Pest control question for Chicago area

[Ting Liu]

One tenant in Chicago area claimed there are ants in the apartment.  Her lease specified that tenant is responsible for pest control inside the apartment.  Land is responsible for pest control outside the building and public area of the building. Therefore, I have sprayed the building almost once a week for the past several weeks. She claimed that there are still ants in the apartment and wanted me to hire pest control. So I told her that she should take care of pest control issue according to the lease. Well, she claimed that she would take legal action. Here is my question.  Should I hire professional pest control to spray her apartment even though the lease specified she should be responsible for inside apartment? Meanwhile, is it legal what specified on the lease? Thank you!