Solitary Bumblebee

This tiny bumblebee has been living on the marigolds for the last week. It keeps catching my eye as I walk past because it is so small, barely 5mm long. That makes it much smaller than a blowfly or a honeybee.
The markings are similar to Bombus terrestris. There is a nest about 2m away from the marigolds, with worker bees coming and going all day.

Although it goes off to forage it comes back to spend the nights, which is not how bumblebees usually behave. The flowers partly close up at night and one cradles the bee.