#9 shot works fine on wasps but is too small for a snake.
7 or larger. Take the shot cup out and replace with wads as the shot cup spins and produces a hole in the middle of the pattern.
For my last birthday, my son got me a "bugasalt" gun as a joke. It's a spring-powered air shotgun that fires a pinch of table salt. At ranges of up to a couple of yards, it will finish off flies and wasps. Very effective inside the house, and the salt doesn't break windows or damage anything. The salt is not a problem to clean up afterwards. It's kind of fun to blast the critters with it.