Interesting. I've never looked for "nesting flies"..in fact I've never heard of it..........but what you are saying is making sense because where the worst problem with them outside is where all the awnings are and so they get in every time a door is openned or there is a tear in a screen.
About winter setting in and nature taking it's course - of that I am aware!!!!!
My apologies, I did not mean to hijack your thread (but I just couldn't resist)
It's too late! You've hijacked the thread and I'm hitting the report abuse button now.... :) Just kidding!
Of course flies have to nest. Haven't you just watched a fly. They fly around and light. They can't keep flying. Even they must light. And by placing a residual based product under the eaves and awnings. They light on the product, then clean themselves, ingest the product and it's adios.