Can I deduct flea treatment from security deposit?

[Joe P.]

Recently bought my first rental property in Pennsylvania and sent the tenant a non renewal notice.  About 5 days after she vacated I discovered fleas in the apartment.  She had a cat which was permitted on the lease.  About 2 weeks after discovering the fleas I had a pest control company spray.  He made a note on my invoice that he did in fact find fleas and they appeared to have been there at least a couple months based on how many were present.  I deducted the treatment from her security deposit and also sent her a copy of the invoice as proof of the flea problem.  She of course is not happy about it and says she never had any fleas and she is blaming it on stray cats living under the front porch.  What would you do in this situation?  Am I wrong to withhold the treatment cost from her deposit?  Neither I nor the pest control guy believe an infestation like this could have happened in such a short period of time.