Bot Flies Missouri Department of Conservation

May 23,2024

Bot flies are chunky, beelike flies usually with rounded heads. Adults are not commonly seen. The larvae are short, pudgy, segmented grubs that live as parasites in the tissues of animals. Those that live just under the skin often form a bulge (called a warble); typically, there’s a small hole in the center of the welt through which the larva’s breathing tubes extrude. The larvae of some types of bot flies live in the nasal cavities of deer; others inhabit the digestive tracts of horses, under the skin of cattle, and so on. One large group parasitizes rodents and rabbits. As with other true flies, bot flies have only a single pair of wings. Other tips for recognizing them as true flies (and not the bees they mimic) include tiny antennae, large eyes, and a pair of knobby structures (halteres) where the second pair of wings would be. Some common Missouri bot flies include: Deer bot flies, or deer nose botflies (Cephenemyia spp.), 5 species in North America. Adults are bumble bee mimics. Females flick newly hatched larvae into the nostrils of deer or elk. The larvae move through the sinuses into the throat and at the base of the tongue, where they burrow into the tissues and develop. When mature, they leave the host and pupate in soil. The species C. phobifer, whose name translates to “fearsome deer botfly,” is apparently the most common representative in our area. Rodent and lagomorph bot flies (Cuterebra spp.), about 25 species in North America. Oval flies about the size of bumble bees. Parasitize rodents and rabbits. Females deposit eggs in the tunnels or burrows of their required hosts. The eggs hatch when the warm body of a likely host brushes by, and the larva grabs on. They burrow head-first into the skin of the host, eating the flesh, causing a bulging, tumorlike “goose egg” on the surface of the host. Small, snorkel-like anal spiracles protrude from the hole in the skin, allowing the larva to breathe. They are certainly not good for their hosts, but they rarely cause death. Horse bot flies (Gasterophilus spp.) include 4 species native to Europe that now occur worldwide. Adults resemble honey bees. They mostly parasitize horses, mules, and donkeys. They usually deposit eggs onto the fur of the host, which then bites or licks the fur, ingesting the parasites. The different species develop in different parts of the host: the stomach, the rectum, the intestines, the throat, and so on. When mature, they usually exit the host along with the feces. Once on the ground, they exit the manure and burrow into the soil to pupate. These can be serious parasites of horses. Horse bot flies have been known to occasionally infect people, and those who work with horses in summer should avoid rubbing their eyes after combing or washing animals and be sure to wash their hands well after handling horses. The sheep nasal bot fly (Oestrus ovis) and the common cattle grub (Hypoderma lineatum) are bot flies that afflict the livestock species indicated by their names. Similar species: As adults, the various types of bot flies may be confused with several kinds of flying insects, including other groups of true flies as well as the various groups of bees they all mimic. Among the true flies that might be confused with bot flies are bee flies, flower flies, deer flies, tachinid flies, and robber flies. Then there are the many groups of true bees that these flies mimic: bumble bees, apid bees, andrenid bees, megachilid bees, longhorn bees, and more.