Black Flies

Because of their tiny, serrated mouthparts, female black flies inflict painful bites. The ears, neck, head, and abdomen of cattle are favorite feeding sites. In addition to local reactions (redness, itching, wheals) at the bite site, there may be general conditions that vary in intensity with the sensitivity of the animal and the number of bites. Attacks by large numbers of black flies can cause severe damage and high mortality in livestock. People may be similarly attacked.

Death from black fly attack apparently results from a toxin in the saliva, which increases capillary permeability and permits the fluid from the circulatory system to ooze into the body cavity and tissue spaces. The animal rapidly succumbs to a mass attack but can recover quickly if protected from further attacks. Reduced milk, meat, and egg production may result from less extensive attacks. Certain species of black flies sometimes cause losses in poultry, either by direct attack or through transmission of Leucocytozoon spp. In Africa, Simulium damnosum and S neavei are important as vectors of Onchocerca spp. S neavei is an important vector of O volvulus. In Central America, S ochraceum, S metallicum, S callidum, and S exiguum are important vectors of Onchocerca spp. S ochraceum and S metallicum also are vicious biters.

Black flies are most often collected in the field and not found on the animals. Adult flies can be identified by their small size, humped back, prominent venation in the anterior region of the wings, and tiny, serrated mouthparts. Identification of black flies to genus and species is probably best left to an entomologist.

If public funds and trained supervisory personnel are available, large-scale control of black flies is possible by treating breeding streams with an approved larvicide. However, black fly control is difficult because of the large number of flowing water breeding sites. Streams can be treated using the natural product, Bacillus thuringiensis var israelensis, a product with no mammalian toxicity.

Treatment of streams and rivers involves techniques similar to those used by mosquito abatement programs. As a rule, pesticides should not be used owing to their potential negative effects on the environment. Pesticide treatments involving water surfaces or large land areas are subject to governmental regulation and must be performed with due regard for possible deleterious environmental effects and residues in food products.

Adult black flies are small enough to pass through window screens or may come indoors on or within a pet’s hair coat. More often, the adult female flies prefer to feed outdoors and during the daylight hours. Because black flies feed predominantly during the daylight hours, it is wise to limit exposure of pets to swiftly flowing streams. Pet owners concerned about black fly bites may use over-the-counter insect repellents. Aerosols containing pyrethrins may provide only temporary relief.

Because area-wide control of black flies is difficult and expensive, livestock producers frequently resort to the daily use of repellents to protect their animals. Extension entomology personnel should be contacted for the latest approved recommendations and withdrawal times.