Common Name: Horse bot fly
Scientific Name: Gasterophilus intestinalis (DeGeer)
Description: Adult flies are brownish, hairy, robust and about 2/3-inch long, superficially resembling honey bees except for having only one pair of wings. Wings of the horse bot fly have faint smokey spots on the wings. Fully-grown larvae (maggots) are ½ to 2/3 inch long and have yellow-white to pinkish thick, tough skin. They are blunt at one (the back) end, and taper to the other (front) end which bears a pair of strong, hook-like mouthparts. Each body segment is ringed with strong spines.
Several other species of bot flies occur on horses (chin fly or throat bot fly, Gasterophilus nasalis (Linnaeus); and, lip or nose bot fly, G. haemorrhoidalis (Linnaeus)). They can be identified, in part, by the shape of the eggs.
Life Cycle: Larvae develop in the digestive tracts of host animals during the winter. In the late winter and early spring months, full grown larvae are found in host’s feces. From there, they burrow into the soil and form a puparium from their last stage (instar) larval skin. They transform into adult flies inside the puparium and emerge in 3 to 10 weeks. Adults are active from mid-summer through fall. Adult females glue eggs on the hairs of horses, particularly to hair on the front legs but also on the belly, shoulders and hind legs. Eggs hatch in 10 to 140 days with the proper stimulus (moisture, heat and friction) caused by the horse licking or biting egg-infested hair. Tiny first stage (instar) larvae enter the mouth and burrow into the tongue for about 28 days before they molt and travel to the stomach where they remain for 9 to 10 months, developing into the third stage (instar) after about 5 weeks. There is one generation per year.
Habitat and Food Source(s): Maggots have mouth hooks that tease tissue apart in the digestive tracts; adults do not have functional mouthparts. Horses, mules, donkeys are primary hosts. Adult female flies, attempting to lay eggs on host animals, cause horses to flee and resist fly “attacks” (hovering, buzzing and striking), occasionally resulting in injury. Larvae live in the digestive tract, injuring the tongue, lips, stomach lining and intestine. They apparently feed on the inflammatory products produced by the host in response to their presence. Infestations cause mechanical injuries and an infected ulcerous condition that progressively starves the host animal.
Pest Status: Adults cannot bite or sting and are harmless to man and animals although horses react evasively to egg-laying attempts by female flies; larval or maggot stages feed internally, in the digestive tracts of horses.
Literature: Metcalf et al. 1962.