A ‘swarm’ of bumblebees is usually a mating ritual by Bombus hypnorum males
I have had many different bumblebee species nesting in my wildlife garden and found many wild nests over the years. In 2001 (Goulson and Williams) found the first recorded Bombus hypnorum bumblebee in the UK. After that BWARS set up a B. hypnorum monitoring scheme. and I eagerly awaited their arrival in my own garden. Prior to them nesting there, I had only ever seen a few worker B. hypnorum bees, when I visited other gardens, nature reserves, etc, down south. I had no idea or any experiences of their behaviour. I had heard that they defended their nests with vigour! I was unsure exactly how this would translate in reality and was rather cautious if not a little intimated by this comment! How far this aggressive defence extended beyond their nest I was not aware. Since 2013 I have had them nest every year, with some years having several nests. Every year I receive many phone calls, emails and even visitors to my house from concerned people about a ‘swarm’ of bees outside a part of their house.
Nest defence
One day I watched a rather persistent green bottle fly try to enter an occupied nest box. A nest guarding bumblebee ran from inside the nest entrance to the outside and tried to ‘ram’ the fly, shooing it off, (though I assumed she would have bitten it). The green bottle simply flew up and moved a couple of inches away but remained near the nest entrance. It attempted to enter the nest via the entrance hole. It was chased again and again by the same bumblebee with the green bottle simply flying out of harm’s way, only to try and enter the nest entrance from a different angle. It did this several times, in the end, the green bottle got the message and flew away. So I thought that did not seem so aggressive!
‘Swarming’ – a nest defence strategy?
On at least on one occasion I have had three B. hypnorum nests in my garden at the same time. The middle nest would largely be ignored by the males (I now know why as there were no virgin queens inside) as this particular nest was a failure in terms of queen production with only a few workers produced. The other two were very active. I watched as individuals would leave one ‘swarm’ and join the nearby ‘swarm’. Was this perhaps part of a diligent nest defence strategy, searching for potential dangers to the nest? On hot sunny days, many more bees were present and less on cooler days. From very early in the morning to almost dusk, they performed this dancing swarming behaviour. For me, this was a completely new behaviour displayed by the bumblebees and I thought well worth filming. I made several videos. I found it to be a rather strange behaviour, something new to me and decided to research the defensive strategies of these bees. Then whilst about 8 feet away from one nest, whilst watching the spectacle, I was stung by a B. hypnorum bumblebee just under the eye. Part of their defence strategy? Filming was put on pause!
Research
Goulson states that ‘B. hypnorum are notoriously aggressive. They regularly attack intruders and will pursue intruders for some distance (from Bumblebees and their ecology). I also found a useful article about their behaviour and nesting from the Bumblebee Conservation Trust, ‘Introducing the Tree Bumblebee’ (Bombus hypnorum) (updated version linked). I did find out that vibration of their nest would initiate an aggressive defence of the nest. So knocking the nest, banging a door of a shed if a nest was inside, would more likely initiate such behaviour. A nest up high under your eves or loft is not likely to be vibrated. So me standing just 8 feet away and not vibrating the nest at all, why was I stung?
I was rather excited to see my first dancing drone display and to be honest I still am! I have filmed several now over the years. Workers would return to the nest with pollen and simply bypass the swarms. Some bees would leave the swarm and go to the ivy behind the nest box, probably for a rest then rejoin the swarm. Queens leave, many are pounced upon and queens were regularly seen returning to the nest.
Misinterpretation of behaviour
I read up about the swarms of bumblebees outside bumblebee nests. It stated that they are drones performing an ‘aerial dance’ that looked like a cloud close to the nest entrance. This behaviour is technically called ‘nest surveillance’ and is a mating characteristic of B. hypnorum. Everything I read fitted my observations. ” Bees join and leave the ‘cloud’ as they move from nest to nest: they are probably following a ‘patrol route’. I had totally misinterpreted the behaviour, being too obsessed and very wary of their ”defending their nests with vigour’ and me being stung to re-enforce this belief! It was obvious that these bees dancing around were not interested in me at all. Some even flew into my face. They were harmless, stingless males. After reading this it all fell into place and I filmed with impunity!
Why was I stung then?
I then got to thinking about how and why I was stung. Whilst watching the swarming spectacle, the sting was not of the same severity as the B. dalmatinus, also known as the tomato-house bumblebee, I had several years ago, (as supplied with the sale of a bumblebee nest box!). That one went straight for me directly, from the entrance hole, which was only a foot away. What was I doing there? Yes, you got it, to film it!
This time I was standing in the direct flight path to the nest entrance. I saw this particular bee launch herself out from the nest box, drop down and start to climb up, it all happened so quick. I tried to move out of her way and she collided with me. The sting was not a full-blown sting, more of a slight prick really. The bee simply carried on with her outward flight and did not return to the nest box as the B. dalmatinus had straight after she stung me. This sting was not a deliberate attack upon me. More my own fault for being directly in the flight path and not moving out of the way quick enough. Knowing that the swarms were males, therefore stingless, knowing that I did not vibrate, knock or otherwise disturb the actual nest itself, knowing my mutual defence strategy was wrong, full of confidence and now out of the direct line of flight, I made this video of the ‘dancing drones’!
Refs. Besides my own observations, videos and experiences;
Hill, C. (2013), “Introducing the tree bumblebee, Bombus hypnorum“, Bumblebee Conservation Trust information sheet.
For more information and to help save bumblebees join the Bumblebee Conservation Trust.
BWARS are recording the spread of B. hypnorum which you can submit a record here and join here
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