Advice on lease language regarding rodents

[Tom McGee]

Looking for advice on how landlords handle pests and rodents in lease agreements. How do you structure your leases regarding tenant - landlord responsibilities?  As a landlord, how specific do you get and at what point, if any, does tenant become responsible fro treating the issue (other than communicating to the landlord of issues). 

I realize laws change from state to state.  This is a Pennsylvania question.