Who is responsible to pAY FOR EXTERMINATION of bed bugs in pa?

i recently started getting bit at night so i contacted an exterminator he determined i had bed bugs so i contacted my landlord and she told me the tenants on the first floor had recently discarded a sofa they got from the garbage that had bed bugs so she contacted terminex and when she found out how much it would cost she said it was the tenants responsibility so i can get with the other tenants and we could split the cost who's responsibility is it