Black Flies

Black Flies

Blackfly
The image of the black fly is from Purdue University

Unfortunately we may be seeing a few more of some pests soon - the dreaded black flies, which can become a big problem after heavy spring rains. The most common of the small biting flies, adult black flies can be an annoyance and sometimes produce serious effects for people and animals. Black flies usually feed on birds and some livestock (e.g., horses) but may bite humans. Black fly outbreaks are associated with areas with sustained running water. Unusually heavy and sustained spring run-off can contribute to large population increases and subsequent biting problems.

Adult black flies (Simulium spp.) are small with a humpbacked appearance, which is why they are sometimes called "buffalo gnats." Simulium vittatum, a species which favors birds, is sometimes called a "turkey gnat". The image below is from Purdue University.

All black flies require cool, running water for development and favor sites with cobbled bottoms (pebbles and small rocks) that are largely clear of silt. The black fly larvae attach themselves to rocks or other submerged materials and feed on organic particles that they filter from passing water. Trailing vegetation or rotted aquatic plants also are attractive to black flies, providing sites for the larvae to attach for feeding. Breeding may occur in rivulets formed by the flooding of fields.

The black fly life cycle can be rapid, taking about three weeks from egg laying to maturation of the adult. Only the female bites, the blood meal being used to provide protein for egg maturation. Adults live about two weeks. Populations can grow very rapidly. Two to four generations may be produced annually. Individual females may lay several hundred eggs.

Adult black flies are migratory, commonly flying many miles from larval breeding sites. As an extreme example, migrations of more than 90 miles are reported in Canada. Black fly attacks on people, cattle, horses and pigs tend to be concentrated around the ears and head. In addition to the blood loss, effects of the insect saliva can cause a variety of problems, with swelling and intense skin irritation most common. Allergenic asthma, nausea, and more systemic effects can also occur, a condition known as "black fly fever."

Species that attack birds feed mostly around the eyes. The intense annoyance can cause animals to become greatly agitated and exhaust themselves in attempts to escape. Unlike mosquitoes, black flies are day feeders. Biting attacks tend to show some periodicity. During sunny, warm days peak attacks occur in mid-morning and then have a more intense phase in evening, ending at dusk. However, biting greatly intensifies at the onset of storms and may persist all day when overcast conditions occur.

Black fly control is difficult due to the highly migratory adult stage and their extensive breeding habitat. In terms of personal protection, choice of clothing can be important. Black flies are highly attracted to dark colors, so wear light colored clothing. Light-colored hats that cover the ears are an important precaution. The repellent DEET (diethyl toluamide) is somewhat effective in preventing black fly bites, although swarming gnats may still be annoying even when you use a repellent. To reduce attacks on poultry, keep the birds in a darkened barn during the day. Usually fans or some other means of cooling the birds is needed.

Larval control is practiced in some areas where chronic black fly problems occur and breeding areas are known. This involves metering of Bacillus thuringiensis var. israelensis into the flowing water where larvae occur. Trade names include Bactimos and Vectobac, the same products used for larval control of mosquitoes. However, as previously noted, the breeding sites can be many miles from where adult insects are causing problems. Adult control is problematic, again due to the migratory behavior of the insects. It is likely that permethrin-based products are among the best. These are effective against most fly species and are labeled for mosquito control and for fly control of livestock. As always, be sure the pesticide product you are considering is labeled for the pest and site. Read the label carefully and follow directions.

Source: Biting Flies by W.S. Cranshaw, F.B. Peairs, and B. Kondratieff. Colorado State University Extension Fact Sheet No. 5.582

Image of Keith Jarvi
Keith Jarvi
Extension Educator - Crops & Integrated Pest Management
Keith Jarvi has been with the University of Nebraska since 1979. He received a Master’s Degree in Entomology from North Dakota State University and a Bachelor of Arts degree in Biology from Gustavus Adolphus College in St. Peter, MN. His area of focus is Crops and Integrated Pest Management. As far as insect id he has seen a lot of interesting specimens, but there are always a few surprises every year.

Contact Keith at:
Dixon County Extension
57905 866 Rd
Concord NE 68728-2828
(402) 584-3819