Horse Flies in Arizona — The Painful Biting Fly That Targets Livestock and Humans

Horse flies are among the most painful biting insects in Arizona — females use blade-like mouthparts to slash skin and lap up blood. Here's how to identify them and protect yourself.

Horse flies (family Tabanidae, genus Tabanus) are among the most aggressive biting flies in Arizona. Unlike mosquitoes, which use a needle-like proboscis to pierce skin, horse flies have blade-like mouthparts that slash through skin to create a wound from which they lap blood. The bite is immediately painful and can cause significant local swelling, itching, and in sensitive individuals, a systemic allergic reaction. Only females bite — they require a blood meal to develop eggs. Males feed on nectar and pollen. Arizona's most common species include the black horse fly (Tabanus atratus), one of the largest flies in North America at up to 28 mm, and several smaller striped species. Horse flies are most active from late spring through early fall, with peak activity during the hottest parts of the day. They are strong fliers capable of traveling several miles from their larval habitat and are attracted to large, dark-colored moving objects — which is why they persistently pursue people, horses, and livestock.

Palo Verde Root Borer Quick Facts

Horse fly larvae are aquatic or semi-aquatic predators that develop in the muddy margins of ponds, streams, irrigation ditches, and flooded fields. In Arizona, the Salt River, Verde River, and the extensive canal system throughout Pinal County provide abundant larval habitat. Larvae are elongated, segmented, and cream to brown in color; they prey on other invertebrates and small organisms in the mud. The larval stage lasts one to three years depending on species and conditions. Pupation occurs in drier soil adjacent to the water, and adults emerge in spring and summer. Horse flies are important prey for dragonflies, birds, and parasitic wasps, and their larvae play a role in aquatic food webs. However, their economic impact on livestock operations and outdoor recreation is significant — a single horse fly can cause a horse to lose up to 300 mL of blood per day when populations are high.

Personal protection is the most effective defense against horse flies. Wear light-colored, long-sleeved clothing — horse flies are strongly attracted to dark colors and movement. DEET-based repellents provide moderate protection but are less effective against horse flies than against mosquitoes. Permethrin-treated clothing offers better protection for extended outdoor work. For horses and livestock, fly sheets, fly masks, and ear nets reduce biting significantly. Sticky traps (Manitoba traps or Nzi traps) baited with CO2 or octenol can capture large numbers of horse flies and are useful for monitoring population levels. Chemical control of adult horse flies is difficult because they are strong fliers that re-invade from surrounding areas. Larviciding aquatic breeding sites is generally not practical or environmentally advisable. If horse flies are a persistent problem around your property, Pest Control Bros can assess the situation and recommend targeted solutions — call for a free consultation.

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