I spent an interesting time in the garden this afternoon watching dozens of wasps buzzing around one of my oak trees.   I think there must be a substance on the leaves that attracted them.    They were even attacking each other when one disturbed another on the same leaf, they dropped to the ground entangled and then flew back to carry on.   That's when I thought I'd seen enough and beat a hasty retreat.
I checked the oak tree on the river bank and there were no wasps there at all.