These are Eastern Cicada Killers, a type of solitary wasp that feeds on.., you guessed it, cicadas. Each wasp lives alone, but in the summer, the males gather into swarms to compete for territory and females. The males do not have stingers, and although fly very close, they do not harm people. Still, suddenly finding yourself in the middle of a swarm of huge wasps can be intimidating, if you don't know that they are harmless.