Those tiny yellow specks on your horse’s legs may look harmless, but they are anything but. One of the most persistent seasonal pests affecting horses is the bot fly (Family Gasterophilidae), and those small yellow dots are actually bot fly eggs. While adult bot flies cause mild irritation on the coat, the real concern begins when the eggs are ingested.
Once digested, bot fly larvae develop in the digestive tract, where they can lead to gastric irritation, ulcers, inflammation, and other internal health issues. Seeing bot fly larvae in manure can be alarming, but understanding their lifecycle and how to respond makes all the difference in keeping your horse healthy.
What are Bot Flies?
Bot flies are parasitic insects that rely on horses to complete their life cycle. Unlike common flies that breed in manure, bot flies attach their eggs directly to a horse's hair, most commonly on the legs, chest, shoulders, and lower body.
Licking, biting, or rubbing these areas creates warmth and moisture that trigger egg hatching. The larvae then enter the horse's mouth and migrate into the digestive tract, where they attach to the stomach or intestinal lining.
After the bot flies spend several months attached to the lining of a horse's stomach or intestines, they eventually reach full maturity. At this stage, the larvae naturally detach from the digestive tract and are expelled from the horse in the manure.
Once expelled, the bot flies’ larvae do not continue feeding, reproducing, or developing in manure. Instead, they leave the manure quickly and burrow in the surrounding soil, where they form pupae. After pupation, adult bot flies emerge and begin the cycle again by laying eggs on horses during the next active season.
What to do if you find Bot Fly Larvae in Manure?
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Deworm effectively. After the first hard frost, it is recommended to administer a dewormer containing ivermectin or moxidectin. These medications target bot fly larvae in the stomach before they reach full maturity, reducing the risk of gastric irritation and other digestive problems.
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Remove manure regularly. Consistent manure removal from stalls, paddocks, and turnout areas reduces the chance of larvae pupating near horses and continuing the bot fly life cycle. Composting manure at sufficiently high temperatures can destroy larvae and pupae. Avoid spreading uncomposted manure on horse pastures, and keep high-traffic areas around feeders, gates, and waterers especially clean.
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Remove bot fly eggs from the horse’s hair. During fly season, check your horse daily for yellow eggs attached to the hair. Use a bot knife, pumice stone, or grooming block to gently scrape off sticky eggs. Warm water and a sponge can help loosen them first. Removing the eggs from your horse before he ingests them will ensure they do not gain access to his mouth and digestive tract to continue their life cycle.
What Do Bot Fly Larvae Look Like in Manure?
These larvae, most commonly seen in manure from late fall to early spring, are yellow to reddish-brown and measure about ½ to ¾ inch long. Their bodies are segmented, firm, and elongated. Although they are sometimes mistaken for worms, these larvae are noticeably thicker and more rigid than typical roundworms.
Their appearance in manure is most common from late fall to early spring, especially after deworming or seasonal larvae release.
Symptoms and Signs of Bot Infestation in Horses
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Restlessness and discomfort
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Stomach irritation or gastritis
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Constipation or diarrhea
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Decreased appetite
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Dry or poor-quality coat
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Colic related to gastric inflammation
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Mouth rubbing or irritation
Preventing Problems in the Future
Preventing bot fly infestations requires consistent care and strategic timing. Effective strategies include:
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Regular grooming and egg removal - Helps prevent horses from ingesting eggs.
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Seasonal deworming at the right time - Targets stomach larvae before they turn into adults.
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Frequent manure management - Reduces the number of larvae that can pupate near horses, and reduces reinfestation risk.
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Ongoing review of parasite control strategies - Keeps your herd protected throughout the year.
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Proper use of mesh sheets and fly masks - provides a physical barrier to keep the flies away.
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Application of fly repellents - rotate products if flies continue to be a problem.
A common concern among horse owners is whether bot larvae can infect other horses. The good news is that once bot larvae are passed in manure, they cannot directly reinfect horses.
Discovering bot fly larvae in manure can be unsettling, but it provides valuable insight into your horse’s parasite exposure. With proper management, bot flies are highly controllable. Taking action now helps protect your horse’s digestive health and reduces the likelihood of reinfestation in future seasons.
