It is incredibly easy to mark and measure bumblebees without having to kill them. I have marked and measured hundreds and have never been stung and have never killed a bee while marking it.
Bumblebees are very tolerant and co-operative animals to work with.
I used this method while doing my field work for my thesis at Aberdeen Univesity, it requires no expensive or specialised equipment and will allow you to recognise indivdual bees for at least 15 days after marking. As different species of bumblebee vary in size and strength a device is necessary that can restrain any bumblebee, big or small. Secure the net over the top of the pot with a length of tape. This mesh size is small enough to prevent escape of even small bees and large enough to allow the bee to be marked between the threads. The plastic top of an aerosol cap can be used to form a base that can be moved up inside the flower pot, to trap the bee between the mesh and the top of the aerosol cap.