For the last few weeks, Tree bumblebee males have been travelling full speed around the garden marking leaves and stems with pheromones to attract queens. The males and new queens have been produced by bumblebee nests as they are reaching the end of their life cycle. Males leave their natal nest and then hang around the entrance of nests possibly hoping for a queen to emerge. Workers are most likely found dead than alive once the males and queens fly from the nest as their role is completed.
I watched a Tree bumblebee nest in my local wildlife garden, as common in this species, they chose high places, and nest boxes and under eaves are the most easily spotted nest sites. A cloud of males hovered around the entrance.
Today I found the mating pair above on hogweed, the first I find for this species. It is hard to believe that the Tree Bumblebee was not found in N England 10 years ago, now it is so common and widespread. Here is the distribution map for this species from BWARS B. hypnorum project.
More info on B. hypnorum in BugBlog here.