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University of Minnesota Extension

Black flies

Quick facts

  • Black flies bite people and many different animals.
  • Black flies are active spring through summer and bites are most common during spring.
  • Wear a long-sleeved shirt, pants and a hat to avoid black fly bites.
  • Repellents are not always effective but may provide some help. 

How to identify black flies

Black fly adult

Black flies are also called gnats or buffalo gnats. Thirty species of black flies occur in Minnesota. All species are very similar in appearance to each other. 


  • Small (1/16 inch)
  • Stout, dark colored, rounded back
  • Broad wings, short legs
  • Few hairs on body


Black fly larva
  • Worm-like and legless
  • End of abdomen is enlarged
  • Two fan-like structures near the head

Biology of black flies


Black fly bites

Black flies can be a serious problem because they bite. When black flies are active they can develop into very large swarms.

  • Black flies use blade-like mouthparts to slash the skin and feed on blood. 
  • Bites are concentrated on exposed areas of skin, especially along the hairline, feet, ankles and arms. 
  • Bites can produce reactions from small red spots and little or no irritation to a lot of irritation and swelling.
  • Sensitivity varies from person to person. 
  • Black flies can attack many different domestic and wild animals, including birds. 
  • In extreme circumstances, black fly bites have killed animals through severe blood loss.

Sometimes black flies are nuisances by just swarming around people without biting. Because they like to swarm around heads, they might crawl into the nose, ears, eyes and mouth.  

Behavior and habits

  • Black flies are most active a couple of hours after sunrise and a couple of hours before sunset.
  • They are most active on calm days and in sheltered areas (wooded areas). 
  • They are less of a nuisance on windy days and in open areas.
  • Black flies are attracted to dark colors such as navy blue.

Can black flies spread disease?

In Minnesota, black flies are not known to spread diseases. In other parts of the world, they can spread serious diseases. ​​​​​​

How to protect yourself from black flies

It is very difficult to prevent black flies from biting, especially when they are in large numbers.

There is no control measure to reduce black flies in an area since they can fly from up to 10 miles away or more.

To reduce black fly bites try these suggestions: 

  • Avoid areas with high black fly populations, such as lowlands, areas with dense vegetation or sheltered and shady areas.
  • Avoid being outside when black flies are most active, generally at dawn and dusk.
  • Wear white or brightly colored clothing. It is less attractive to the flies than dark-colored clothing.
  • Cover up bare skin with shoes, socks, long sleeves shirts, long pants and hats.
  • Wear a head net, similar to a bee keepers veil. You can find them in outdoor stores and gardening catalogs.
  • Insect repellents, such as DEET, are not always effective but may provide some relief.

Jeffrey Hahn, Extension entomologist

Reviewed in 2019

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