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Black flies

Jeffrey Hahn

About 30 species of black flies occur in Minnesota. Black flies, also referred to as gnats or buffalo gnats, can be a serious problem because of their ability to bite. They use blade-like mouthparts to slash the skin and feed on the blood. Fortunately black flies are not known to transmit diseases to humans in Minnesota. Some black flies are just a nuisance, flying around us without biting. Black flies are a problem from spring through summer in nearly all areas in Minnesota.


Adults are small (1/16 inch), stout, dark colored insects with a humpbacked appearance. They also have broad wings, short legs, and are relatively hairless. The larvae are legless and worm-like. The posterior third of the larva is noticeably swollen. Larvae possess a pair of fan-like structures near their head to help collect food particles in the water.


Eggs are laid directly onto the water or on the leaves of aquatic plants and objects in rivers, streams, and other running water. Once they hatch, the larvae attach themselves to stones, grass, branches, leaves and other objects submerged under the water. In Minnesota, black flies develop in large rivers (e.g. Mississippi, Minnesota, Crow, and Rum rivers) as well as small streams. Most larval black flies develop under water for 10 days to several weeks depending on water temperature. After about a week as pupae, they emerge as adults. Adults live for about three to six weeks. Most adults can fly about 10 miles from their breeding source but can be carried considerably further on air currents.

Here is a list of the more common black fly species that you may encounter:

Both males and females can feed on nectar for flight energy, but only females bite to take a blood meal. Black flies are most active a couple of hours after sunrise and a couple of hours after sunset. Although they are strong fliers, they are less of a nuisance on windy days and in open areas than on calm days and in sheltered areas (e.g. in wooded areas).

Black flies often swarm around a person's head because they are attracted to carbon dioxide in the breath. Black flies are also attracted to dark colors such as navy blue. Bites are concentrated on exposed areas of skin, especially along the hairline, feet, ankles and arms. The bites can produce a variety of reactions ranging from little or no irritation to considerable irritation and swelling. Sensitivity varies from person to person.


It is very difficult to prevent black flies from biting, especially when they are abundant. Because they can move into an area from up to 10 miles away or more, there is no practical control to prevent black flies from migrating into an area. When you are out in an area where black flies are present, there are several strategies to try to reduce their bites.

When possible, avoid areas with high black fly populations, such as lowlands, areas with dense vegetation or sheltered and shady areas. Also, try to avoid times when black flies are most active, generally at dawn, and dusk.

Wear white or brightly colored clothing, which is less attractive to the flies than dark-colored clothing. Cover up bare skin with shoes, socks, long sleeves shirts, long pants, and hats. You can also try wearing a nylon head net, similar to a bee keepers veil. You can find them in outdoor stores and gardening catalogs.

The use of insect repellents, such as those with DEET (N,N-diethyl-m-toluamide) are not consistently effective but may provide some relief. Products containing a moderate amount of DEET (35%-60%) are as effective as those with a high content (90%-95%).

Related information

Published in Yard & Garden Brief March 2000

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