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Bot Fly

Bot Fly
Bot Fly Adult
Horse stomach with Bots
Horse Stomach with Bots


Two species of bot flies are serious pests of horses in the Northeast. During summer and fall, the flies lay eggs on the hairs of horses. The common bot fly, Gastrophilus intestinalis, hovers over the animal, darts from side to side, and cements eggs to hairs preferably on the forelegs, but also on the mane, shoulders, belly, neck, and flanks. The throat bot fly, Gastrophilus nasalis, darts at the animal and deposits eggs on the hairs under the jaw. Flies appear about the middle of June and are present until the first killing fall frost. Eggs usually can be found on horses as soon as flies are seen, and egg laying continues as long as adults are present.

Even though bot flies cannot bite, horses fear them. Fright and irritation caused by egg-laying adults or newly hatched bots may cause animals to go out of control and inflict damage to themselves or anyone trying to handle them. On warm, sunny days, animals on pasture fight the flies from morning until late afternoon. While horses do this, they are unable to graze; as a result, they may lose weight and suffer from lack of proper nourishment.

To hatch, common bot fly eggs require moisture and friction from the horse's licking. Under these optimum conditions, the eggs hatch in 2 to 7 days, and the larvae burrow into the horse's lips, gums, or tongue. This causes horses to dip their lips into the water of watering troughs, then rub their lips and noses violently on the side of the tank, the ground, fences, or other nearby objects. After remaining in the mouth for 3 to 6 weeks, the larvae pass to the stomach and intestine, where they remain attached with no change in position until the following summer. These attachments cause inflammation, which interferes with digestion and the passage of food. Several hundred larvae may be found in one animal, and the degree of damage done by their feeding is roughly proportional to the numbers present. The mature larvae then release their hold and pass out with the feces. When they reach the soil, they pupate and remain there for 15 to 70 days. Adult flies start emerging in early summer. The total life cycle for any of the bots is 10 to 11 months.


Significant control can be accomplished during the fly season by clipping the hairs to remove the eggs. Also, approximately 2 weeks after the first frost has killed all of the adults, apply warm water rinses (120°F) to the animal's coat to induce hatching of the eggs and the subsequent death of the young larvae.