Most blood-suckers are not fictional vampires; they are small, disease-transmitting pests. More than 30 species are in group known as blood-letting flies, and most of them will travel long distances from their nests to reach their targets. Some are more than nuisances, whether it is because of their painful bites or their ability to transfer disease. For the horse fly, with its large head and eyes and razor-sharp mouth, it is both.
Horseflies breed and lay eggs in vegetation overhanging moist, muddy areas like swamps or bogs. When hatched, the larva drop down and burrow into the mud along the banks of the waterways. Though the horsefly larva primarily feed on snails, earthworms, organic debris and insects, they are cannibalistic and will also feed on other horsefly larva if the opportunity arises. The larva usually matures in late spring, after wintering in the mud, and then moves out of the wet area to enter the pupal stage, which can last six to 12 days. Once they emerge, adult flies immediately begin mating or seeking meals by focusing on what attracts their interest.
Both male and female horseflies feed on nectar and plant juices, but the female horsefly has an extra nutritional requirement: blood. The female horsefly needs blood for her eggs and she will target any mammal to get it, including humans. Their bites are sharp and painful because their mouth parts actually cut through the skin to reach the blood. These bites are not only painful, they can be dangerous and spread disease to both animals and people.
Horseflies sight their targets visually and are attracted to dark, moving objects. One suggested method of controlling horsefly bites is to wear light-colored garments when you are in areas where horseflies may gather, particularly on warm days.
The carbon dioxide breathed out by horses, dogs, people and other mammals is also attractive to horseflies. It helps them identify their warm-blooded targets even if they are not moving.
Controlling horseflies is difficult because it is impossible to eliminate their breeding grounds without destroying the environment. Insecticides are, for the most part, useless against horseflies, because they will enter an area -- unharmed -- directly after it has been sprayed. Suggestions for controlling the horsefly include trapping and eliminating the females before they have a chance to breed, and using devices designed for insects attracted to moving objects. You can purchase repellents for use on animals, but what works on one animal could be harmful to another. Use caution, read all warning labels, and consult a veterinarian or local agricultural office for more information.