We need your state to give you a definite answer. The general rule is that L pays to control insects & pests unless the infestation was caused by T. Usually this means T left out food & it attracted cockroaches. If rats are an ongoing problem, L can't charge you for them, any more than L could charge you for maintaining the driveway.
Write L back (certified, return receipt) protesting the charge and demanding a full refund. If L doesn't pay up, sue in small claims court. Also, check your residential landlord tenant act to see if you are entitled to double/triple the $150 refund.
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This is not legal advice and you are not my client. Double check everything with your own attorney and your state's laws.
[This message has been edited by Tracey (edited June 20, 2000).]