THE MERCK VETERINARY MANUAL
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Horse Flies and Deer Flies

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Tabanus spp (horse flies) and Chrysops spp (deer flies) are large (up to 3.5 cm long), heavy bodied, robust dipterans with powerful wings and very large eyes. They are swift fliers. These flies are the largest in the dipteran group in which only the females feed on vertebrate blood. Horse flies are larger than deer flies; many horse flies are highly colored. Deer flies are medium sized; they have a dark band passing from the anterior to the posterior margin of the wings and a yellow to brown abdomen with black patches and longitudinal bands.

Adult horse flies and deer flies lay eggs in the vicinity of open water. Larval stages are found in aquatic to semiaquatic environments, often buried deep in mud at the bottom of lakes and ponds. The developing larval stages have sharp mandibles and are predatory on smaller vetebrates and invertebrates; they will even lightly nip at the toes of people wading in shallow water. This is an unusual dipteran fly in that both larval stages and adult flies are capable of inflicting pain as they bite.

Adults are seen in summer, particularly in sunlight. Adult females of both species feed in the vicinity of open water and have reciprocating, scissor-like mouthparts, which they use to lacerate tissues and lap up the oozing blood. They consume 0.1–0.3 mL of blood at a single feeding. Bites are painful and irritating. These flies feed primarily on large animals, such as cattle and horses, which become restless when the flies are present. Site preferences include the underside of the abdomen around the navel, the legs, or the neck and withers. Horse flies and deer flies feed a number of times in multiple feeding sites before they become replete. When disturbed by the animal's swatting tail or by the panniculus reflex, the flies leave the host, yet blood continues to ooze from the open wound. These flies may act as mechanical transmitters of anthrax, anaplasmosis, tularemia, and the virus of equine infectious anemia.

These flies can be identified by their large size, powerful wings, compound eyes, and lacerating scissor-like mouthparts. Species identification of intact adult and larval horse and deer flies is probably best left to an entomologist.

Horse flies and deer flies are the most difficult to control of all of the blood-sucking flies. Many of the adulticide compounds used for other biting flies can kill both horse and deer flies. However, because these flies are intermittent feeders that alight on the host for only a short time, they may not be exposed to these compounds long enough to be affected. Thus, larger doses of the compounds may be required.

Horse fly traps have been effective when used around cattle confined to manageable areas. For livestock, pyrethroid pour-ons function as limited repellents. Self-application techniques are usually not effective for horse and deer flies.

Manipulation of these flies' aquatic habitat has been attempted by removing unnecessary woody plants from residential areas or draining wet areas. Application of insecticides in the water may have detrimental environmental effects.

Last full review/revision August 2013 by Charles M. Hendrix, DVM, PhD

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