Black flies are persistent and irritating pests which swarm around humans and other animals. including livestock, poultry, wild birds and mammals. Their biting can disrupt outdoor activities at home, recreation and work areas, as well as impact on livestock production. Black flies get into the nose, eyes, ears and mouth and crawl into clothing when abundant. On cattle and horses, the ears seem to be the favorite feeding location. Flies lacerate the skin and suck blood. A strong anticoagulant in the saliva facilitates bleeding of the host. A lesion may form at the bite and the bitten area itches. Black flies serve as vectors of human and animal diseases in some areas of the world. An important protozoan parasite of poultry, Leucocytozoon simondi, is transmitted by black flies in the United States.
Black flies are small (usually 1.2 to 3 mm), dark flies with short legs. Because of their distinct humpbacked shape, they are sometimes called buffalo gnats. The wings are broad and the antennae are about as long as the head. Male black flies have larger eyes than females. Some species have white markings.
Black fly eggs are 0.18 to 0.46 mm long and somewhat irregular in shape although they are oval in some aspects. Pale white at first, the eggs darken as the embryo matures.
Black fly larvae are slender, 5 to 15 mm long, whitish brown to blackish and with a distinct head and anterior proleg (Figure 1). The head has slender antennae and two brushlike structures called cephalic fans.
Black fly pupae are 2 to 3 mm long with a respiratory organ on the thorax which protrudes from the open end of the cocoon. If removed from the cocoon, the wing pads, legs and other features of the adult fly can be seen, although these structures are closely appressed to the body.
Nearly all streams of North Carolina and surrounding states are inhabited by one or more species of black flies, particularly areas where water flows rapidly and tumbles, such as pond spillways. Some black fly species occur in slow moving streams, in swamps or in rivers. Even a small stream can produce enough black flies to cause a considerable nuisance.
Some species of black flies have four to seven generations per year. Black fly adults are active throughout most of the spring , summer and fall and even fly during warm spells in winter. Females lay their eggs after a blood meal. Eggs are deposited in masses on objects near water or under water, particularly at the crests of falls and spillways of impoundments. Some species lay eggs in water. Such eggs are carried downstream until they come to rest on the bottom. Tiny, newly hatched larvae either remain near the eggs or spin a silk thread and drift with the current until the thread becomes tangled in a suitable substrate. The larvae anchor themselves to the substrate by spinning a silk pad and clinging to the pad by a ring of hooks on the hind end. Most black fly larvae feed on organic matter filtered from the moving water with brushlike structures on the head called cephalic fans. As the larvae grow, they develop through six instars. Just before pupating, the larva spins a silk cocoon which is usually shaped like an open cone. The open end faces downstream which keeps sediment from filling the cocoon. The new flies emerge from the pupae through a slit in the top and rise in a bubble to the surface. Males and females feed on nectar of alfalfa and other flowers. Females of some species must feed on blood to produce eggs successfully.
Because black flies are widespread native insects, their eradication from any one locality seems unlikely. These insects are highly mobile and readily move away from breeding grounds in search of a meal. Spraying of individual yards, e.g., spraying shrubs or weedy areas or outdoor fogging, is not likely to be of much help. ULV applications of insecticides for adult black fly control may offer some temporary relief. Effective chemical control of black flies targets the breeding sites, which means it must be a community-based project. Options for chemical control of larvae are extremely limited. The only available effective treatment is a microbial pesticide containing Bacillus thuringiensis israelensis or Bti, which must be metered into the water in weekly treatments during periods of peak black fly activity. These products are also effective against mosquitoes. They provide for larval control while having little impact on other aquatic insects. These treatments can be expensive and require training to perform effectively and safely. For these reasons, large-scale larval and adult treatments are typically done by public operators hired by the municipality.
The best defense is personal protection in the form of repellents (see Insect Repellent Products). Children and pregnant women should use repellents sparingly. Black flies usually do not enter buildings in large numbers. During heavy black fly flights, you can reduce stress on livestock by sheltering the animals in stables or barns.
Publication date: May 1, 2005
Revised: Dec. 12, 2022
Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by NC State University or N.C. A&T State University nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your local N.C. Cooperative Extension county center.
N.C. Cooperative Extension prohibits discrimination and harassment regardless of age, color, disability, family and marital status, gender identity, national origin, political beliefs, race, religion, sex (including pregnancy), sexual orientation and veteran status.