I have flystrips all through the house........and hanging water fly traps outside.
The weather has been so weird this summer that it's beyond weird. Friends just 20 miles south haven't seen a flea at all, but wre inundated with ticks...and we haven't seen a tick.........the flys have been different. Those with central A/C don't have the flies inside, just outside. those of us without central a/c have both.
You shouldn't have to worry much longer. You're far enough north that the weather will make it's own change very soon. Once you get your first real frost, then most of the flies are gone.