Most bumblebee mites are harmless…
Most bumblebees have many tiny mites clinging to their bodies. In most cases the mites are difficult to see, but sometimes they can cover large parts of the bumblebee’s body.
The good news is that most of the mite species that live with bumblebees are fairly harmless to them and are simply clinging to the bumblebee so that they can be transported to new nests. When in the nest, the mites usually feed upon the wax, pollen, nest debris, and other small insects, so do not feed on the bees. Then, when they reach a certain stage in their life cycle, the mites cling to worker bees, and are transported onto flowers. From these flowers, the mites then attach to other visiting bees, and are transported to new nests.
However, the mites may present a problem if an individual bumblebee becomes so heavily infested that it is unable to fly because of the weight of the mites. If this happens, you can try to remove some of the mites by gently brushing them with a child’s paintbrush.
Others, however… Some mite species can be more harmful. For example, one species, Locustacarus buchneri, lives in the tracheal system of queen bumblebees. This species lays up to 50 eggs in the respiratory system of the bee, and the young develop inside the queen’s body. It is not known if infections like these definitely do harm the bees though.