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It can take up to five days for the bot egg to incubate into a maggot. Within 10 days of being laid, the egg will either hatch spontaneously or will be stimulated into hatching by the horse's warm, moist tongue during self-grooming. Once the bot fly maggot or first instar larvae gains entry to the horse's mouth, it attaches to the lining of the mouth, tongue or gums. This burrowing action can irritate the horse's mouth and cause pockets of pus or loosen the teeth. Some horses may experience a loss of appetite. Others may be seen rubbing their muzzles in response to the irritation. The first instar larvae will remain embedded in the mouth for approximately a month.

Stable management techniques aimed at parasite reduction will also help reduce bot fly populations. Begin by removing and composting late winter and early spring manure, including horse droppings in the pastures and arena. Since the bot pupates in the soil at this time of year, spreading manure to expose it to the elements may not be sufficient to kill immature bot flies. The heat created by composting helps to kill the larvae before they reach the soil and will eliminate those bots pupating in the compost pile.

Since a bot fly female can travel to find host horses, properly managing manure may not be effective for controlling adult bot flies, especially if neighboring farms aren't quite so diligent. In this case, removing the bot fly eggs that are attached to the horse's coat may be the best method for preventing the bot larvae from gaining entrance to the horse. Bot fly blocks and bot egg knives are inexpensive tools for bot egg removal and are readily available at most tack shops and online tack retailers. Some equestrians simply use sandpaper or a knife to remove the eggs, but care must be taken with the latter so as not to cut the horse. These tools are used to scrape or cut the bot egg off the hair shaft. The eggs should be gathered and properly disposed of as these eggs are still viable. Naturally, it's better to remove the eggs in areas where horses won't be eating. With this method, it's prudent to check the horse's coat daily for new deposits of eggs.

An alternate method is to wash the bot fly eggs with warm water (110ºF to 112ºF) to encourage hatching of the larvae. The bot larvae will need to be rinsed away or the area sprayed with an insecticide in order to kill the larvae. It should be noted that using this method does not remove the cemented-on egg casings and freshly laid eggs may not be developed sufficiently to hatch.

Although it's rare, bot larvae can also penetrate the skin, eyes or the oral tissue of humans. The larvae will die within a few days, but can cause irritation and itching. Precautions, like wearing gloves or hand-washing, should be taken when removing bot fly eggs and grooming or working with horses contaminated with bot eggs. 



Attack of the Bot Fly


The information contained within this article does not constitute medical advice. Please consult your horse's veterinarian for further information on equine internal parasites.

In late winter or early spring, the larvae migrate to the rectum and either pass out with the feces or reattach to the rectum wall before passing out of the horse's body. Horses infected with bots at this stage may become restless. Once the larvae have passed out of the body, they pupate in the manure or burrow into the surrounding soil. Here they will remain for approximately one to two months before emerging as adults. 

The female bot fly adheres her small yellow eggs or nits to the hair shafts of the horse. 

After molting into the second instar larvae, the bots will migrate to the horse's stomach. Here they will firmly attach themselves to the stomach lining using their abrasive mandibles and sharp hooked mouth parts. While overwintering in the stomach or intestines, they will continue to rob the horse of nutrients and molt into the third instar larval stage. In addition to damaging the intestinal tract of the horse, large numbers of bots can create blockages that can induce colic. A heavy infestation of bot larvae can also lead to anemia, gastritis, ulcers, peritonitis, and in the worst cases, rupture of the stomach.

Like a World War II fighter pilot, the Bot fly is the only internal parasite of the horse that attacks from the air. Perhaps you've seen these honeybee lookalike insects as they dive bombed your horse in an attempt to adhere their eggs to the hair shafts of your horse's coat. Perhaps you've found their tiny yellow or grayish-black eggs stuck to various areas of your horse's body. While these little eggs seem harmless enough, the larvae of the bot fly causes intense damage to the intestinal tract of your horse and robs him of vital nutrients. In addition, some horses become so frightened or annoyed by the female bot flies' behavior that they injure or harm themselves. If that's not disturbing enough, the larvae of bot flies can also enter human tissue in our eyes, mouth or skin. Luckily, horse owners can counterstrike the attack of bot flies by understanding the bot's life cycle and they can protect their horses by developing an effective deworming schedule.

As adults, bot flies neither feed nor bite. They begin the adult stage of their life cycle when they emerge from the soil in summer or early fall. During their short seven to ten day adulthood, the female bot fly's sole purpose is mating and reproduction. Once she finds a suitable male, she will then travel up to several miles to find appropriate equine hosts. The female can disperse several hundred eggs per day and will often choose several horses in an effort to increase her odds of successful propagation.

The area of the horse's body where the female lays her eggs depends upon the species of bot fly. The common bot or Gasterophilus intestinalis prefers to deposit her eggs on the shoulder, flank, front legs or mane of the horse, while the nose bot (Gasterophilus nasalis) and throat bot (Gasterophilus haemorrhoidalis) specifically target the lips or under the jaw respectively. 


Since the bot fly only produces one generation per year, one or two well-timed dewormings can help control this internal parasite in horses. Horse wormers containing Ivermectin are the boticides of choice for killing all three stages of the bot larvae. The ideal time to administer boticides is dependent upon geological location. In warmer climates, horses can be wormed for bots approximately one month after the adult flies begin laying eggs, then again in the fall. In northern areas of the United States, it's common practice to use a boticide thirty days after a killing frost has eliminated the adult bots and again in the spring before the third instar bot larvae pass out of the horse's body. Horse owners are urged to consult with their local veterinarians for help in setting up an effective deworming schedule for their horses. Whenever any anthelmintic or boticide is administered, strict adherence to the manufacturer's directions and precautions is essential for maximum effectiveness and safety.

Bot flies are distributed throughout North America. Dispersal is enhanced by transporting infected horses during the late larval stages. Subsequently, horses can pick up nits or bot fly eggs when they are at showgrounds, on bridle trails or even at the trainer's stable. Although fly masks and sheets can deter the female bot from certain areas of the body, she will inevitably choose the more desirable location on the forelegs for depositing her eggs. Additionally, the female bot neither bites nor lands on the horse making insecticides ineffective in stopping her. While she may seem to be an unstoppable foe, we can diminish the damage caused by her offspring with a few simple steps!