Horse Bot Activity Nearing an End

The first hard frost marks the end of horse bot activity for the year and signals the time for treatment.

Horse bot flies look and act like bees, buzzing and droning as they hover around waiting for a chance to glue their tiny eggs or nits to body hairs of horses, donkeys, and mules. The erratic noise they produce during movement can be frightening. Animals can injury themselves as they run from the disturbance or attempt to relieve the irritation of newly hatched bot larvae burrowing into the skin. However, the real problems caused by these insects results from the months that the larvae spend as internal parasites in the digestive tract.

There are three species of horse bots. Their life cycles essentially vary only in where they attach their yellow to gray eggs to the host.

  • Common horse bot eggs most often are attached to hairs on the fore legs but can be found on the outside of the legs, the mane, and on the flanks.
  • Throat bot eggs are attached to the long hairs beneath the jaws.
  • Nose bot eggs are stuck to hairs on the upper and lower lips.

It is easy to see how horses can be spooked by flies buzzing around these areas and how they may injure themselves or people working or riding them at the time. Depending on the species, females deposit from a few hundred to 1,000 eggs during their life time.

Bot eggs hatch after a 2- to 5-day incubation period, often stimulated by warmth and moisture from the animal’s tongue. Newly hatched bot larvae enter or are taken into the mouth. They spend about 3 weeks in soft tissue of lips, gums, or tongue. The bots then migrate to the stomach or small intestine where they use sharp mouth hooks to attach to the lining of the organ. Bots can damage the lining of the stomach or small intestine, interfere with the passage of food, or cause other gastrointestinal disorders. They spend about 7 months there before passing out in the feces. The mature larvae enter the soil below the dung pile and pupate. In 2 weeks to 2 months, depending upon the season, they emerge as adults.

The adults do not have functional mouthparts, so they cannot feed. Females go to horses only to lay their eggs. Much of the egg-laying is done during August and September. A hard frost is needed to assure the end of fly season each year. While bot flies may or may not be noticed around horses, it is easy to look for nits or eggs on the animal’s coat. Virtually all horses in Kentucky are likely to be infested.


Most of the pest’s life cycle occurs in the horse. Consequently, an insecticide, applied internally, is necessary to provide effective control. Consult your veterinarian about an appropriate parasite control program.


by Lee Townsend, Extension Entomologist



Posted in Livestock Pests