Drain flies / larvae / worms

[Left-is-Hot]

Got tenants complaining about flies and black worms coming out from under the toilet. I'm pretty certain that the gasket is loose under there. So I'm going to just replace the toilet (it's an old one) while I'm at it. I figured I'd have them flush some bleach down there a couple of hours before I arrive. Then once I lift the old toilet out, how should I treat this drain? What should I pour down there to treat these pests?

 

[plumb_bob_square_pants]

Most likely they are not coming from the drain. The wax seal is probably leaking and they are living/breading under the toilet.

 

[SHEPLMBR]

septic system or city sewer?