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Bumblebee hibernation

Most bumblebee queens should be hibernating now. Hibernation in UK bumblebees usually lasts 6-9 months, so when you consider that queens only live for about a year, this means that more than half of their life is spent in hibernation!

It’s not surprising then that bumblebee queens are remarkably well adapted to the extremes they face during hibernation. For example, they survive the freezing temperatures by producing the chemical glycerol in their bodies. This acts as ‘antifreeze’, preventing the water in their bodies from turning into ice. It allows them to tolerate temperatures down to minus 19 Celsius!  They also avoid starvation by storing energy as fat inside their large bodies, and filling their crop (similar to our stomach) with a honey-like nectar solution from their mother’s nest before they leave.

Before a queen goes underground she has to find the perfect place to spend the winter. Bumblebees exploit a variety of places for hibernation, but generally tend to prefer north-facing banks. This northern exposure means that the soil won’t be suddenly warmed by winter sunshine, and that the bees are only awoken when the soil becomes consistently warmer in the spring.  Many of you will have found bumblebees hibernating in unexpected places though, including compost bins, plant pots, and even some rolled-up curtains!

To hibernate, the queen digs a tunnel into the soil, using her legs and mandibles. The depth of tunnel varies, but is usually 2-15cm deep. At the end of the tunnel, she excavates a small chamber, where she will remain until the soil becomes warm, indicating the arrival of spring. During hibernation, she will be in a dormant state, and use just enough energy to keep her body alive. In the photo to the right, you can see a queen bumblebee digging her hibernation hole.

You may have noticed that some species appear much earlier than others. For example, the Buff-tailed bumblebee, Bombus terrestris, usually emerges first, often as early as February. This may be because of differences in preferred hibernation sites, which means that some species get warmed up by the rising temperatures sooner than others.

Whatever the reason for their emergence, it’s always a joy to see the first queen of the year, so that’s something to keep us going through the long winter!

The text for this blog first appeared in our Buzzword newsletter in November 2012. Members receive this newsletter three times per year. If you'd like to join BBCT, click here.

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