The Bottom Line on Bot Flies in Horses

Bot flies, botflies, or simply bots, are parasitic flies that use your horse as part of their life cycle, beginning with bot eggs. Even though bot flies spend part of their life inside your horse, they are not worms. The seasonal life cycle also influences the best time to look for evidence of bot flies in horses and when to treat for bots.

Table of Contents

 

yellow bot eggs on a horse knee

Those little yellowish dots are botfly eggs. They are super sticky!

 

Bot Flies – What Are They?

 

  • Parasites use another creature for nutrients, safety, growth, or multiplication and harm the host directly or indirectly. Compare this to mutualists, which are two creatures that benefit from the relationship. This situation is also known as symbiosis or a symbiotic relationship.

Parasites vs. mutualist

 

  • In horses, parasites could be tapeworms, strongyles, or pinworms. These internal worms steal nutrients from your horse, often creating physical blockages in critical parts of your horse’s digestive system. External parasites of horses include ticks, the ringworm fungus, flies, and mites.

 

  • Mutualist creatures that benefit horses are the microbes in the hindgut that digest fiber, or birds that feed on parasitic insects of horses. Equestrians who care for horses are another great example. Both horses and humans can benefit from their relationship.

 

The bot fly verdict

 

  • When it comes to bot flies in horses, they are parasitic. They are part of the Oestridae family of flying insects, and botflies are a significant nuisance to horses. They resemble bees and often appear as furry-bodied, winged yellow, gray, or sometimes brown bugs.

 

bot knife on a table

The bot knife is a handy tool to have in your grooming kit. 

 

The Different Types of Botflies

 

  • Three types of bot flies can invade your horse, each at different times of the year. These various species are distinguished by where they lay their eggs, among other things.

 

The common horse bot fly

 

  • These pests, officially called gasterophilus intestinalis, deposit yellow eggs on your horse’s forelegs, chest, shoulders, and undercarriage, like the belly and flanks. This happens at the end of summer or early fall.

 

  • The common botfly eggs can incubate for 7-14 days, and your horse needs to stimulate them during grooming to continue their life cycle.

 

The throat bot fly

 

  • Gasterophilius nasalis is the scientific name of the throat botfly. As their name suggests, these eggs land near your horse’s jaw and throat latch. Their trip on your horse begins in late spring and may continue into early summer.

 

  • Unlike common bots, the eggs do not need any interference from your horse to hatch. They also incubate for 4-6 days.

 

The nose bot fly

 

  • The rarest of the flies creating a botfly infection is the Gasterophilus haemorrhiodalis. These parasites like to place their eggs on the nose hairs of a horse. Unlike the yellow eggs of other types, these eggs are black. They need moisture from your horse, or their drinking and eating, to hatch. This speedy process takes 2-4 days.

 

  • Another trait of the nose bot is spending some time attached to your horse’s rectum before finally dropping to the ground.

 

The Life Cycle of Botflies in Horses

 

  • There are four stages in the life cycle of a bot fly. From an egg, the fly transforms into a larvae, then a pupae, and finally an adult. And the cycle begins again!

 

The egg stage

 

  • The adult female botflies make it their mission to lay about 500 eggs on the end of the horse hair. She will aim for the legs, neck, and mouth area and glue her eggs to your horse, mule, donkey, or zebra. For some reason, she doesn’t like cows, which is good to know if you have them.

 

  • Now, your horse starts to help this reproductive cycle. Those common botfly eggs are stimulated as your horse rubs their legs with their nose.

 

  • Your horse’s pasture mates may do the same while mutually grooming.

 

  • Bot fly eggs are sticky and itchy, which makes your horse want to groom that area. Most eggs are yellow, but the color of the egg can range from white to orange. The eggs intentionally deposited by the nose bot fly are black.

 

  • Eggs from adult flies are hard to see on gray horses.

 

The larval stage

 

  • After the eggs are disrupted, or not, as is the case with the throat botfly, they hatch into larvae.

 

  • As your horse is grooming or chewing those itchy areas, the larvae end up in your horse’s mouth.

 

  • In the mouth, they will burrow into the tissue of the tongue and gums with specialized hooks.

 

  • They live in the mouth for a while, then the common bot larvae migrate to the top of the stomach near the esophagus and attach themselves there. The nose bot fly larvae attach where the stomach meets the small intestine

 

  • Then, they rest for 8 months to a year. Just chilling out.

 

Larvae do not lay eggs, so having fecal exams won’t tell you if your horse has bots in their stomach.

horse manure pile in a paddock with fencing and a horse in the background

Horse manure is chock full of parasite parts!

 

 

The pupal stage

 

  • After marinating for up to a year, the larvae are released from your horse’s digestive system and pass with manure.

 

  • Once on the ground, they become pupae and dig themselves into the ground for a few weeks, where they transform into flies.

 

The adult stage

 

  • Once a botfly emerges from the earth, it only lives for a few days. The females cannot feed and instead focus on reproducing and finding a horse host.

 

 

Signs of Bots in Horses

 

  • It’s almost impossible to see clinical signs of botfly larval infestation in your horse. Horses may have significant larvae in their bodies without any outward signs.

 

  • The most obvious thing you may see are eggs glued to your horse. But, depending on location, your horse’s coat color, and the type of parasite, you may not see these eggs.

 

Signs of parasitic infections in horses

 

  • It’s more likely that you will notice something off with your horse, and it could be any number of parasites, including bots, or it could be something else entirely. Only in rare cases will you have more obvious signs like a stomach rupture.

 

  • Anytime your horse shows unusual or new behaviors, contact your vet to narrow down possible causes. The following symptoms of bot flies are also indicators of SO MANY OTHER THINGS.

 

You may notice:

 

    • Dullness to the coat
    • Ulcers or wounds around the tongue or gums
    • Reluctance to take the bit
    • Weight loss
    • Colic

 

two hands holding a palmful of composed horse manure

Compost is a great way to take care of all of that POOP.

 

 

Treatments for Bot Flies in Horses

 

  • While fecal egg counts and targeted deworming are the protocol for intestinal worms and have replaced a regular deworming program based on different products throughout the year, it’s not as easy to treat fly bots for horses as you can’t confirm their presence in your horse.

Dewormers

 

  • Equine dewormers are the best way to rid your horse of botfly larvae.

 

  • Look for avermectin dewormers, which are named ivermectin or moxidectin. The treatment process with an ivermectin product may need repeating.

 

Weather is on your side (sometimes)

 

  • Frost will kill any stages of bot that are not in your horse, so this is the time to deworm.

 

  • Waiting until after a frost will treat existing larvae in your horse, and the frost will prevent any adults from living and planting eggs on your horse.

 

  • This timing assures your springtime manure pile won’t be full of bot pupae. If you don’t have weather on your side, talk with your veterinarian about a bot plan of attack for the most effective treatment schedule.

 

Bot Fly Infestation Prevention

 

  • Preventing equine parasites may be the ultimate game of whack-a-mole, but small things will add up.

 

Pasture management

 

  • Scrape or pick your horse’s manure from pastures and paddocks. Reducing the amount of manure and heating it in compost removes many parasite eggs, larvae, and pupae, including the larvae of botflies.

 

  • Rotate pastures to allow the grass to rest and help with manure management. Many of the worst parasites can’t survive good pasture management.

 

pumice stone grooming block

Grooming blocks are pumice stones that make for great bot fly egg removal. 

 

Grooming practices

 

  • Daily grooming is horse health care! Daily grooming and egg removal are essential, as some eggs can hatch in a few days.

 

  • The easiest ways to remove eggs are with a grooming block, bot knife, or bot comb to remove eggs.

 

  • If your horse has extra fuzzy hair, consider clipping them in warmer weather to make eggs easier to see and remove.

 

  • Extra sticky eggs often benefit from marinating in grooming oil or water to help them loosen.

 

  • Cover your horse with fly sheets and fly sprays. While there are no traps for botflies, you can provide physical barriers.

 

  • Practice good fly control. The female botfly is a crafty bug, and overall, good sanitation and fly control methods can reduce botflies in horses.

 

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Farnam Slick 'N Easy Horse Grooming Block
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Use as directed by your vet

02/08/2025 09:13 pm GMT
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