In every residential lease, there's what we lawyers call an "implied warranty of habitablity." Basically, your landlord promises that the house or apartment is fit to live in.
A pest infestation that exists because of structual defects or improper treatment (not because a tenant leaves trash and garbage around) may be a violation of the warranty.
Breach of this implied warranty could relieve a tenant of the obligation to pay all or any rent.
You should consult with a Landlord tenant lawyer for an answer to your specific legal question.
Note: This is general information and is not meant as legal advice for your situation. This answer doesn't create a lawyer/client relationship between us.
Leo M. Mulvihill, Jr.
Attorney and Counselor at Law
2424 E York St, suite 316
Phila. PA 19125