When I was about 8 I was picking Strawberries out side of Portland Or. Anyway the field was partly flat and then rose up a hill and continued on.
Well heard this loud noise and looked up to see a black cloud coming from the upper field.
Not sure what it was at the time but instinct told me to lay flat between the rows. It proved to be a good choice as a large swarm of Bumblebees swept over the entire field.
Several of the pickers took off running and all got numerous stings.
Only time I have ever seen something like that.