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Black Flies

image of black fly

New Hampshire is home to approximately 40 species of black flies. Of these species, only 4 or 5 are considered to be significant human biters or annoying. In some cases, black flies may not bite but are extremely annoying as they swarm about the head or body. Only the females bite and fortunately most species feed on birds or other animals.

Habits & Life Cycles

Black flies breed exclusively in running water. Some species live in large, fast-flowing streams; others live in small, sluggish rivulets. Almost any kind of permanent or semipermanent stream is occupied by some species.

Large black fly populations indicate clean, healthy streams since most species will not tollerate organic pollution. Since New Hampshire is well known for its clear streams, it is not surprising that black flies also are abundant, and at times a serious annoyance to residents and visitors.

Females lay their eggs on vegetation in streams or scattered over the water surface. The eggs hatch in water and larvae attach to rocks, leaves, grass or other submerged objects. Mature larvae pupate underwater emerging adults ride bubbles of air to the surface and fly away. Adults mate near the breeding site and fmales, who need a blood meal so they can lay eggs, begin their search for blood. Once they have fed and digested, they lay eggs in a suitable stream habitat.

Black fly species in New Hampshire exhibit two types of life cycles. One type overwinters stage in the egg stage. The eggs remain submerged in streams over the winter and hatch in spring when water temperatures reach about 40° to 50°F. The larvae develop during early spring and adults emerge in spring or early summer. Some species have more than one generation per year.

The second overwinters in the larval stage. Larvae hatch in the fall when stream temperatures are falling and develop during the winter months. Pupation occurs in early spring when water temperatures reach about 37° to 38°F. Adults emerge from late April through May and females deposit eggs in in May and June and remain in the water until fall when they hatch. In general, biting black fly species tend to breed most abundantly at the outlets of ponds and lakes, perhaps because of more abundant food or warmer temperature.

There are only 2 species of black flies in New Hampshire consistently and abundantly bite humans. These are Prosimulium mixtum and Simulium venustum. Simulium venustum, the so-called "white-stockinged" black fly emerges in early to mid-May in southern New Hampshire and remains a pest until late May. In the north, it emerges in late May to early June and can remain abundant until the end of June in some areas and even into July in higher mountain localities.

Simulium jenningsi is a late season species and is most annoying along the larger rivers in northern New Hampshire in August and September. They swarm around the head in large numbers but they do not usually bite.


Individuals should not try to control black fly larvae. Treating a small area, or part of a single stream will do little to alleviate a black fly pest problem. The State of New Hampshire requires a special permit to apply pesticides to surface waters, but because black fly nuisance problems are rarely confined to one location, state regulators are not likely to grant permits to individuals for black fly control. A commercial license to apply pesticides is also required before a permit is issued.

Control of black files in the adult state is usually not practical, unless you wish to obtain temporary relief (hours) on local property. Fogging to control black flies in the adult stage may provide a brief period of relief, but because adults are strong fliers, relief usually is only temporary. Sustained control is not possible using this method.

Proper clothing offers good protection against black fly bites. Keep shirt sleeves and front closely fastened and tuck trousers inside socks or high boots. Zippered front shirts will keep flies out better than button shirts. Light colors such as orange, yellow and light blue are less attractive to black flies than dark ones such as green, brown and red. Shoulder-length head nets are sometimes useful. These can also be impregnated with repellents.

Black flies are active only during the day. They do not bite at night. Depending on weather, black flies tend to be more active at certain times of day. Activity peaks tend to occur around 9:00 to 11:00 AM and again from 4:00 to 7:00 in the late afternoon and early evening, or until the suns falls below the horizon. They tend to be most active on humid, cloudy days and just before storms. If possible, avoid activity during times when black flies are most active. Early morning, midday and late evenings are the best times to work outside.

Chemical Repellents:
The same repellents used for mosquitoes are effective against black flies, but apparently do not last as long against black files. Since black files tend to crawl up sleeves and under socks and trousers repellent applied around the wrists, ankles and belt line will prevent files from crawling under clothing and biting. Repellents should be applied as needed for alleviating black fly attacks. Garments impregnated with repellents may also be effective in preventing black fly attacks. Currently the most effective ingredient is N,N-diethyl-meta-tolumide, or DEET. This material is not recommended for use on infants. Some people, especially, young children, can be sensitive to DEET, so use it with caution. Skin reactions, eye irritation, slurred speech, confusion and seizures have been reported.

Reviewed and edited by UNH Extension Entomologist Dr. Alan Eaton, 9/01

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