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Bot Flys, Are Out

Posted: August 25, 2015

Three species of bots are considered serious pests to horses.

Bot Fly

The bot is the larval stage of the horse bot fly

  • Bot flies (Gastrophilus species) vary in where they lay their eggs on the horse and how the eggs hatch.
  • In general, they lay eggs on the forelegs, shoulders, and lips during the summer and fall. Where the eggs are laid describes the three species: common bot fly, chin bot fly and nose bot fly.
  • The eggs hatch when the horse licks and rubs them. Once in the mouth, the small bots burrow into the gums and tongue where they remain for approximately one month and then migrate into the stomach.
  • Upon entering the stomach, the larvae attach themselves to the mucus membrane and remain there for the next 8-10 months. They detach themselves in the spring and pass through the feces.
  • Once outside the host, the larvae pupate. Mature flies emerge in four to six weeks. The flies mate, the adult females lay eggs, and the cycle begins over again.
  • The flies do not bite, but the egg-laying process is annoying and unpleasant to horses.
  • The principal damage caused by the bot is to the stomach lining. In extreme cases, bots can cause a stomach rupture or fatal colic if they block the stomach valve to the small intestine.

Consult your veterinarian for drugs available

  • Ivermectin is the de-worming drug of choice for internal control. Be sure to follow the directions on the label before using it.
  • Remove (by clipping or scraping) the yellowish bot eggs from the horse’s hair before they hatch. Then sweep them up and dispose of the clippings and eggs away from feeding areas. Warm water will cause eggs to hatch, allowing small larvae to be killed before they enter the mouth.

Contact Information

Ann M. Swinker, Ph.d.
  • Extension Horse Specialist (Retired July 2017)
Phone: 970-389-5066