Bot flies don't land on the horse - they shoot those ugly eggs out of their tail shoot - like a torpedo - and that is no joke. http://www.ctahr.hawaii.edu/oc/freepubs/pdf/LM-10-3.pdf
Notice in this link, the words "while hovering" are used
Point-being there isn't a fly spray made that keeps them from laying eggs on the horse.
What I have noticed is that it seems every other day is "Busy egg laying day". I also notice that, as we get closer to Fall, the stickier the "stick-um" is that holds the eggs onto the horse. I deal with bot flies during the entire warm season and have noticed the eggs don't stick on the horse near as well in late spring as they do when we're getting closer to Fall. The above link also talks about nose and throat bot flies.
The eggs left in the nose and throat area can't get licked so how do they get into the horse's system one might ask? They crawl their own way into the mouth<--just when you thought things couldn't get any worse.
I see them on my horses' jowels every year - some years worse than others.
Except for attracting dirt, the mineral oil is a good idea.
I will also use it or some kind of ointment on the jowels just in case I missed any bot eggs in that area.
I also keep that hair shaved pretty close so they're easy to find. Which works for finding ticks, as well