Black fly

Black flies (also called buffalo gnats, turkey gnats, or white socks) belong to the family Simuliidae within the Culicomorpha infraorder. This family is related to biting midges, non-biting midges, and the Thaumaleidae. More than 2,200 species have been formally described, with 15 known only from fossils. The family splits into two subfamilies: Parasimuliinae, which has just one genus and four species, and Simuliinae, which includes all the others. Over 1,800 species fall under the genus *Simulium*. These flies are typically small and black or gray, with short legs and antennae. Adults mainly feed on nectar for flight energy, but females of most species also need a blood meal from birds or mammals—including humans—to produce eggs. They are a widespread nuisance, and many U.S. states run programs to control their numbers. Black flies also transmit several diseases, most notably river blindness, caused by *Simulium damnosum* and *S. neavei* in Africa, and by *S. callidum*, *S. metallicum* (Central America), and *S. ochraceum* (Central and South America) in the Americas.

**Ecology**

Eggs are laid in running water, and larvae attach themselves to submerged rocks. Breeding success depends heavily on clean water; pollution harms it. Larvae use tiny hooks at the ends of their abdomens to grip the substrate, employing silk holdfasts and threads to move or stay in place. They have foldable fans, called mouth brushes, around their mouths. While feeding, these fans expand to catch passing debris—small organic particles, algae, and bacteria—and the larva scrapes the catch into its mouth every few seconds. Black flies rely on flowing water (lotic habitats) to bring food. They pupate underwater and then emerge in an air bubble as flying adults. Trout often prey on them during emergence. In South Africa, larvae of some species are known to hitch rides on mayfly nymphs (phoresy).

Adult males and females drink water and carbohydrates like nectar or honeydew for flight energy. Females of most species are anautogenous—they must bite birds or mammals to get blood for egg production; male mouthparts cannot pierce skin. Different species prefer different hosts, sometimes reflected in their common names. They feed during the day, especially when wind is light. Some African species can travel up to 40 miles (64 km) from breeding sites in search of blood, while others stay closer. In the few species where females do not bite, they use nutrients stored from the larval stage to produce eggs. Male black flies are rarely seen. Depending on the species, black flies may have one generation per year (univoltine) or multiple (multivoltine). The number of generations in a pest species often matches the intensity of human control efforts. Research at Portsmouth University (1986–1987) showed that *Simulium* larvae create highly acidic conditions in their midguts. This environment suits bacteria that digest cellulose. Since insects cannot break down cellulose on their own, these bacteria allow the larvae to turn cellulose into simple sugars, providing nutrition for both the larvae and the bacteria. This symbiotic relationship is a key adaptation, as fresh-flowing streams alone would not supply enough food for growing larvae.

**Regional effects of black fly populations**

In wetter parts of northern North America—including parts of Canada, New England, Upstate New York, Minnesota, and Michigan’s Upper Peninsula—black fly numbers surge from late April to July, bothering people doing outdoor activities like gardening, boating, camping, backpacking, or walking. They are also a major nuisance in mountainous areas. In Canada, black flies harm livestock, causing weight loss in cattle, and the saliva of one species can trigger anaphylactic shock and death. Pennsylvania runs the largest black fly control program in North America, benefiting both residents’ quality of life and the state’s tourism. In England, the Blandford fly (*Simulium posticatum*) was a public health problem near Blandford Forum, Dorset, due to its large numbers and painful bites. It was eventually controlled with targeted applications of *Bacillus thuringiensis israelensis*. In 2010, a summer surge of bites blamed on this fly sent many people to the hospital. New Zealand’s biting “sandflies” are actually black flies of the species *Austrosimulium australense* and *A. ungulatum*. In parts of Scotland, various black fly species bite humans mainly from May to September, found in mixed birch and juniper woodlands, lower pine forests, moorlands, and pastures. Bites most often occur on the head, neck, and back, and they frequently land on legs and arms.

**Public health**

Only four genera in the family Simuliidae—*Simulium*, *Prosimulium*, *Austrosimulium*, and *Cnephia*—include species that bite people; other species prefer other mammals or birds. *Simulium*, the type genus, is the most widespread and vectors several diseases, including river blindness. Mature adults can disperse tens or hundreds of miles from their freshwater breeding sites, using their own flight and prevailing winds, which complicates control. Swarming behavior can make outdoor activities unpleasant or unbearable and can affect livestock production. During the 18th century, the “Golubatz fly” (*Simulium colombaschense*) was a notorious pest in the region.