I tried garlic years ago, and it didn't help any at all. The only thing I've found that helped (trying to stay away from chemicals - mostly out of laziness and Dancer goes berserk if she sees a spray bottle) was yellow salt blocks.
We were over run with horseflies this summer. Our horses, especially the paints who had a LOT of white on them, had bloody necks and shoulders from the horseflies. Flysprays kept the regular little flies at bay, but only irritated the horseflies, and they came right back. In desparation, we put out the yellow salt blocks. Vet said the sulphur in them wouldn't hurt the horses at all. Within three weeks, the horse flies had all but disappeared and the bloody necks and shoulders were healing. The little flies had greatly reduced their numbers and our horses were happy campers.
I was afraid they would stink from the sulphur, but they just smell like horses - maybe just a bit stronger. I can live with that!
Plain Old Dee, horses Dancer and Rain I believe in dragons, unicorns, good men and other mythical creatures!