Protecting Horses From Horse and Deer Fly Bites

Horse and deer flies are large biting flies that are annoying pests of horses cattle humans and wildlife. Horse and deer flies have many regional names including yellow flies greenhead flies gad flies stouts dun flies moose flies whame flies buffalo flies and elephant flies. Adult female flies feed on blood and can transmit pathogens including those that cause equine infectious anemia Foil et al. 1983 and tularemia Klock et al. 1973 Petersen et al. 2009.
Figure 1 Horse flies A B and deer flies C D are stoutbodied often brightly colored biting flies. Photo credits A Kondor83 Shutterstock Standard License B Brett Hondow Shutterstock Standard License C Christina Butler Flickr CCBY2 0 D HHelene Shutterstock Standard License

Biology
Horse flies are large flies that generally belong to the genera Tabanus or Hybomitra whereas deer flies are smaller flies that mostly belong to the genus Chrysops Figure 1. In some areas of the country these flies are collectively called Tabanids. Both groups are stoutbodied flies with varying patterns and large eyes that often are brightly colored. Their large eyes allow them to distinguish movement and shapes that contrast against the background. Darkcolored animals provide the flies with a clearercontrasting visual target and can attract more flies than lightcolored horses in pastures.
Horse flies and deer flies have specialized mouthparts that cut the skin to produce a pool of blood. They are strong fliers and aggressively try to bite. Females will lay eggs on vegetation adjacent to wet habitats like marshes streams or ponds. Larvae develop in the mud in these areas.
Veterinary Concerns
Horse and deer flies are a nuisance that can upset horses and horse caretakers although bites may cause raised welts and blood spots Figure 2. The loud flying sounds and bites can cause horses to mount defensive behaviors like biting at their sides

stomping kicking swishing their tails bucking and running. Evasive behaviors like fence walking or running can increase when horses are pressured with flies. These behaviors can compromise your horses health when done excessively. For example excessive stomping can lead to hoof damage and potential lameness.
Horse and deer flies can also transmit the virus that causes equine infectious anemia EIA Foil et al. 1983. When flies feed on horses infected with EIA the virus can travel on the mouthparts of the fly to an uninfected horse transmitting the virus. Tularemia has also been reported to be transmitted by horse and deer flies Klock et al. 1973 Petersen et al. 2009.
Control
Control of horse and deer flies can be challenging. Horse and deer flies develop in water and are strong fliers and habitat modification in these environmentally sensitive areas is difficult. Therefore control is usually focused on intercepting adult flight or protective measures for horses against adult bites.
Turnout and Pasture Management
Horse and deer flies are active around wooded areas especially if there is some form of water nearby. These flies are active in the sun and avoid areas of shade. Runin shelters or large canopy trees located away from wooded edges can give horses refuge from the flies Raymond and Rousseau 1987 Figure 3. If pasture space can be increased this will allow horses to naturally move away from bitingfly pressure. Some species of horse and deer flies are territorial. Their range may include only one pasture or portion of a pasture and sometimes bites can be avoided by turning horses out in another pasture away from horse and deer fly activity. Turning horses out at night instead of the day can reduce biting because horse and deer flies are daytime biters.
Trapping
Horse and deer flies use their vision to locate their hosts. Current