These flies, which are usually large and heavy-bodied, are important pests of domestic and wild animals and, at times, of man. They rarely occur indoors. Horse flies are known under a variety of names such as greenheads, deer flies and others.
Only the females are bloodsucking, the males feeding for the most part on plant juices. The eggs are deposited on plant foliage, rocks, etc., usually in damp situations.
Most of the larvae are aquatic or live in moist soil. The larvae often occur in the mud and at the bottom of ponds and ditches. Pupation usually occurs in the mud at the edge of these areas. Some of the larvae are predacious on immature insects and other small animals. The entire life cycle may require from approximately three months to as much as two years.
Gerry (1949) controlled the salt marsh greenhead fly by using DDT sprays. Many other species of horse flies are very difficult to control because of their habits. Those which may spend weeks in the larval stage in the dry soil may be controlled by drenching the soil with pyrethrins, but the cost is excessive. Those that live under rocks in running streams cannot be killed without danger to the environment and even the larvae which normally live in marshy areas are difficult to poison.