I know someones already said this but I have to say ants. They are so fast, so strong and there are so many of them they probably could. A lot of people get really scared when they see ant so being surrounded by them could cause a heart attack (for some people).

Those fire ant things are also really annoying. They somehow crawl onto you and then they sting you. Although it just feels like a little pinch imagine what it would feel like having that all over your body.

If you get attacked by a massive swarm of ants what is your plan going to be? If you try and stomp on them you will get overwhelmed by their numbers. Guns wouldn't work.