This is the best fly trap I have ever used.  The reason it works so well is the bait smells like a dead carcus.  The flies swarm to it.  I have had mine out for a month and it is FULL of flies.  The whole bottle fills up with dead flies.  I made the mistake of putting it outside a kitchen window and the whole kitchen smelled of dead carcus.  So put it so the smell doesn't get in the house.