Yesterday I spent some time deleting photo files from my computer. I was just about to trash this one when I saw two odd-looking dots on the back of the bumble bee. When I enlarged it, I decided the dots looked like mites. If I had noticed them at the time, I would have tried for a better shot. Instead, this is what I have:
It turns out these mites are not uncommon. They are most likely in the genus Parasitellus and can be seen singly or in large groups on the thorax of bumble bees.
From what I’ve pieced together, these mites live in the nests of bumble bees and disperse by hitching a ride on foraging bees, preferably queens. The phoretic stage seen in the photo is a deutonymph—one of the larval stages—not a mature adult.
The mites spread when robber bees or cuckoo bees enter the nest, when deutonymphs are left on a flower and hitch a ride on a different bee, or when the mites simply switch from one bee to another. Occasionally they are found in the borrows of small animals and even in honey bee hives, where they do not survive.
It is unclear if the mites do any harm to the bumble bee. In fact, some sources suggested the mites may be predatory on other mites and thus be beneficial to bumble bee colony health.