The adult horse bot fly is a brown, hairy fly with only one pair of wings, measuring about 2 centimeters. In the summer months, we often see and hear horse bot flies—strongly resembling bees—buzzing around horses. Bot flies are sometimes mistaken for horseflies.
Female bot flies lay their yellow eggs on the horse’s forelegs and shoulders. Each fly lays about 1,000 eggs. The horse licks up these eggs. Larvae then hatch and enter the mouth, where they penetrate the mucous membranes. After several weeks, the larvae migrate to the stomach, where they attach themselves to the stomach wall. They remain there throughout the winter.
In late spring, the larvae detach from the stomach wall and leave the body via the manure. The larvae burrow into the soil and, after several molts, the adult fly emerges and the cycle starts again.
Bot fly larvae cannot be detected through manure testing, as bot flies lay their eggs on the outside of the horse. Therefore, we always recommend administering a preventive deworming treatment once a year, after the first frost. The first frost kills off free-living bot flies, eliminating the risk of reinfection after treatment.
As a Worm&Co customer, you will receive an email from us indicating the best time to administer the end-of-year treatment.
Horses carrying an infection can become very ill. Symptoms include inflammation of the mouth and tongue, stomach inflammation in the form of gastric ulcers, and reduced appetite, which can lead to weight loss and lethargy. Severe infections may also result in anemia, diarrhea, and, in the worst cases, colic.
It is important to prevent an infection, or at least minimize the risk. For this, the eggs need to be removed so that the horse cannot lick them up. Special blades and blocks are available for this purpose, for example from our partner Horsefitshop. If you are unable to remove the eggs, you can wash the horse with lukewarm water mixed with vinegar, which makes it easier to remove the eggs.
We recommend treating once a year, but there are also situations where you may choose not to treat preventively. This decision can be made, for example, for horses that have carried few or no eggs or for horses that have already been treated during a specific period. The most important thing is to use common sense and closely monitor your horse’s health.
Feel free to contact us if you have questions about bot fly larvae. We are happy to provide you with personalized advice and to think along with you!
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