Huh - I accomplish pretty good natural fly control just by spreading poo regularly during fly season so it dries out and flies don't lay eggs in it. I also can't prove it, but I think my many anthills on the property also feed on fly larvae. I sure see a lot of ants moving to and from manure piles.
Well I'm tossing it in the trash. I don't want to be responsible for tagging and keeping up with where they go:')