This is an urban legend. I had a very bad fly problem earlier this summer in TX and a native Texan neighbor suggested the bag of water trick. It did absolutely nothing. I stopped by our Sears hardware store and they were selling fly traps - it's basically a plastic cup and a lid with special holes that flies can get in to but not out of. You mix a green powder with water to produce a sometime foul smelling liquid and before you know the cup if filled with hundreds of flies. Works like a charm. The cup filled with dead flies does get disgusting after a while though.
Are maggots canibals? Will a bag of dead flies get maggots?
My husband likes to leave his Corona bottles outside. The central Texas flies seem to be more drawn to it than the dog food. Of course, one of the reasons we have so many damn flies is because of the dog. Fecal matter in the yard doesn’t help the problem but the empty Corona bottle sure gives the flies a delicious place to hang out until they drop dead. Never tried the bag method but I’m ready to try anything to keep the disease carrying pests away.