Nearly every Bumblebee I saw in Slaidburn on Wednesday had lots of mites clinging to them. On the Bumblebee Conservation Trusts web page they say that generally mites are harmless. The mites will probably have hibernated with the young queen and when she makes a new nest they will feed on wax, pollen, nest debris and any other small insects. At some stage they will hitch a ride on foraging worker bees who carry them to flowers where they wait to attach themselves to another visiting bumblebee and so travel to a new nest.

Heavy infestations might make flying difficult for the bee and they suggest trying to remove some with a childs paintbrush...

There is another mite Locustacarus buchneri that lays around 50 eggs in the bees respiratory system where they hatch and develop, the trust says its not known for sure if they are harmful but that must be one heck of a head cold.