Horse Fly

Horse flies are the fly family Tabanidae. Horse flies (genus Tabanus) are considerably larger than deer flies (genus Chrysops).

Horse flies are with huge heads (large eyes in males) and from 3/4 inch to over an inch long. Smaller species are brown, black or gray, and often have brilliant green eyes. The eyes are sometimes crossed with reddish-gold bands that disappear when the fly dies.

Larger species are brown to black and may be slightly striped. Horse flies have their antennae divided into three parts, the third being long and composed of five to eight rings. Horse flies usually have clear wings.

Horse flies usually attack livestock.Eggs are dark, shiny, spindle shaped and in layered masses (tiers) of a few to several hundred on vegetation in or hanging over the water. Fully grown larvae are cylindrical, tapering toward both ends, whitish or yellowish gray, banded with black or brown and a fleshy elevated ring on each body segment. They are tough skinned (leathery), up to two inches long and are often used as fish bait.

Eggs: Eggs are laid in masses ranging from 100 to 1000 eggs. Eggs are laid in layers on a vertical surface such as overhanging foliage, projecting rocks, sticks and aquatic vegetation. Aquatic vegetation is most preferred. A shiny or chalky secretion, which aids in water protection, often covers eggs. The vertical surfaces on which the eggs are deposited are always directly over water and wet ground favorable to the development of larvae. The female will not deposit egg masses on vegetation that is too dense. Eggs are initially a creamy white color but soon darken to gray and black. Eggs are cylindrical in shape and measure from 1 to 2.5 mm in length. Eggs hatch in five to seven days, depending upon ambient weather conditions, and the larvae fall to the moist soil and water below.

Larvae: Larvae use a hatching spine to break out of the egg case. The larvae are aquatic, semi-aquatic or terrestrial. Chrysops spp. are termed “hydrobionts” and are found in areas with high water content. Tabanus spp. prefer dryer substrates and are “hemi-hydrobionts”. The larvae taper at each end and are usually whitish in color, but also can be brownish or green depending on the species. Black bands are found around each segment of the body in many species. The larva breathes through a tracheal siphon located at their posterior end. The larva has a small head and 11 to 12 additional segments. Larvae pass through six to nine stadia. The time spent in the larval stage can last from a few months to a year.

The larvae of Chrysops feed upon organic matter in the soil. Tabanus spp. feed upon insect larvae, crustaceans, and earthworms. Even though the Tabanus spp. are considered to be carnivorous and cannibalistic, reports of as many as 120 larvae per square yard have been found. The larva moves into the upper 2.5 to 5.0 cm of the soil, where it is drier, when it is ready to pupate. Within two days after moving to the surface the pupal stage is reached.

Pupa: The pupae are brown colored, rounded anteriorly, tapering posteriorly, and have leg and wing cases attached to the body. There is a row of spines encircling each abdominal segment. A pupal “aster” consisting of six pointed projections is located at the apex of the abdomen. The pupal stage generally lasts from two to three weeks.

Life Cycle :Winter is spent as partially grown larvae that pupate in spring and begin emerging as adults in late spring and summer, varying by species. Females often lay eggs in specific locations, such as on vegetation overhanging water. Eggs are laid in masses that darken to brown or black before larvae hatch out and drop to the ground or into water. Larvae are generally whitish, spindle-shaped and develop through six to thirteen stages (instars) over one or more years before pupating. One generation per year occurs for most species.

Damage:Tabanids lie in wait in shady areas under bushes and trees for a host to happen by. Sight is the main host finding mechanism, but carbon dioxide and odor also play a role. Moving objects, especially if dark colored, are most prone to attack. Attacks occur during daylight hours with a peak beginning at sunrise and lasting three hours. A second peak is two hours before sunset and commences shortly after. Attack frequency is low on overcast days or at temperatures below 22 and above 32 ºC. On livestock, biting occurs on the abdomen, legs, and neck. Tabanids inflict deep wounds that cause a flow of blood. The mandibles and maxillae penetrate the skin in a scissor-like action. Anticoagulants in the saliva are pumped into the wound and the blood is ingested through the sponging labella.

Pathogens may be transmitted from flies that are disturbed while feeding on one animal and begin feeding on another. It is known that deer flies can mechanically vector Tularemia and Loa loa, and horse flies transmit Anthrax. Fly attacks result in lowered gains and low milk production in livestock animals. In 1976, estimated losses in the United States were at 40 million dollars. One cattle ranch in Kentucky lost an average 100 lbs. per animal due to tabanids. It is not uncommon to see as many as 100 flies feeding on an animal at one time. Twenty to thirty flies feeding for six hours are capable of taking 100 cc of blood.

Control: There are no chemical controls for horse fly larvae, which develop in mud around edges of ponds and small streams. However, breeding can be suppressed by removing vegetation around pond edges to inhibit egg laying. To control adults, direct insecticides at shrubbery and other resting sites.

The relatively large size of the flies, which increases the dose of insecticide necessary to produce mortality.The brief feeding period on the animal when it would be in contact with a treatment on the animal, the continued emergence and host seeking of females of numerous species over a relatively long period the ability to fly from emergence sites that may be a considerable distance from the host to be protected and the wide range of larval habitat which precludes, except in limited situations, larval control.

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