I think the bees from three of my hives have suddenly discovered Himalayan balsam somewhere nearby. I have seen the occasional bee with a white back in recent weeks but in the past couple of days there have been hundreds of them. Today dozens were flying in every minute.
Interestingly they seem also to be collecting white pollen in their pollen baskets, something I haven't noticed before. I had assumed they didn't like the white pollen but there is a lot being brought in at present.
I don't know where the balsam is but the bees have obviously found it within flying distance. The other two hives either haven't found it or are busy with other flowers somewhere else.