Black Flies - ENT/rsc-16

Black Flies and Their Control

By: Charles Apperson and Michael Waldvogel, Entomology Extension 

Insect Note - ENT/rsc-16

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. 

DESCRIPTION

Adult - 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. 

Egg - 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. 

Larva - Black fly larvae are slender, 5 to 15 mm long, whitish brown to blackish and with a distinct head and anterior 

proleg (see diagram at right). The head has slender antennae and two brushlike structures called cephalic fans. 

Pupa - 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. 

BIOLOGY

Distribution - 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. 

Life History - 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. 

CONTROL

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 

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Black Flies - ENT/rsc-16

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. 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. 

Pest information and control recommendations presented here were developed for North Carolina and may not be appropriate for other states or regions. Any 

recommendations for the use of chemicals are included solely as a convenience to the reader and do not imply that insecticides are necessarily the sole or most 

appropriate method of control. Any mention of brand names or listing of commercial products or services in the publication does not imply endorsements by North 

Carolina Cooperative Extension nor discrimination against similar products or services. All recommendations for pesticide use were legal at the time of publication, 

but the status of pesticide registrations and use patterns are subject to change by actions of state and federal regulatory agencies. Individuals who use chemicals 

are responsible for using these products according to the regulations in their state and to the guidelines on the product label. Before applying any chemical, always 

obtain current information about its use and read the product label carefully. For assistance, contact the Cooperative Extension Center in your county. 

Distributed in furtherance of the acts of Congress of May 8 and June 30, 1914. North Carolina State University and North Carolina A&T State University commit 

themselves to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, or disability. In addition, the two 

Universities welcome all persons without regard to sexual orientation. North Carolina State University, North Carolina A&T State University, U.S. Department of 

Agriculture, and local governments cooperating.

Last updated - 5/29/05 

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