A couple of weeks ago I returned home from work to find that my mother-in-law had been trying to reach me. The reason? Her window cleaner had reported that there were bees in a bird box attached to the side of the house, and she wondered if I could help find out what they were.
Without even seeing them I was pretty certain of what they would turn out to be – Tree Bumblebees! The window cleaner had said they definitely weren’t wasps, and a nest box is too small for Honey Bees to set up home in, so Tree Bumblebees were the most likely candidate, as they are a species that particularly love bird boxes.
Tree Bumblebees are a species that’s relatively new to the UK, having first been spotted in the South in 2001. In the years since, they have spread to colonise much of the rest of England and part of Scotland.
Their rapid spread across the UK has been put down partly to their adaptability – they forage on a wide range of flowers and are aerial nesters who are not too demanding in terms of suitability of nest sites. Oh, and the fact that in the UK we love our bird boxes, which are reminiscent of a Tree Bees favourite nesting spot – a hollow space within a tree. This means they love urban gardens and are a frequent visitor to many.
Tree Bumblebee recorded sightings 2007, from the NBN Data Gateway –
Tree Bumblebee recorded sightings 2016, from the NBN Data Gateway –
I went round the next day to take a look, and verified that they were Tree Bumblebees. The male bees swarming around the entrance hole was a dead giveaway – behaviour that is common at this time of year as male bees hang around hopefully to see if an un-mated Queen will emerge. My mother-in-law is not the greatest fan of bees, mainly because her daughter is badly allergic, and as the box was just outside her patio door their presence meant she was prevented from being able to leave the door open in the heat. So I quickly volunteered to move the box containing the nest to my garden a mile down the road to look after for the rest of the season.
That weekend we waited until after dark to move the bees. We arrived armed with a sponge which I used to block the entrance hole before removing the box from the wall. Then we undertook a fairly nervous car journey with the nest encased in a carrier bag – an insurance policy in case the sponge fell out! The bees within buzzed loudly as they knew something was up.
On returning home I quickly hung the box up on a nail I’d stuck in the side of the garage earlier. The sponge was left in place – if it’s removed in the dark the bees may emerge and then not be able to relocate the nest. I went to bed that night happy that the move had gone well, but worrying about whether our new garden tenants would have enough oxygen within the nest to see them through the night(!)
The morning came and to our surprise, the local male Tree Bumblebees had located the nest and were beginning to swarm around the entrance.
I nervously removed the sponge blocking the entrance hole to the sound of the now familiar buzz rising as the bees inside felt the nest box move. I was convinced that the bees would pour out to see who or what was interfering with their home so beat a fairly hasty retreat, but only a couple flew out. They immediately began taking orientation flights – flying in larger and larger circuits around the garden and then returning to the nest before setting out again. This allows the bees to map the location of all the local landmarks (specific plants, the compost bin etc) which will allow them to find their way home again.
It was a couple of days later that I finally got a closer look at our new guests. And I got a bit of a surprise – Tree Bumblebees are a very distinct bee with a bright orange thorax. There’s no other UK bumblebee that looks like them so they are very easy to identify.
There are more uncommon Melanistic variants though, where the orange is either faded or missing, meaning the bees are almost completely black. And that’s what we had!
They are really stunning, and of course their unusual attire means I have to call them ‘Goth Bees’.
They have now been in the garden for a week, and seem to be getting used to their new surroundings. On one particularly hot day this week I arrived home to find a group of bees clustered around the entrance hole, using their wings to fan warm air out of the nest.
These bees are excellent thermoregulators, fanning the nest to cool it if it’s too hot, or clustering together and using their flight muscles to ‘shiver’ and raise the temperature if it’s too cold. They keep the temperature within one degree of 30 degrees centigrade at all times.
Many more male Tree Bumblebees are visiting daily – some days there are too many to count. The females can get quite grouchy about this – yesterday I saw one female repeatedly grab male bees and physically fly them away from the nest. It didn’t seem to stop the tenacious males returning, however!
The fact that the males are here means that it’s almost the end of the season for this nest. The new Queens will emerge and mate, before leaving the nest for the final time and hibernating until next year. The bees within the nest will die off and the Tree Bumblebee cycle ends for this year. My father in law can then have his nest box back! I’m going to miss these little bees once they are gone, so I’m going to site an empty nest box in this location, containing a little moss and animal fur to encourage Tree Bumblebees to inhabit it next year. Who knows, maybe one of this year’s new Queens will take up residence?
It may only be a few weeks now that I get to spend with these bees, so I’m going to make sure I enjoy them while I can 🙂