Bed bugs in apartment, who has to pay for the exterminator?

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I've recently been finding some bedbugs in my room in my apartment. Under city law, who has to pay for an exterminator? This is a two flat building and the landlord occupies the other unit if that matters at all. Not sure on who is responsible for the cost, but I have 2 friends who have had to deal with eliminating bed bugs and both swore by Diatomaceous Earth to get rid of them