First and foremost, ALL of my horses are on Strongid C2X, daily dewormer and they are paste wormed twice a year, as per the Strongid Instructions. So, I do not believe that is the cause of the bot flies.
Now, I can not speak for the two surrounding horse farms that have numerous horses. I guess they hatch them and we have to remove the bot their flies lay on ours.
Our area of the country is high in bot Flies...perhaps being the lower midwest / south. Did not have bot issues out in Virginia.
I guess I will keep doing what I have been ... debotting the horses everyday until the stupid flies die off (Of which I have personally taken 2 out already).