Bots in horses are the parasitic larvae of bot flies, a group of insects that spend most of their life cycle inside a horse’s digestive system. The most common species worldwide are Gasterophilus intestinalis and Gasterophilus nasalis, though at least nine species exist. These flies don’t bite or feed as adults. Instead, they lay eggs on a horse’s hair or nearby grass, and the resulting larvae make their way into the mouth and eventually the stomach, where they attach to the lining and feed for roughly 10 months before passing out in manure.
How Horses Pick Up Bots
The process starts with an adult bot fly, which looks like a fuzzy bee and tends to hover around horses during warm months. The fly’s only job as an adult is to reproduce. Female bot flies glue tiny yellow or cream-colored eggs onto individual hairs, and the exact location depends on the species. G. intestinalis, the most common type, deposits eggs primarily on the lower forelegs and occasionally on the hindlegs and belly. G. nasalis, G. haemorrhoidalis, and G. inermis prefer the head, laying eggs near the mouth and between the jaws.
The eggs hatch when stimulated by warmth and moisture, which usually happens when a horse licks or rubs its legs and muzzle. The tiny larvae enter the mouth, burrow into the gum tissue and tongue, and eventually migrate down to the stomach. One less common species, G. pecorum, takes a different route entirely: it lays eggs on grass rather than on the horse, so larvae are ingested during grazing.
The Full Life Cycle Takes About a Year
Once bot larvae reach the stomach, they attach to the stomach lining using small hooks and feed there for approximately 10 months. This is the longest and most consequential phase of the life cycle. When fully grown, the larvae release their grip and pass out with the horse’s manure. On the ground, they burrow into the soil and form pupae. Adult flies eventually emerge from the pupae, mate, and begin laying eggs on horses again. The entire process from egg to egg generally takes one full year, meaning most horses deal with one generation of bots per season.
The incubation period for eggs is usually short, but cold weather can delay hatching. Viable eggs have been found on horses long after the adult flies have disappeared for the season, which is why you might spot those yellow specks on your horse’s legs well into autumn or early winter.
Where Bots Attach and What Damage They Cause
Bot larvae cause two rounds of irritation. The first happens in the mouth, where freshly hatched larvae burrow into the gums and tongue tissue. Horses may show discomfort while eating or resist the bit during this phase, though many show no obvious signs at all.
The more significant damage occurs in the stomach. Clusters of larvae attached to the stomach lining create localized inflammation and can cause ulceration at the attachment sites. In heavy infestations, this irritation can extend beyond surface damage. Severe cases have been linked to intestinal twisting (volvulus), diarrhea, and in rare instances, perforation of the gut wall leading to peritonitis. Most healthy adult horses tolerate a moderate bot burden without dramatic symptoms, but young horses, older horses, and those with other health issues are more vulnerable to complications.
Externally, the most visible sign of bots is the eggs themselves: tiny pale yellow dots cemented to hairs on the legs, belly, or face. The adult flies are also hard to miss. Their persistent hovering around the horse’s head and legs causes obvious agitation, and horses will often stamp, shake their heads, or try to run from the flies during peak egg-laying season.
Removing Bot Eggs From Your Horse
Physically removing eggs before they hatch is one of the simplest ways to reduce the number of larvae your horse ingests. Several tools work well for this. A bot knife, a blunt curved scraper designed specifically for the job, is a popular choice. Grooming blocks intended for removing dirt and loose hair also scrape eggs off effectively. For more stubborn eggs, a pumice stone or lava rock provides extra abrasion. Some people use sandpaper, and others go with a warm damp cloth, since the warmth can loosen the egg’s cement-like attachment to the hair shaft.
Whichever method you choose, checking the legs, belly, chest, and face regularly during fly season and scraping eggs off before the horse can lick them makes a real difference in how many larvae end up in the stomach.
Deworming for Bots
Chemical treatment targets the larvae after they’ve already been ingested, since there’s no practical way to treat the adult flies themselves. Ivermectin is the standard choice, with studies showing 95.4% effectiveness against third-stage bot larvae in the stomach. Moxidectin, another broad-spectrum dewormer commonly used in horses, is far less effective against bots, with efficacy ranging from just 0 to 20.4% in comparative research.
Most deworming programs include a dose of ivermectin timed for late autumn or early winter, after the first hard frost has killed off the adult flies. This timing ensures that all the larvae deposited during the season are in the stomach and vulnerable to treatment, and no new eggs will be laid afterward. Treating too early in the season means your horse can pick up new larvae after the dewormer has cleared the system.
What the Seasonal Pattern Looks Like
Bot fly activity follows a predictable seasonal rhythm in most climates. Adult flies are active from late spring through early fall, with egg-laying peaking in summer. Horses ingest larvae throughout this period, and the larvae accumulate in the stomach as the season progresses. After the first frost kills the adults, no new eggs appear, but the larvae already in the stomach continue feeding through winter and into spring. They detach and pass in manure as the weather warms, pupate in soil, and emerge as new adults to restart the cycle.
This pattern means your horse carries the highest larval burden in late fall and winter, which is exactly why the post-frost deworming window matters. Egg removal, on the other hand, is a warm-weather task. Checking your horse’s legs and face every few days from June through October (depending on your region) catches eggs before they become an internal problem.