I really don't know, Donna, but I'm thinking it's just the heat and humidity. Here in Chicago we're having a problem with gnats which I don't ever remember hearing about. I think it's just all the rain and humidity. If they're not too bad, maybe wait until the weather gets cooler and dries out a bit and see if the flies disappear before changing litter. Although I don't know if the flies might be a health hazard to the kitties......