We had a problem at the beach house with hundreds of flies getting into the house. We had to leave the house open for a few hours after renting it to a tenant with a cat so that we could air it out and the house filled with flies. We placed this trap outside for over a week hoping that it would do something but it caught only about 10 flies, while more and more keep getting into the house every time we open the door to go outside. Useless.