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Where there are horses,
there are bots

Horse bot flies are covered with black and yellow hairs, bee-like in appearance and about the size of a honey bee. They are found throughout the United States, wherever horses are. Horses, including donkeys and mules, are the normal hosts, but dogs, rabbits, crows and (rarely) people have become infected.

Bot flies don’t bite or feed on horses, but they do lay their eggs on them. The larvae make their way into the horse’s stomach. That is where the most serious damage in done by the larvae. A few bots will cause little damage; however large numbers of bots can cause a horse to become run down, unthrifty and injure itself as it attempts to relieve the irritation from burrowing activities of newly hatched bots. Horses have also caused injury to themselves or people working or riding them when the bot flies are buzzing around them.

Life Cycle

There are three species of bots: horse bot, nose bot and throat bot. Their life cycles are very similar, except in where they attach their yellow to black eggs to the host. Common horse bot eggs most often are attached to hairs on the forelegs but can be found on the outside of the legs, mane, belly, shoulders and flanks – basically anywhere the horse can easily reach with its tongue. 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. Depending on the species, females deposit from a few hundred to 1,000 eggs during their life time. 

Eggs of the common horse bot hatch after a 2 to 10 day incubation period, often stimulated by warmth and moisture from the animal’s tongue. Eggs of the other species may hatch without stimulation. Newly hatched bot larvae enter or are taken into the mouth. They spend about 3 to 4 weeks in soft tissue of the lips, gums, or tongue before they molt. 

The bots then migrate to the stomach or small intestine where they use sharp mouth hooks to attach to the lining of the organ. They spend about 7 months there until they molt again and are fully matured. They will then detach and pass out in the feces. 

The mature larvae enter the soil below the manure pile and pupate. In 2 weeks to 2 months, depending upon the season, they emerge as adults in the late summer or fall. The life cycle takes one year.

The adults do not have functional mouthparts so they cannot feed. Females go to horses only to lay their eggs. Most of the egg-laying is done during August and September but may continue until the first hard frost.


Infestations can produce symptoms varying from mild to severe, such as: irritation of stomach membranes; ulceration of stomach; peritonitis; perforated ulcers; colic; mechanical blockage of stomach resulting in stomach rupture; esophageal paralysis; and squamal cell tumors. The larvae developing in the stomach have also been shown to cause severe anemia. In addition, the first stage larvae migrating in the tongue and gums have been shown to cause gum and tongue inflammation, pus pockets and abscesses in the mouth.


While bot flies may or may not be noticed around horses, it is easy to look for eggs on the animal’s coat. Clipping hairs that harbor eggs is not a practical solution for these pests. Sponging areas of the forelegs where eggs are attached with warm water (110o to 112oF) may stimulate some eggs to hatch and the small larvae can then be washed off or if it is a cool day, they will die of exposure. This is of limited value and would have to be repeated frequently because new eggs are attached daily while the flies are most active.

There are some insecticide formulas containing pyrethroid insecticides that are labeled to wipe on horses to kill bots flies. They will need to be applied throughout the egg laying season as recommended by the product.

Since most of the pest life cycle occurs in the horse, effective control requires breaking the life cycle of the fly. Consequently, an insecticide, applied internally, is necessary to provide this control. During the winter after the first hard freeze (unless symptoms of bot infestation are apparent earlier) is the recommended time to use an oral larvacide to kill the larvae in the stomach. Check product labels carefully; all equine deworming drugs do not necessarily control horse bots. Before purchasing any product, read the pest list on the label and note any precautions regarding product use. 

Insecticides with organophosphate or cholinesterase inhibiting products, such as those containing dichlorvos (Vapona), coumaphos (Co-Ral), or tetrachlorvinphos (Rabon) should not be applied to horses at the same time, or within several days of treatment. The product label will give specific restrictions. Ivermectrin, the active ingredient in Zimectrin Gold controls bots and other internal parasites and is not a cholinesterase inhibitor. No supplementary bot control material is needed when using products that have ivomectrin as the active ingredient.

Bots and Humans

On rare occasions, bots can invade the oral tissue of humans. By grooming or petting an area where bot eggs are attached can result in larvae on the hands and subsequent transfer to the person’s mouth, nose or even eye. The small larvae may burrow beneath the skin and cause an uncomfortable sensation. Since they have difficulty developing in people, it is very rare that the larvae is found in the stomach. 



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Date Last Updated January, 2006