Who is should pay when a tenant who has been in his apartment for three months claims he has bedbugs. Meanwhile all the other apartments where check and there is no sign of bedbugs. The problem lies only in the one apartment. Know the who building needs to be treated. The tenants that lived in the apartment the year before had no problem either. This apt. Is in pa.

Extermination is usually the responsibility of the landlord.

You can try documenting all this, especially dates, and billing him for extermination, and suing in small claims if he refuses to pay (you may or may not win). Of course you are responsible for all the other tenants, so you might as well get it done now and worry about blaming him later.

Who is should pay when a tenant who has been in his apartment for three months claims he has bedbugs. Meanwhile all the other apartments where check and there is no sign of bedbugs. The problem lies only in the one apartment. Know the who building needs to be treated. The tenants that lived in the apartment the year before had no problem either. This apt. Is in pa.

Let me guess. You wrote this but failed to proof it?

Is this what you are trying to say?
"Who... should pay when a tenant who has been in his apartment for three months claims he has bedbugs [?] Meanwhile all the other apartments were checked and there is no sign of bedbugs. The problem lies only in the one apartment. No other building needs to be treated."