Winter-active bumblebees: Still flying when it’s freezing

Bumblebees make the quintessential sound of summer. That lazy, droning buzz on the edge of earshot is mentally linked with hot days, sunshine, ice cream… so why was I hearing it now, in a garden centre in mid-November as I searched for inspiration for next year’s allotment? Well, it was a bumblebee, all right, but not just any old bumblebee – this was a winter-active bumblebee, or WAB for short.

In southern Europe, a proportion of bumblebees have always remained active through the winter, building their nests in autumn and producing new queens in spring. But in Britain, such behaviour was unheard of, historically – winters were too cold and flowers too sparse. But then came global warming, and gardening. The first British winter-active bumblebees were seen in the early 1990’s, and they have been a regular fixture ever since, increasing in number and geographic scope. But how have they managed it?

Over the past few decades, winters have become milder, particularly in the south of the UK, and especially so in towns and cities (the ‘urban heat island effect’). This shift has allowed the Buff-tailed bumblebee (Bombus terrestris) to become ‘winter-active’ in the same way as their Mediterranean cousins. In the UK, we define ‘winter-active’ as out of hibernation between November and February inclusive – while a few species stay out late (like the Common carder bumblebee (Bombus pascuorum) queen I saw at the garden centre) and others, such as the Tree bumblebee (Bombus hypnorum), often start the year early, appearing in February, only the Buff-tailed seems to actually have winter-active nests.

Bumblebees are pretty cold-tolerant thanks to their size, furry jackets, and a few other clever tricks, but they still need food.  That’s where the gardeners come in. Urban areas, in particular, offer a surprising amount of pollen and nectar through the winter, thanks to increased planting of ornamental shrubs like mahonia, winter-flowering heathers, and witch hazel. These nectar-rich plants provide reliable fuel for winter-flying bumblebees even when natural wildflowers are scarce – although the Botanical Society of Britain & Ireland (BSBI) New Year Plant Hunt suggests that even native species are increasingly found in bloom in the depths of winter. For these cold-weather bumbles, gardens and parks effectively create pockets of habitat where they can survive the colder months.

Being able to do something doesn’t always mean you should, however. Winter-active colonies still face challenges: short daylight hours, sudden frosts, and periods of heavy rain all reduce their foraging time, while snow can blanket low-lying plants or nest entrances. And cold weather increases the amount of energy the bees need to fly, and to keep the nest toasty, which in turn increases their need for nectar. We still don’t have a great idea how well these winter-active colonies are actually doing – to be successful, a nest must at least produce a replacement queen for the next generation. But winter-active colonies produce males and new queens in spring, when it’s virtually impossible to tell if a queen out and about has just emerged from a nest, or from hibernation. Male Buff-tailed bumblebees are very difficult to reliably tell apart from workers, especially if you’re not looking for the differences. We do know that some nests are successful – there’s a steady dip of sightings of mating pairs of Buff-tailed bumblebees early in spring, when at least the males must be from overwintered nests (those from summer nests die off in early autumn).

But they must be doing something right. Sightings keep coming in every winter, from as far north as Inverness, although the south-east and south-west regions have by far the most records. And that’s where you come in. Every winter bumblebee sighting helps us understand how species are responding to climate change, where winter activity is happening, and which plants are most important for supporting them. Even a single observation contributes valuable data.

So if you spot a bumblebee on a winter walk – even on Christmas Day! – please record the sighting on iRecord. Uploading a sighting takes just a couple of minutes, through the website or via an app. A photo is ideal, but not essential. Your records help build a clearer picture of how bumblebees are changing their behaviour in a warming world. And the more we understand, the better we can protect them.

So this winter, keep an eye out. Bumblebees may be tougher – and the bumblebees busier – than you realised.