What can I legally do if I moved into an infested unit and when I notified landlord, she asked if I wanted my unit fumigated but it never happened?

In California, you are allowed to spend up to one month's rent on paying for repairs [I assume this would qualify] if after giving notice the landlord does not within a reasonable period of time makes the repair [give notice by a certified letter]. You can also complain to the local health department. But you have to be careful, as the landlord may then try to terminate the lease or evict you.

Withhold rent