To Overcome Botflies, Horses Need Your Help

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Bot flies are bee-like insects that fly around the horse barn, the horse rider, and the horse itself. They are irritating and annoying but don’t bite. The problem with bot flies are the eggs they lay. A female can lay up to 1,000 eggs at a time and a favorite spot to land is your horse; the legs, belly, flanks, shoulders and manes are ideal hiding places for the eggs. While grooming itself, the horse will lick these eggs off its hair and skin, causing the eggs to hatch in the mouth of the animal.

When bot fly eggs are ingested, and hatch, they often burrow into the lips and mouth of your horse. Eventually they make their way to the stomach where they grow even more and may block absorption of nutrients. After a few stages of growth, the larvae detach from the intestinal walls and are excreted in manure. They burrow into the soil or dried manure and mature before the adult emerges in the summer and fall. The cycle begins again as the insect matures and lays eggs on your horse.

The biggest cause of concern with botflies is the irritation that the larvae may start by burrowing in the mouth of the animal and in the prevention of nutrient absorption and possible ulcers. Horses may lose weight due to their reluctance to graze because of the pain in their mouth from the burrowing larvae in the tongue, gum, or lips. In the stomach, the larvae can cause obstruction of the flow of food, colic, or even perforations of the stomach or small intestine wall.

An ivermectin-based product is often used to treat bot fly loads…usually timed to occur in late summer and after the first hard frost of the year (typically sometime in November). By utilizing this timing, it is unlikely that the horse will be infected again and the number of larvae that pass through the manure is minimized. Follow your veterinarian’s directions for routine worming in your horse to prevent extreme bot fly loads. Removal of the eggs, often with a bot fly knife, can aid in the decrease of bot flies in your barn.

Stefani Garbacik is a Livestock and Forage Extension Agent with the North Carolina Cooperative Extension. If you need any further assistance with horses or livestock concerns, please contact her at or (919) 731-1525.