Originally Posted by hazeljane

http://www.monsterflytrap.com/

We have a blind dog who can't negotiate a double dog door, so we keep the inner door open in the summer, and just have a cut out in the outer door. And flies!!

These things work pretty well. I have one about 25 feet outside of that door, and while they aren't all gone, it keeps the population down. Of course, like all fly traps, they are disgusting, so maybe you have to disguise it.