I have flies every summer as I don't have screen doors. I leave the doors open during nice weather so I don't have to let the dogs in and out. I found that flies of all sorts as well as wasps and hornets congregate on my windows. I go around with my vacuum hose and suck them all up every day after closing the doors for the day. It takes care of about 98% of them and then I cook dinner.
I'm waiting until after killing frost to wash the insides of my windows as fly spit will continue to accumulate until then.
"Never doubt that a small group of thoughtful, committed citizens can change the world. Indeed, it's the only thing that ever has."--Margaret Mead