This week I’ve had two enquiries from colleagues at the University of Northampton asking advice on what to do about colonies of bumblebees that have set up home in their roofs.  In both cases these were nests of the Tree Bumblebee (Bombus hypnorum), a species that only colonised the British Isles in 2001 and has since spread rapidly (see this post from last year for a more detailed account).  Because of their association with human settlements they are significant pollinators of garden produce: over the past few weeks I’ve been watching them pollinating the raspberries in our garden and we now have a large crop.

But having a bee nest in your home is, for many people, a real concern.  I thought it might be useful to discuss the issue by quoting from the email correspondence I had with my first colleague, Paul.

Paul wrote:   I wonder if you can give me some advice. I returned home from holiday on Saturday to find that a colony of bees had taken up residence in a roof space above my front porch. The bees are not domestic honey bees but large bumblebees with white rears. I am not sure how many there are, they buzz furiously when I close the door…..  They are not in the house and I cannot see them from my loft…..so they are not causing a problem at the moment other than a moderate dead rabbit smell in the porch.

I am considering contacting the local council pest controllers, but fear they may just gas and kill them as they are not honey bees. What would your advice be, would it be safe to leave them alone, if so how long are they likely to stay, how large is the colony likely to become, are they likely to cause any damage or mess?

Here’s my response:   From your description they are almost certainly Tree Bumblebees which often use loft spaces, bird boxes, etc. As the name suggests they naturally nest in holes in trees. The colony is not likely to get much bigger though over the next few weeks you may find males patrolling the front of the nest, waiting for the virgin females to emerge so that they can mate. That sometimes makes the colony seem larger than it actually is – there are not likely to be more than about 150 bees in there.

I’ve had Tree Bumblebees in my roof a few times and they’ve never caused any damage. All bumblebee colonies die over the winter and the newly-mated females fly off and hibernate. So by late August or September (perhaps earlier if the weather ever gets warmer….) the bees should have gone. At that time you could seal the entrance to the roof space, though they are unlikely to return next year (although it’s not unknown).

Yes, a pest controller would kill the colony. But they are unlikely to be aggressive unless you stick your fingers in the nest hole! My advice is to let them be and take pride in your own bee colony – they are very discerning and don’t nest just anywhere 🙂

So there you have it: my advice is, leave them alone.  Of course if you or your family have a particular sensitivity to bee stings you may need to think carefully about this advice, but in my experience bumblebees are only aggressive if they feel directly threatened.  In over 25 years of field work focused on bees and other pollinators, I’ve only ever been stung a few times, and mainly by honey bees.

UPDATE: A commenter on Facebook had a great suggestion, that I provide a link to Dave Goulson’s nice little video showing what the inside of a bumblebee nest looks like – so here it is.