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Certain species of adult black fly females are fierce biters, whereas others are strictly a nuisance by their presence around one's nostrils, ears, arms, hands, and other exposed skin areas. These flies can discourage people from remaining in or visiting certain recreational areas for fishing, camping, hiking, golf, etc. when the black fly season occurs. Children are especially susceptible and may be severely bitten while adults in the same area are scarcely aware of the flies. Most complaints occur in early spring (April to June) in hilly areas with swiftly, flowing streams. Bites may appear where clothing fits snugly against the body, leaving a ring of bites just above or below the belt line.
After the black fly finishes feeding, bleeding may continue for some time. At first, the bite site appears as a small, red, central spot surrounded by a slightly reddened, swollen area. Next, the area becomes increasingly itchy, swollen and irritating, sometimes for several days. Some black flies readily attack people, whereas others prefer domestic animals or birds, often feeding during the daylight hours and sometimes into the night. Flies may become so abundant as to be drawn into the air passages of livestock, occasionally resulting in death. It is believed that allergic reactions to bites may be caused by histaminic substances in the fly's saliva. These flies transmit a disease of filarial worms, onchocerciasis, which causes blindness in people in Mexico, Central America and Africa in addition to protozoan parasites, leucocytozoonosis to turkeys and wild birds. They may be potential transmitters of encephalitis. It is suspected that the expansion of black fly populations in Ohio is likened to improvement in stream and river water quality in recent years. As with many aquatic insects, black flies are very sensitive to water pollution.
Most species of adult black flies are about 1/8-inch long (2 to 5mm), black gray or even yellow colored, broad clear winged without hairs or scales with heavy veins near the anterior wing margin, have short 11 segmented antennae, large round eyes (no simple eyes) and the thorax (middle body region) is strongly convex, giving a humpbacked, gnat-like appearance.
Small creamy-white eggs (rather triangular) about 1/32-inch long (0.1 to 0.4mm) are deposited on the water surface or attached in compact masses to stones and vegetation in shallow fast-running water (riffles) in streams and rivers. Larvae, black to light brown colored, cylindrical, about 1/4-inch or more long (10 to 15mm), are quite active and abundant, sometimes appearing as moss. Pupa are boat or basket-shaped cocoons up to 1/8-inch long (2 to 5mm) in the water.
Black flies often occur in enormous numbers in the spring and early summer months, especially in the northern latitudes. Bites can be extremely painful, and their mouthparts are somewhat similar to those of a horse fly (bladelike and piercing) in the female. Mouth parts are rudimentary in the male. On people, they crawl into sleeves, under neckbands, around boot tops and other vulnerable places, especially favoring the head just beneath the rim of a hat. Bites can cause swelling and numb soreness for many days. There are records of both domestic animals and people being killed in a few hours through venomous bites and blood loss. Death can result from suffocation as a result of plugged nasal or bronchial tubes and allergic reactions.
Flies usually bite during the day in outdoor shaded or partially-shaded areas. They do not bite indoors or late at night. Some fly 7 to 10 miles from the breeding sites, or are blown by wind even further to feed on warmblooded animals and people. Flies usually bite for about three weeks before they die. Dark blue cloth attracts more flies than white cloth.
Females deposit from 150 to 500 small, shiny, creamy-white eggs on submerged objects in the stream such as on water plants, rocks, twigs, leaves, etc. or simply scatter the eggs over the water surface. Eggs darken then hatch in four to five days at water temperatures of 70 deg F. Eggs deposited in the autumn do not hatch until the following spring when the water warms.
Young larvae attach themselves to submerged objects, molting six times as they grow. They are elongate with the hind part of their bodies swollen. A head fan sweeps food material into the mouth. They retain their position in the water by means of sucker-like discs and tiny hooks at the tip of the abdomen. Also, they may spin a fine thread which aids in anchoring them. Winter may be passed as larva. Pupation occurs in a cocoon, open at one end. Adults emerge in two to three days when the water is warm. They are capable of immediate flight and mating. The entire life history spans about four to six weeks, depending on species, water temperature, available food, etc. There may be four generations per year.
Black flies are attracted to mammals by the carbon dioxide and moisture in exhaled breath, dark colors, convection currents, perspiration, perfumes, toiletries, etc.
There is little that the individual homeowner can do to control black flies. Bites can be treated with soothing lotions as well as corticosteroids to relieve pain and itching and help lesions resolve. If the reaction is mild, oral antihistamine therapy may suffice, but severe reactions involving shock may require epinephrine (consult a physician).
Some avoid outdoor activities during the black fly season. One can purchase hats with fine mesh netting extending over the face and shoulders from camping supply and mail order houses. Repellents offer some relief depending on the individual, species of fly, temperature, humidity, time of day, etc. There are times that flies will bite regardless of the repellent and concentration used. Protection sometimes lasts for two to two and half hours. Any of the following repellents can be used such as deet (Cutters, Diethyl-toluamide, Off), citronella oil (Skin So Soft), ethyl hexanediol or dimethyl phthalate. As a whole, the individual can merely resort to household sprays, aerosols, repellents and screens (60 mesh bolting cloth) to cope with this pest. Some persons indicate that unpainted aluminum "hard hats" attract black flies. A thin film of fuel oil smeared over the outer surface traps flies in the oil, giving the wearer much protection.
The best methods of control are directed toward reducing the number of black fly breeding areas. Removal of vegetation and other objects in streams will cut down the number of larvae. Temporary damming of water can reduce populations, as immature stages need swift, running water. Larvae will die in 10 to 24 hours in calm non-running water.
Fogging for black flies provides temporary relief since only those flies that contact the fog are killed. Later migrants into the treated area are unaffected. Daily treatments of pyrethrins usually is necessary.
Treatment of breeding areas in rivers, streams, etc. is effective when the application timing is correct. Large scale efforts are needed such as by a government agency skilled in coping with black flies. It is necessary to treat large areas where larvae occur by helicopter or airplane. Also, crews spraying streams and other bodies of water, and fog machines and mist blowers to kill the adults, is helpful.
Other labeled insecticides for application only by Public Health officials and trained personnel of Mosquito Abatement Districts for control of adult black flies in residential and recreational areas include permethrin (Biomist, Permethrin), chlorpyrifos (ULV Mosquito Master), and hypermethrin (Culicideth).