The temperatures and amount of snow we experienced in these winters were not unusual four decades ago when I was a child. In more recent years the climate has changed and we are used to experiencing much milder winters.
Hibernating bumblebees are well-adapted to the sort of temperatures we had in the 2009/10 and 2010/11 winters, and it has done them no harm, however it is interesting to see what effect it had on the winter-active colonies. Most, it seems, were wiped out but a few survived. This is all the more reason for us gardeners to get interested in systematic bumblebee recording and bumblebee conservation. The more information we can gather about the reactions of bumblebee populations around the country to our unpredictable weather, the more that can be done to conserve them. If you take part in the British trust for Ornithology's weekly Garden Birdwatch you can submit winter bumblebees sightings on their garden bumblebee module; you can also submit sightings to BWARS, and a fact sheet is available from Hymettus. (Details of these organisations on my LINKS page.)
The photo shows a B. terrestris queen with her typical buff-coloured tail. She is shown visiting Erica arborea (European Tree Heather) in March.
How I got involved
I originally became interested in this subject in 2005, when I noticed bumblebee workers (non-reproductive females that forage for nectar and pollen) visiting an Arbutus (Strawberry Tree) in my garden in November and December. According to the books then available this was not possible, because bumblebee colonies die out in late summer and only individual young queens (fertile females) survive the winter in hibernation to found new colonies the following spring. The presence of worker bees on a regular basis suggested that there was an active nest nearby.
At that time I was studying Ecology and Conservation as a part-time student at Birkbeck College, University of London. I had to choose a dissertation subject and I decided to investigate the phenomenon of winter bumblebees. I collected data by regularly walking a 1km fixed route (transect) in the area of suburban London where I lived at the time, to record any observations of winter bumblebee activity in 2006/7. The exercise produced a lot of data.
I repeated the exercise the next winter, 2007/8. I was able to arrange a transect in Kew Gardens, which passed many winter-flowering plants. Again, this produced a lot of data. These data have been incorporated in a scientific paper that you can view HERE
Detailed research into the mechanisms behind, and consequences of, the winter generation in B. terrestris is currently being undertaken by Dr Tom Ings from Queen Mary, University of London, who is also continuing the survey work at Kew Gardens.
Tom's website is at http://webspace.qmul.ac.uk/tcings/
Where do bees go in winter?
Bumblebees: Colonies die out at the end of the summer. Only young fertile females (queens) survive the winter, in hibernation in nooks and crannies under the ground, in a deep sleep. The winter-active bumblebee colonies described on this page are an exception to this rule, and seem to be a recent phenomenon in the UK, involving only one of our 24 species.
Solitary Bees: All adults die as soon as their period of mating and nest building is over. For most species, this period is a window of activity lasting couple of months or so in spring or summer. Different species are active at different months, usually coinciding with the flowering of their main forage plants. Solitary bees' offspring survive the winter as mature larvae, pupae, or as dormant young adults (depending on species) in a state of suspended animation inside their nest cells. Again depending on species these can be holes or tubes in wood, dead stems, holes in sandy cliffs or in crumbly mortar, or holes or chambers underground.
Honeybees: These are a domesticated species and in the UK are now almost wholly dependent on human intervention. Colonies survive intact inside beehives and can last for several years with help from a beekeeper. Feral colonies left to their own devices (e.g. in hollow tree trunks) can also survive the winter but at present many of them seem weakened by the Varroa mite and less able to survive in the long term.
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© Marc Carlton 2011. You are welcome to print this page, for personal use or for non-commercial, not-for-profit educational purposes. All other reproduction prohibited without permission of the author.