I have this fly problem in our house, we kill about 30 a day, even here in the cold north. TG they are slow now and we get them easily. I am wondering, where are they coming from? The only thing different this year is the 2 Guinea Pigs. Could the flies be breeding in the bedding, and appearing that way? The whole house is shut up for the winter, and no food left out. It is grossing DH out, he already thinks I am a lazy housekeeper.