OM- if Heather took both of us she would not have a fly, tick or mosquito to deal with! I have had both deer flies and horse flies in the Wind Rivers. But I spend little time on horse trails and a lot of time above timber. Wind is the best repellant. And if you stay high you will usually have a wind. Flies are also like mosquitoes- if you stay clean and cool it helps a bit. Body armor - that is solution! I used to wear thick, tight weave army surplus wool pants (classic 60's and 70's clothing)and never had a problem. Flies are most common late season. First the ticks get you in the spring lower elevations- very prevelent in sage brush (the large red Rocky Mountain ticks,not the small black California coastal kind), then the skeeters get you (July), then the flies get you (late August- very bad in Sept). Does make you wonder why we backpack at all! Honestly, the worst mosquitoes I have encountered were in the Sierra and the worst flies in New Hamshire and the worst ticks in Idaho.