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Focus on Flies: A Bot Fly by Any Other Name...

Though less common after the invention of ivermectin dewormers, bot flies can still pose potential irritation and health hazards to your horse. Unlike larvae of other species of flies around the stable and pasture, bot fly larvae grow inside your horse rather than outside in manure, trash or spilled feed. The adult flies look like bees (but have only two wings), do not feed and usually live only a day or two. After mating, females glue their eggs directly to the hair of one or more horses, often to the great annoyance of the horse(s), and then die.

woman deworming a horse

The most common species in the U.S. (Gasterophilus intestinalis) lays up to about 1000 eggs, mostly around the horse’s front legs, shoulders and flanks, and the eggs hatch after about a week in response to the horse licking them. Females of the other two species (Gasterophilus nasalis and G. haemorrhoidalis) lay fewer eggs under the chin, or on the nose and lips, where they hatch on their own.

After hatching, the bot fly larvae burrow into the horse’s tongue or gums for nearly a month, with the resulting irritation and sometimes infection potentially causing the horse to go off feed. After molting, the bots move to the gut, where they cluster in the upper part of the stomach or in the intestines just below the stomach (depending upon the species). In the gut they use two fang-like mouth hooks to hold them in place, rasp the gut lining with their flattened mandibles (mouthparts), and feed on fluids from the abraded gut lining.

After hatching, the bot fly larvae burrow into the horse’s tongue or gums for nearly a month.

This irritation causes the gut lining to swell around the crater-like feeding site, yet horses generally do not show symptoms unless the infestation is large enough to interfere with the passage of food. Rarely, dangerous perforations of the gut can occur, and occasionally large bot infestations can obstruct the gut and result in colic. After sitting in place for about 9 months to a year, the bots release their hold and are passed with the feces, usually burrowing into the soil to pupate, where they remain for a month or two before emerging as an adult fly.

IverCare 1.87% ivermectin paste dewormer effectively treats and controls both the oral and gastric stages of bot fly larvae in your horse. Because bot flies can lay eggs on your horse nearly anytime during warmer weather, it is important to maintain a regular deworming schedule according to the product labeling or the advice of your veterinarian.

“Gad” is an old English term for a stick used to prod livestock — and bot flies can certainly do that — leading to the synonymous common name “gadfly.” “Gadding” is a term used to describe the strong reaction horses often have to the female bot fly attempting to lay eggs: In trying to avoid the buzzing fly, horses may sprint off at full gallop, tail in the air, with the fly chasing behind. This can result in injuries when the panicked horse runs into a fence or other obstacle. Gadfly is also an apt term used to describe certain people, with one definition according to the Merriam-Webster online dictionary being: “a person who stimulates or annoys especially by persistent criticism,” no doubt making the listener want to gallop away as well!

IverCare is a registered trademark of Farnam Companies Inc. Merriam-Webster is a registered trademark of Merriam-Webster, Incorporated.

By William B. Warner, Entomologist and Sr. Specialty Product Development Mgr.


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