Deworm your horse with an Ivermectin product. It will kill all blood sucking ticks as well as get the Bots that should be migrating by now.
Then, follow up with 5% Sevin Dust when you start to see any more of them. Put the Sevin Dust in an old sock, tie it shut and pat the horse with it anywhere you see them. Be sure to get the base of the mane and the dock of the tail really good.