Welcome to the Forum. Let’s see if I can help you.
From your photo you probably have a colony of the Red Tailed Bumblebee, Bombus lapidaries (sorry, but my darned auto-speller keeps making the penultimate letter an e, when it should be a u !). There would be two other possibilities, B. rupestris a Cuckoo, and B. ruderarius which is fairly rare, but except by having red hairs on the pollen baskets looks identical to B. lap.
A B. lap colony is going to run out of steam around late September. The nests are going to be 1 - 3 litres in size, which fits with the void size of your concrete blocks. Usually they will be re-using an old mouse nest, or it might be an accumulation of dry leaves or straw. If you were to touch or otherwise gently disturb the actual nest, you should her a a hissing sound, or buzzing sound. To make things more complicated, bumblebees quite frequently walk to their nest from their “flight point” - which I guess is the crack between the blocks in your photo - but frequently the nest would be next door to the entry point. A strong colony could contain somewhere around 150 or so bees. This species is not all that strongly defensive (you might see this as aggressive) in defence of the colony.
If the blocks are laid without mortar, could you not remove the blocks above and around the flight point until you see / hear the bees react ?
Then stop. To keep the nest dry, it would be worth putting a board, slate etc over the block that contains the nest, to keep the rain out.
Re-reading your email, what I have described should fit pretty well with your current action plan.
If you decide to actually move the nest, do it at night, working by red light (a cycle rear light). Put it in a wooden box. Put that on-site within about 3 metres of the original location; alternatively, move them away about 2 miles away, so the don’t recognise their new location and come back to where they used to live.
I’ll look up some more detailed instructions for a colony move later if you need them.
I hope the above information helps !
Thanks for replying, Clive- I do appreciate it. We have two problems.
The first is that moving would be almost impossible, as it would mean (if the nest is where we assume- if it isn’t, then we’re well and truly stuffed) moving the block on top and then moving the block that actually contains the nest. (I assume that scooping a nest out of the block cavity is out of the question). Given what they weigh, I don’t see doing that without upsetting the bees sufficiently to cause a bee riot; it would certainly be impractical to try it at night. (By the way- the wood fragment you can see in the photo is actually a long plank sandwiched between the top and second course down, presumably to add stability, and I think the bees’ nest is tucked under this plank, so no problem with the weather for the nest while we are working round it).
So, if we can’t move it, can we leave it until it leaves us? Answer, sadly, is probably not. We have building work that MUST push on over the summer, and the best I can offer is damage limitation. We can work round our guests until perhaps the beginning of August. The crucial question is, is there any point, for the bees’ genetic survival, in waiting a bit rather than pushing on with full demolition right away? Would the colony have produced any fertile females by August? I hate the thought of destroying the nest, but if we have at least allowed them time for some reproduction then the damage done wouldn’t be as great. So- back to my original question- how long is ‘long enough’ if I want to let the bees make some genetic profit out of the colony?
I’m sure this is the sort of calculation you’d much rather not have to make, but if you can give me some idea of how production of fertile females is likely to proceed over the summer, I’d be very grateful.