This is relatively new to the UK, having migrated from the European mainland. It was also my first sighting of this bee, it is unmistakable with it's tawny orange thorax and black abdomen with white tail.
This was the first time I had seen a female of this cuckoo bee, I had several of the males the previous year. It was feeding on the same Rhododendron as it's victim, Bombus lapidarius. It was perhaps a little larger than B. lapidarius queen, it's buzz was a little different but otherwise difficult to tell apart by sight unless the two were together. I noticed that B. lapidarus queen was wary of me where B. rupestris took no notice. Cuckoo bees don't have pollen baskets on their hind legs as they don't make their own nests, instead they utilise their host's resources.
This was strangely the only sighting I had of a male last year, which was unusual as I had several feeding the previous year on Dahlias. It made a quick landing and a quick getaway! It is similar to the male B. lapidarius, except the face is all black where B. lapidarius has yellow hairs on the face.