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More than 3,700 horsefly species are described worldwide. One of the world’s largest horsefly species is Tabanus americanus, the American horsefly. Adults range from 22 mm to 33 mm in body length. The body color ranges from dark brown to nearly black on the dorsal surface, with pale margins between abdominal segments. Much of the body is covered by fine, felt-like setae or hairs (pubescence). The ventral surfaces are lighter gray, and the front of the head and legs are brownish yellow. Horseflies have large compound eyes that are bright metallic green or blue in living flies. Males and females can easily be distinguished by the degree of separation of the eyes. In females, the eyes are obviously separated, and in males the eyes are connected across the entire front and top of the head. The mouthparts of a female horsefly comprise a set of long, saw-toothed blades for slashing flesh and narrow accessory structures that form a sponging apparatus for lapping up blood. Mouthparts of males are short and blunt, forming a sponging structure. The antennae are five-segmented, short, and the terminal segments are forked near the base and narrower towards the tips. The forewings are large, brown along larger veins, with transparent membranes between the veins. As with all winged members of the order Diptera, the hind wings are reduced to small, knobbed structures called halteres, used for balance in flight.
American horsefly larvae are long, cylindrical, yellow maggots with obviously segmented bodies indicated by enlarged ring-like structures. The head is located at the narrow, anterior end of the body, but is not obvious, and the posterior end is blunt. Larvae may reach 50 mm in length when fully developed. Females deposit eggs in dark oval masses up to 25 mm in length comprising many eggs each. Egg masses are deposited on plants near larval habitats and are glued together by a shiny, adhesive substance.
Almost 100 species of the family Tabanidae have been documented in Louisiana. Many species are similar in appearance and require technical expertise to distinguish. The American horsefly can be easily distinguished from all other species based on its large size, brown body color and mainly transparent wings. Only the black horsefly (Tabanus atratus) approaches the American horsefly in size, but is uniformly black in color, with dark brown to black wings.
As with other members of the fly family Tabanidae, American horsefly adult females require a blood meal to reproduce, although they also will feed on nectar. Males feed only on nectar and other plant fluids and do not bite to obtain blood meals. Adult American horseflies are large, agile, day-active hunters that use chemical cues and vision to locate food sources. As with all adult members of the order Diptera capable of flight, flight propulsion is provided by the large forewings and flight muscles that fill most of the interior of the thorax. Agility is aided by the small hindwings, which are highly modified into small knobbed gyroscopic structures (halteres). Females typically feed on the blood of larger mammals, including livestock, deer and other large warm-blooded animals, including humans.
Horseflies undergo four developmental stages — egg, larvae, pupae and adult — a pattern that occurs in all insects with complete metamorphosis (holometaboly). Egg laying occurs during mid to late summer in Louisiana in shady, moist environments. During early spring, larvae hatch, and the predacious maggots burrow into mud, moist soil, rotting logs and damp vegetation. Larvae are less associated with wetlands than those of most other horseflies and have been recovered in well-rotted logs in moist areas of forest but also occur in wetlands such as pond edges. Maggots are predators of other invertebrates and occasionally larger animals such as small amphibians. Larvae pupate in soil after several growth stages (instars). Adults emerge during late spring and summer.Adults are highly mobile and may fly 5 to 10 miles during dispersion events but typically stay near appropriate larval habitats.
The American horsefly is not host specific and will attempt to feed on any large warm-blooded animal. Infestations of these large blood sucking flies create havoc for livestock management. Horsefly mouthparts include serrated knife-like components that lacerate the host’s skin, causing intense discomfort. Blood flows from the wounds, aided by anticoagulants from the saliva, and is imbibed. Each female requires up to 0.5 ml of blood to reproduce. Feeding can last several minutes. American horseflies prefer shaded areas such as edges of pastures and forests.
Horseflies vector a variety of livestock diseases, although the specificity of transmission among different horsefly species is not well documented. Equine infectious anemia is a serious tabanid-transmitted disease in horses in the southeastern U.S.
Horsefly feeding on livestock may be temporarily mitigated though the use of pyrethroid and synthetic pyrethroid livestock pours. Researchers suggest moving livestock from infested areas or covering the animals with lightweight sheets to limit contact with biting flies. Additionally, traps that attract and reduce populations have been used with some success. Traps that release carbon dioxide to lure horseflies chemically have been used for years. More recently black ball traps have shown some promise. These traps utilize a black ball that resembles the belly of an animal. When exposed to the sun the ball heats up, and horseflies detect the infrared heat and are attracted to the ball. The ball may be coated with a sticky substance to trap flies or a conical canopy over the ball can be installed to direct flies into a jar located at the top, where they are trapped. These traps should be positioned some distance away from livestock so that biting flies are lured away from the animals. Colors and patterns are important considerations in designing horsefly decoys, with blue, red and black more attractive than other colors, and solid-colored surfaces more attractive than those with stripes.
Schyler Thibodeaux
Christopher E. Carlton
