Blackfly

Black fly
A black fly (sometimes called a buffalo gnat, turkey gnat, or white socks) is any member of the family Simuliidae of the Culicomorpha infraorder. They are related to the Ceratopogonidae, Chironomidae, and Thaumaleidae. There are over 1,800 known species of black flies (of which 11 are extinct). Most species belong to the immense genus Simulium. Most black flies gain nourishment by feeding on the blood of other animals, although the males feed mainly on nectar. They are usually small, black or gray, with short legs, and antennae. They are a common nuisance for humans, and many U.S. states have programs to suppress the black fly population. They spread several diseases, including river blindness in Africa (Simulium damnosum and S. neavei) and the Americas (Simulium callidum and S. metallicum in Central America, S. ochraceum in Central and South America).

ECOLOGY

Eggs are laid in running water, and the larvae attach themselves to rocks. Breeding success is highly sensitive to water pollution. The larvae use tiny hooks at the end of the abdomen to hold on to the substrate, using silk holdfasts and threads to move or hold their place. They have foldable fans surrounding their mouths. The fans expand when feeding, catching passing debris (small organic particles, algae, and bacteria). The larva scrapes the fan's catch into its mouth every few seconds. Black flies depend on lotic habitats to bring food to them. They will pupate under water and then emerge in a bubble of air as flying adults. They are often preyed upon by trout during emergence.

Adult males feed on nectar while females also feed on blood. Some species in Africa can range as far as 40 miles from aquatic breeding sites in search of their blood meals, while other species have more limited range. Different species prefer different host sources for their blood meal, which is sometimes reflected in the common name for the species. They feed in the daytime, preferably when wind speeds are low. DEET-based insect repellents may attract greater numbers of black flies; permethrin products designed for ticks are effective but can only be applied to clothing, limiting their utility.

Black flies may be either univoltine or multivoltine depending on the species. The number of generations a particular pest species has each year tends to correlate with the intensity of human efforts to control those pests.

Work conducted at Portsmouth University by Bob Harris in 1986-1987 indicates that Simulium sp. create conditions within their mid guts that are highly acidic. This acidic environment provides conditions that are ideally suited to bacteria that metabolise cellulose, insects cannot metabolise cellulose so the presence of these bacteria allow cellulose to be metabolised into basic sugars , this provides nutrition to the black fly larva as well as the bacteria. A symbiotic relationship that indicates a specific relationship as fresh flowing streams could not provide sufficient nutrition to the growing larva in any other way.

Regional effects of black fly populations

In the wetter parts of the northern latitudes of North America, including parts of Canada, New England, Minnesota, and the Upper Peninsula of Michigan, black fly populations swell from late April to July, becoming a nuisance to humans engaging in common outdoor activities, such as gardening, boating, camping, and backpacking. They can also be a significant nuisance in mountainous areas.

 Black flies are a scourge to livestock in Canada, causing weight loss in cattle and, in some cases, death.
 Pennsylvania, in the United States, operates the largest single black fly control program in North America. The program is seen as beneficial to both the quality of life for residents and to the state's tourism industry.

 The Blandford Fly (Simulium posticatum) in England was once a public health problem in the area around Blandford Forum, Dorset, due to its large numbers and the painful lesions caused by its bite. It was eventually controlled by carefully targeted applications of Bacillus thuringiensis israelensis.

 The New Zealand "sandfly" is actually a type of black fly.

Role in human disease

The black fly is central to the transmission of the parasitic nematode Onchocerca volvulus, which causes Onchocerciasis, also known as river blindness. It serves as the larval host for the nematode and acts as the vector by which the disease is spread. Transmission of the parasite occurs through the bite of a black fly when feeding on human blood. Simulium pruinosum bite is thought to be involved in the pathogenesis of Fogo Selvagem, the endemic form of pemphigus foliaceus, a bullous dermatosis.