Weston Bee update

Hi all
We've just come back from our holls the bees have filled up the 2 slots we left them each end , we checked the bees again on  Friday and they were fine , they did have a mad half hour this afternoon ( then the thunderstorm came ) . We've noticed tonight that some have been coming back with a white stripe on their backs !

We have googled this and most sites say it is Himalayan Balsam that the bees have been feasting on , catching one bee to photo was a bit of a struggle (for me not the bee).
Anyone any other ideas?

Mike and Monique