THE MERCK VETERINARY MANUAL
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Black Flies

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Members of the family Simulidae are commonly called black flies (although their coloration may vary from black to gray to yellow to olive) or buffalo gnats (because their thorax is humped over the head, giving the appearance of a buffalo's hump). Black flies are the tiniest of the blood-feeding dipterans, 1–6 mm long. They have broad, unspotted wings with thick, prominent veins along the anterior margins. Black flies have compound eyes; eyes of females are distinctly separated, whereas those of the males are contiguous above the antennae. The palps have five segments. Female black flies have scissor-like mouthparts with tiny, sharp, serrated edges. The female flies require a blood meal so that they are able to lay eggs. Males are never bloodfeeders but instead feed on nectar from flowers.

Although there are >1,000 species of black flies, only a few are considered important as pests. Black flies feed on all classes of livestock, wildlife, birds, and people.

Black flies are distributed throughout the world in areas where conditions permit development of the immature forms. Larvae nearly always are found in swiftly flowing, well-aerated water; shallow mountain torrents are favored breeding places. Some species breed in larger rivers; others live in temporary or semipermanent streams. Black flies are particularly abundant in the north temperate and subarctic zones, but many species are found in the subtropics and tropics where factors other than seasonal temperatures affect their developmental and abundance patterns.

Larval black flies are cylindrical and attach themselves by a large posterior sucker. On the anterior end are the mouthparts and a pair of brush-like organs. The larvae are carnivorous. Just below the mouthparts is an arm-like appendage called the proleg. Larvae attach to rocks or other solid objects in the stream, sometimes clinging to aquatic or emergent vegetation. The mature larva spins a triangular cocoon on the floor of the stream. The oblong pupa has one dorsal and one ventral respiratory tube, the branches of which float out of the cocoon.

Black flies produce one to six generations per year, depending on the species and climatic conditions. Adult female feeding activity may last from 2–3 wk to 3 mo. Adult black flies may fly 8–11 miles (12–18 km) from the swiftly flowing streams; migrating windborne swarms have been known to travel ≥250 km.

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 the permeability of the capillaries 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 israeliensis, 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.

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

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