Not a real threat: The smaller you go, the harder it is to resist environmental effects (wind, rain, etc). Go small enough to be unnoticeable, and they'll be impossible to fly in any area that has aircon. Also, because the flight speed is kind proportional to the size of the motor/propulsion, they wouldn't be very fast, so you could probably just... run away? Finally, the payload (bomb) gets smaller too, small enough and a bomb becomes just a minor annoyance, like poking someone with a toothpick, even failing to pierce the skin. A group effort requires communication, communication can be monitored, interfered with, and jammed. Hell, even background noise will be a major issue because you can't make the signals very strong on tiny drones. The smaller you go, you the more fragile the drone. The radiant heat from fire (flamethrower) would probably be enough to fuck them up after a second of exposure (not neccessarily a direct hit). You could probably swat them out of the way with your hand. Now just for arguments sake, let's say that in the future they somehow do manage to make a swarm of these things that don't have any of the problems listed... Well we already have lasers that we use to destroy bits of space junk. If they can destroy bits of metallic rubbish out in space from the ground, I'm sure they can make a miniture laser turret system that'll protect you from these things. Finally, nerve gas is better than any drone and already exists.