Where does it drain into - a sewage tank or a line in the street? Being a floor drain, they could have just run it outside down hill somewhere.
And chances are it does not have a trap in the drain line. Ever get odors from the drain?
Once you find out where the pipe drains to, you may be able to stop them with a pipe snake and foaming pipe cleaner. They are probably living/hatching in sludge/dirt that is in the pipe. If it drains to a sewage tank system, I wouldn't use anymore acid or bleach. Not good for the system.