Horse flies are strong fliers that can move long distances from their wetland breeding sites in search of blood meals. Females slash the skin with their broad, blade-like mouthparts, and then lap up the blood that wells up from the wound. Severe animal reactions to the painful bites interrupt the flies’ feeding so several attempts are usually necessary for the insects to get a complete meal. Continued attack can make animals miserable and hard to handle; some may be injured as they try to escape their tormenters. Reactions to salivary proteins injected while horse flies feed can produce swelling, discomfort, and may become sites for secondary infection. In addition, horse flies can carry anaplasmosis from animal to animal on their blood-contaminated mouthparts.
Eggs of horse flies are laid in batches of up 1,000 on vegetation in wet areas or along stream or pond banks. The larvae are aquatic or semiaquatic and may take 2 to 3 years to reach the adult stage. These wet areas cannot be treated with insecticides so source reduction is not an option. Efforts to reduce bites must target these day-biting flies.
Horse flies will not enter structures so giving horses access to sheltering structures can reduce attacks. The flies are attracted to contrasting silhouettes so commercially available or home-made fly traps may capture some of them.
Repellents or insecticidal sprays may provide some temporary relief or protection but they tend to be short-lasting, so repeated applications are needed. Treated animals still attract flies so the attack pressure remains constant. Horse flies will resume feeding as soon as the amount of insecticide on the animal reaches a tolerable level.
by Lee Townsend, Extension Entomologist