Dealing With Bot Flies

— Written By and last updated by Patricia Burch
en Español

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This has been an interesting fall to say the least. With a very active hurricane season, horse flies hanging around longer than usual, and extra mosquitos that can carry and spread disease. The weather has not changed our visit of pesky bot flies though! Most of what I have seen started earlier this month. But, if you haven’t seen the bot fly buzzing around and hovering over your horse, you might have seen the yellow eggs they left on your horse. The bot fly, which resembles a honey bee, has non-functional mouthparts and does not bite the horse, but can cause significant internal damage to the digestive system. The eggs of the bot fly are what we are more concerned with, since they contain the bot larvae that can be dangerous to your horse’s digestive tract. Bot flies usually lay eggs on the horse’s legs, flank and belly area, but sometimes even in the mane, neck, chest, throat and other areas. There actually are three types of bot flies: the common horse bot fly (Gastrophilis intestinalis), the throat bot fly (G. nasalis) and the nose horse bot fly (G. haemorrhoidalis). The common and throat bot flies are found throughout the U.S., but the nose bot fly is more common in the northern and Midwestern states. The female bot fly can lay between 150-500 eggs in her 7-10 day life cycle. What’s most important is that the eggs are removed promptly, before the horse licks them and the larvae are allowed to enter the mouth and start causing problems.

Bot eggs require two things to hatch: friction and moisture. The horse provides both of these if they lick or scratch an area with their mouth. The small bot larvae will attach to the horse’s tongue, burrowing into the tissues of the mouth. Some of the bot larvae found closer to the head will even emerge and migrate on their own without the horse’s help. It takes about one to five days for the egg to incubate before hatching, so its best to remove the eggs as soon as you see new ones on your horse. After about three weeks, they will leave the mouth area and travel to the stomach and the upper portion of the small intestine. The cycle is complete when the fully grown larvae pass through the horse feces and burrow in the soil to pupate, then emerge as flies after a couple months.

Damages caused by bot larvae can be extensive. Horses may lose weight due to the inability to graze because of the pain in their mouth from the burrowing larvae in the tongue, gum, or lips. They may rub or bite at objects to relieve pain from the burrowing and injure themselves. In the stomach, the larvae can cause obstruction of the flow of food, colic, or even perforations of the stomach or small intestine wall. Ulcers, peritonitis, esophageal paralysis, and even rupture of the stomach can occur in very severe cases.

Controlling bots is not hard, but routine inspection for eggs and frequent removal is required to minimize their effects. Breaking the life cycle is the key. Sponging affected areas of the horse with warm water will cause the eggs to hatch, and including an insecticide with the water will kill any eggs exposed once hatched. If you’d rather not use them, a quick method of removal is to either use a bot knife or clip the area. Oral treatment and in most cases prevention of the horse from bot infestation is done through certain deworming products. Dichlorvos, ivermectin, trichlorfon, and moxidectin are all effective for bots. It is recommended to deworm both in the late summer and immediately after a killing frost for best results. After such an abnormal fall, we will see when that frost comes! Best wishes for controlling those bots in the meantime.

Recommendations for the use of chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by the North Carolina Cooperative Extension nor discrimination against similar products or services not mentioned. Individuals who use chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage and examine a current product label before applying any chemical. For assistance, contact an agent of the North Carolina Cooperative Extension in your county.

** Editor’s Note:  Eileen Coite is the County Extension Director for the N.C. Cooperative Extension, Sampson County Center.