I have to ask; why a hellwasp? This class doesn't have any connections to the Hells so it honestly feels tacked on. Why not just make it so the Swarmlord can gain any appropriate vermin as a familiar and have the Hellwasp as one of the options?
What about swarms of Tiny creatures? Otherwise this ability looks fine but you may be getting it too early. The DR is very limited, but those immunities can be useful. It just makes the class really front loaded for level 1 and attractive as a dip. Also, I think you mean sickened, not sickness.
Why poison use? The Swarmlord can create swarms with poison special attacks and the class is centered around offensive usage of swarms. Just something else that feels kind of tacked on since Poison use is generally a stealth/assassin class thing.
Does this apply to magical or supernatural poisons as well?
What exactly is a "large swarm"?
Does this apply to magical or supernatural diseases as well.
The first part of this is really niche and the second part is definitely too little too late. Flat immunity to Mind-affecting effects would be more suitable honestly.
This...is a really weak capstone ability to be honest. It doesn't really entice someone to take all 20 levels in this class.