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 cranial to the caudal 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 vertebrates and invertebrates; they will even lightly nip at the toes of humans 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. Their development takes 4–5 months, even longer in low temperatures, and larvae can hibernate.
Pathology of Horse Flies and Deer Flies
Adult horse flies and deer flies are evident 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 (1). Bites are painful and irritating, giving rise to weals.
These flies feed primarily on large animals, such as cattle and horses, which become restless when the flies are present. If animals are in harness, they become unmanageable in the presence of horse flies. Site preferences include the underside of the abdomen around the navel, the legs, and 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.
Horse flies and deer flies can act as mechanical transmitters of anthrax, anaplasmosis, tularemia, and the virus of equine infectious anemia. Oozing blood attracts other flies that become mechanical vectors of these infectious diseases as well.
Diagnosis of Horse Flies and Deer Flies
These flies can be identified by their large size, powerful wings, large compound eyes, and lacerating scissor-like mouthparts. Species identification of intact adult and larval horse and deer flies is best left to an entomologist.
Treatment and Control of Horse Flies and Deer Flies
Prevention of exposure
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 might not be exposed to these compounds long enough to be affected.
Horse fly traps have been effective when used around cattle confined to manageable areas. For production animals, 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, such as by removing unnecessary woody plants from residential areas and draining wet areas. Application of insecticides in the water can have detrimental environmental effects.
For More Information
Horse flies and deer flies. University of Kentucky Department of Entomology.
Also see pet owner content regarding flies and mosquitoes of dogs, cats, and horses.
References
Hollander AL, Wright RE. Impact of tabanids on cattle: blood meal size and preferred feeding sites. J Econ Entomol. 1980;73(3):431-433. doi:10.1093/jee/73.3.431
