Members of the family Simulidae are commonly called black flies (although their coloration varies 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 among the tiniest of the blood-feeding dipterans (1–6 mm long; usually 1.5–3 mm) (1). They have broad, unspotted wings with thick, prominent veins along the cranial 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 scissorlike mouthparts with tiny, sharp, serrated edges and require a blood meal to lay eggs. Males never feed on blood 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 production animals, wildlife, birds, and humans.
Black flies are distributed throughout the world in areas where conditions permit development of the immature forms. The eggs are laid in fast running water, just below the water's surface, on stones or leaves. 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; however, 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 caudal sucker. On the cranial end are the mouthparts and a pair of brushlike 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. Mature larvae spin 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 lasts from 2–3 weeks to 3 months. Adult black flies can fly 12–18 km (8–11 miles) from swiftly flowing streams; migrating windborne swarms have been known to travel ≥ 250 km (155 miles) (1). Black flies are active in the morning and evening. During hot days, black flies rest near the ground on the underside of leaves.
Pathology of 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, general conditions 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 rates in production animals. Humans can be similarly attacked. Swarms of black flies can lead cattle to stampede and refuse to graze.
Black flies are a great nuisance to animals. The bites are associated with petechial hemorrhage and lesions that take weeks to heal, in particular on cows' teats. Poultry can be affected and become anemic if attacked by a large number of black flies. Death from black fly attack apparently results from a toxin in the saliva (2), which increases capillary permeability and permits 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.
Decreased milk, meat, and egg production can result from less extensive attacks. Certain species of black flies sometimes cause losses in poultry, either by direct attack or via 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.
Diagnosis of Black Flies
Black flies are most often collected in the field and not found on animals. During the day, black flies hide close to the group on the underside of leaves. Adult flies are dark with short legs and can be identified by their small size, humped back, prominent venation in the cranial region of the wings, and tiny, serrated mouthparts. Wings are broad and spotless. Wings are not hairy, except for bristles on the thick cranial veins. Identification of black flies to genus and species is complex and best left to an entomologist.
Treatment and Control of Black Flies
Destruction of larvae in their breeding sites
Preventing exposure
Insecticides or repellents
Successful black fly suppression programs focus first on surveillance of larval populations and stream treatment with larvicides. Only when nuisance levels become severe are adulticides employed.
Public funds and trained supervisory personnel are required for large-scale control of black flies. However, black fly control is difficult because of the large number of flowing-water breeding sites.
Streams can be treated using the approved microbial larvicide Bacillus thuringiensis israelensis (Bti), a product with no mammalian toxicity. Bti is a naturally occurring bacterium in soil that is the main larvicide used to control mosquitoes and black flies worldwide. Bti produces toxins specifically targeting mosquitoes, black flies, and fungus gnats. Other types of insects, such as honeybees, are not affected.
If notable numbers of black flies are observed, fogging the high-risk area using approved insecticide (eg, permethrin, sumithrin) should be considered, in consultation with public health authorities. Fogging programs use ultralow-volume (ULV) pyrethroids or pyrethrin insecticides and are based on mosquito control operations. Fogging provides only short-term relief and is rarely the best black fly management strategy; black flies can disperse many kilometers from breeding sites and new black flies quickly reinvade treated areas. Fogging is done at dawn or dusk when black fly activity is greatest.
Where regulations permit, long-term population suppression thorough larval control in rivers and streams is the best control strategy. Treatment of streams and rivers involves techniques (eg, Bti-based larvicides) 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 strict 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 come indoors on or within a pet’s hair. More often, adult female flies prefer to feed outdoors during the daylight hours. Because black flies feed predominantly during daylight hours, limiting pets' exposure to swiftly flowing streams is advisable.
Dog owners concerned about black fly bites may use available veterinary products, such as pyrethrin-containing spot-ons and pyrethroid-impregnated collars that are strongly repellent to biting flies. Aerosols containing pyrethrins provide only temporary relief.
Permethrin and other synthetic pyrethroids are highly toxic for cats and cannot be applied to them. Physical protection is the primary recommendation for cats.
Pearls & Pitfalls
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Because area-wide control of black flies is difficult and expensive, 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.
Substantial black fly nuisance activity should be reported to the local vector control district or county health department. Reports should include the location, date, time of day, estimated number of flies, photographs if available, and any impact on livestock, pets, or outdoor activities. Where livestock impact occurs, producers should notify the state department of agriculture or state veterinarian. These agencies coordinate species identification and surveillance and evaluate appropriate control measures.
For More Information
Bti for mosquito control. EPA.
Mosquitoes: larvicides. EPA.
Brühl CA, Després L, Frör O, et al. Environmental and socioeconomic effects of mosquito control in Europe using the biocide Bacillus thuringiensis subsp. israelensis (Bti). Sci Total Environ. 2020;724:137800.
Also see pet owner content regarding flies and mosquitoes of dogs, cats, and horses.
References
Adler PH, Currie DC, Wood DM. The Black Flies (Simuliidae) of North America. Cornell University Press, 2004.
Cupp EW, Cupp MS, Black fly (Diptera: Simuliidae) salivary secretions: importance in vector competence and disease, J Med Entomol. 1997;34(2):87-94. doi:10.1093/jmedent/34.2.87
