Bombus hypnorum?

A lovely sunny but breezy afternoon in the garden.
I noticed this bumblebee on the berberis and was curious about the bare patch on it’s back. Having checked on several bumble bee identification sites I think this is a tree bumble bee, bombus hypnorum. This species is easily identified by its tawny thorax, black abdomen and white tail. It is a recent arrival to the UK from Europe having first been recorded in 2001. I have unfortunately no answer to the question of the bare patch and am happy to hear if anyone may know the reason. I do love the word for the genus bumblebee - bombus