Remove bots with a bot knife. Scrape the bot eggs off with the bot knife away from where your horse grazes. If the eggs drop onto the grass and the horse eats the grass, he or she will also eat the eggs. Pick bot eggs off with your fingernail if you have no tools handy. This is a slow and tedious method.
Most people give up before they have removed all the bot eggs.
Worm your horse one month after the last killing frost. Worm your horse again in the spring to catch any late-migrating bot larvae. Select a wormer that kills bots. Many wormers that kill strongyles do not kill bots. Underestimating your horse's weight will cause you to underdose.