THE MERCK VETERINARY MANUAL
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Stable Flies

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The stable fly, Stomoxys calcitrans, is often called the biting house fly. It is about the same in size and general appearance as Musca domestica, the house fly. It is brownish gray, the outer of four thoracic stripes is broken, and the abdomen has a checkered appearance. It has a bayonet-like, needle-sharp proboscis that, when at rest, protrudes forward from the head. The wings, when at rest, are widely spread at the tips. These flies are found throughout the world. In the USA, they are found in the midwestern and southeastern states.

The larval and pupal forms develop in decaying organic matter, including grass clippings and seaweed along beaches. In the midwestern USA, larvae can be found in wet areas around the edges of hay stacks and silage pits. Where cattle are fed hay, breeding can occur at the edge of the feeding area where hay has become mixed with urine and feces. The life cycle in the field can be completed in 2–3 wk, and adults may live ≥3–4 wk.

Both male and female stable flies are avid blood feeders, feeding on any warm-blooded animal. Stable flies stay on the host for short periods of time, during which they obtain blood meals. This is an outdoor fly; however, in the late fall and during rainy weather, it may enter barns.

Horses are the preferred hosts. The fly usually lands on the host with its head and proboscis pointed upward and inflicts painful bites that puncture the skin and bleed freely. It is a sedentary fly, not moving on the host. Stable flies usually attack the legs and ventral abdomen and may also bite the ears. They can be a problem in cattle feedlots in the midwestern USA. The damage inflicted to cattle is caused by the painful bite and blood loss, and the irritation results in a reduced efficiency in converting feed to meat or milk. In pets, stable flies prefer to feed on the tips of the ears of dogs with pointed ears, especially German Shepherds.

Stable flies are mechanical vectors of anthrax, surra, and equine infectious anemia. They are the intermediate host for Habronema muscae, a nematode found in the stomach of horses.

Stable flies are easily identified by their size (about the same as that of the house fly), coloration, and bayonet-like proboscis that protrudes forward from the head.

The main consideration in stable fly control is sanitation, which can effect up to 90% control. Areas along fence rows, under feed bunks, or wherever manure and straw or decaying matter can accumulate should be kept clean, because these substrates provide the medium in which the larval flies develop. If good sanitation procedures are practiced, chemical control is less likely to be needed. Various insecticides can be sprayed where flies may be resting in barns or on fence rows.

Stable flies feed on the lower portions of cattle, around the legs and belly, including the udder. They usually feed only once or twice daily for short periods, thus minimizing exposure to compounds applied to these areas. Often, insecticides applied to these body regions are rubbed off by contact with dense vegetation and mud or rinsed off when dairy cattle are rinsed before milking. Direct animal application of sprays and dusts may be used in some instances to protect large animals. Insecticides used for direct animal application usually have short residual activity. This type of application is quite labor intensive.

A combination of two compounds, imidacloprid and permethrin, works to repel S calcitrans. Monthly application of this product repels these flies and prevents their blood feeding on dogs, but the product does not kill this type of biting fly.

Last full review/revision August 2013 by Charles M. Hendrix, DVM, PhD

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