There have been bumblebee queens about for a couple of weeks, but this is the first one who's stayed still long enough for me to get a picture. I'm pretty sure she's a buff tailed bumblebee, rather than a white tailed but she zoomed off again before I could get a better look.
The queens are coming out of hibernation now and they are the only bumblebees to survive the winter, so each queen represents a potential new colony. As well as finding food for herself, the queen will be looking for somewhere to start her nest. If you see a bumblebee flying backwards and forwards over the ground and stopping to crawl about for a bit, she's investigating potential nest sites.
Flowers which provide pollen and nectar at this time of year are important for the queens' survival - and even dandelions can be useful. The red tailed bumblebees, in particular, will use dandelions when there aren't many other flowers about, so I always leave a few for them.