That slight nip in the air isn’t the only sign that autumn is on its way—you may also be finding tiny yellow botfly eggs dotting your horse’s forelegs.

“Bots” is the catchall term given to the eggs, larvae and adult stages of the Gasterophilus genus of parasitic flies. At this time of year, the more common botfly species deposit sticky yellow or grayish eggs on the hairs of a horse’s legs; some species also lay eggs on the mane and the underside of the jaw. A single female fly can lay as many as 1,000 eggs between the end of August and the first hard frost.

Warmth and moisture, often provided when the horse licks the area, encourage the eggs to hatch, and the larvae then embed themselves in the horse’s lips and mouth. There they remain about three weeks before they emerge and are swallowed to attach themselves to the lining of the stomach or small intestine with their sharp, hooked mouths. About seven months later, the larvae detach and exit with the manure. After a few weeks, they emerge as adult flies ready to lay eggs and start the cycle anew. Removing manure from pastures two or more times a week will help break the cycle.

An extensive bot infestation can lead to colic, but ivermectin is very effective against this parasite. A single dose given after fly activity ends for the year, November or December in North America, will kill ingested larvae. You can also protect your horse by removing any bot eggs you find before they hatch. Scraping the sticky eggs off of the hairs is easier with the curved, serrated “bot egg knives” made for the purpose or with fiberglass blocks that you run over the legs.

This article first appeared in EQUUS issue #457