A good bot-worm control program includes eliminating bot eggs from your horse. Horses rub their legs with their mouth and may ingest the eggs. Scraping off the eggs with The Bot Knife will help. The Bot Egg Knife's special shape is designed to reach into crevices where bot eggs are found.
It has fine serrated edge that scrapes the eggs laid on your horses (usually lower) legs.
Make sure you do not do this in your pasture where your horse will pick up and ingest the eggs again, thus reinfecting the horse.