Desirai Schild


Fall brings cooler days, turning leaves and the arrival of bot flies.

Beyond being unsightly and annoying, these tiny hitchhikers can pose a real health threat to your horse.

Bots look quite a bit like honeybees but instead of hanging out in the flower bed, these parasites hover around horse’s legs, flanks, shoulders and stomach to lay eggs. The eggs appear in those areas as tiny yellowish dots attached to individual hairs.

The location of bot egg distribution is important because those are locations horses can reach to nip or mouth.

The eggs develop into the first larval stage within five days. They hatch into a maggot with seven to 10 days. The larvae are stimulated by the horse biting or licking the fully developed eggs.

“The larvae get into the horse’s mouth and bury themselves in the tongue, gums or lining of the mouth,” said Dr. Jason Moulton, of the Animal Health Clinic in Blackfoot. “They will be there for about 28 days.”

The burrowing can result in severe irritation, pus pockets and even loose teeth. Loss of appetite also is possible.

When the larvae molt into the second stage they move into the stomach, attach themselves and reside there and in the intestinal tract for nine to 12 months. This can cause blockages and colic. They damage the digestive tract lining and take part of the nutrients the horse consumes.

Once this developmental state is completed, the future bot fly detaches from the horse’s insides and is passed out in manure. This tends to occur in late winter and early spring. The insect gestates in manure for three to 10 weeks and emerges looking for a mate when the weather gets warm.

Heavy infestations can be discouraged by applying insect repellant to the horse’s legs and other vulnerable areas when the bot flies start to appear. The wormers in the Ivermectin family are very effective in both adult and larval stages. The horse should be treated within one month of eggs first being deposited. The second treatment is due in the fall to control the second and third larval stages.

I know some people who administer insecticide through pellets put daily in the horse’s grain.

Dr. Rex Gillespie, of the Cottonwood Veterinary Clinic in Blackfoot, said that intensity might not be necessary in southern Idaho because we get such cold winters. He advises developing and sticking to a regular rotational worming schedule. Rotating types of wormer is important to prevent resistance developing in the worms.

Valley Vet Supply recommends Pyrantal in January and February, Fenbendazole/Oxibendazole in March and April, Ivermectin in May and June, Pyrantal in July and August, Fenbendazole-oxibendazole and Ivermectin in November and December.

I follow that rotation but only do four treatments starting in April and ending in November.

It’s also important to remove the bot eggs before they are ingested. There are specially designed bot knives for this chore. A sharp knife also is effective. Personally, being lazy, I use my cordless clippers and remove hair and all.

Whatever methods you use, do something to keep these nasty critters from hurting your horse.

Desirai Schild has been involved in raising, breeding and showing gaited horses in eastern Idaho for more than 20 years. She may be reached at

Desirai Schild has been involved in raising, breeding and showing gaited horses in eastern Idaho for more than 20 years. She may be reached at