We had billions of the (^%$^%%^$ flies, just about drove me nuts.  BIL was out one day and said "oh, you mean Cluster Flies?  Spray the whole exterior of the house in late Aug. early Sept. with Demand CS."  He works for Plunkett's Pest Control.  I have done it every year since and not only did it take care of the flies but the asian lady beetles as well.  Life's better chemically!

Phil