Phorid flies larvae survive only in moist, decaying organic matter. The first spot should be the floor drain. Fly Larvae will live in the moist film that develops on the sides of a drain and in debris that may become trapped in the drain’s edge. When you see the presence of adult flies around the inside drain this is a good indication that the flies are breeding in that particular drain. The use of a drain treatment and floor mopping agent will help eliminate and prevent any organic matter from accumulating in the floor drain.