Adult Bot Flies are flying insects that are found very commonly around horses. The “bot” is the immature form (larvae) of the adult fly.

In the UK, there are three main species that can affect horses, each relying on the horse to complete the lifecycle. The principal areas where they like to lay their sticky eggs are on the hairs of the legs, neck, and face.

Bots are considered a pest as they are very irritating for the horse. In extreme cases, the bots can even lead to damage to the intestinal tract. So what do we do about them?

What do they look like?

The adult bot fly is like a bee in appearance. They have a brown body with darker stripes, around 1-2cm long. They are also hairy and have a single pair of wings.

Female bot flies have no mouth, so they live off stored reserves long enough to lay their eggs around the horse. The adult female lifespan is seven to ten days.

The bot larvae are around 2cm long with a narrow, hooked end with a round body. These are noticeable as they are a reddish orange in colour. They can be described as “maggot-like” when seen.

The egg’s appearance can vary depending on the species, but eggs will mostly look small, like yellow drops of paint on the hair of the horse (typically only 1.25mm long). Egg laying occurs early in the summer, with each female laying up to one thousand eggs.

How can I spot a bot infestation?

An infestation of bot fly larvae will not show in a faecal worm egg count, so it can be difficult to know if a horse has an infestation, as the eggs are laid externally not internally.

As the lifecycle means the larval and pupal stages are passed out in the horses’ faeces, they are often seen in the spring and summer. Often the best way to detect an infestation is when you are grooming, as eggs can be seen by the naked eye.

What is the lifecycle?

The bot goes through four main stages:

Eggs

The female can lay up to one thousand eggs in her lifetime and can last up to 14 days on the horse.

Larvae

The eggs can hatch within 7 to 10 days of being laid. The larvae are stimulated to emerge by the horse trying to lick or bite the eggs. The larvae either crawl to the mouth or become ingested during grooming. The exceedingly small larvae bury themselves in the tongue, gum or lining of the mouth where they will remain for around 28 days. After 28 days they will enter further larvae stages and travel to the stomach, where they will attach themselves to the lining of the stomach. They can remain here for nine to twelve months, feeding off the stomach contents. They are eventually passed out in the faeces.

Pupae

Once the larvae have been passed out, they will burrow into the soil or dry manure/pasture where they will undergo pupation. This stage of the lifecycle lasts 3 to 5 weeks. It will occur between late winter and early spring.

Adult 

The adult bot fly emerges from the pupal stage after 3 to 10 days during the mid to late summer. After the fly emerges, it quickly needs to find a mate to repeat the lifecycle.

How to treat for botfly

An insecticide can be applied during the fly season to control the adults. In the UK, a permethrin-based spray solution can be applied monthly for fly control, this can be shortened if an infestation is severe. The most widely used formulation contains cypermethrin, a broad-spectrum insecticide. It is a POM-VPS medicine which is available from your SQP and veterinary surgeon.

It is also wise to remove the eggs from the coat before they hatch or are ingested – “bot knives” have a serrated blade and are designed specifically for combing the hair and cutting away the eggs, which can then be discarded.

Once ingested, bot larvae cannot be treated until they reach the stomach. The use of a wormer is recommended at the start of winter as this will kill bot flies and ensure no reinfestation. You could use a moxidectin- or ivermectin-containing wormer as this will also treat the encysted redworms.

If you are concerned about bots, please talk to your vet or SQP for personal;ised advice for your horse!

Further reading:

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