I was photographing a bee on the Willow yesterday and noticed its mites. Apparently mites on Bumble Bees are not a problem, unless the bee is so heavily infested with them that they can’t take off. They’re just hitching a lift to another nest where they will feed on pollen etc. They do kind of spoil the beauty of the bee though. This is the one I saw yesterday on the Willow. 
Here’s another from a couple of week’s back, with quite a large infestation:
I was wondering if it’s to do with the time of the year as you don’t see so many mites on the bees in the summer. Here’s a Bumble from last summer – no sign of mites that I can see:
But looking back through my photos there don’t seem to be a lot of mites around until I get back to 2013. So perhaps it will remain a mystery. Perhaps weather has something to do with the amount of mites around. Here’s are a couple of poor Bumbles with quite a lot of mites in 2013:
