Black flies are also called buffalo gnats because the very tiny fly (5-15 mm) has a humped thoracic region that might look like the hump on a buffalo / bison. Black flies are true flies (Order Diptera) and classified in the family Simuliidae. I learned, during the 2018 black fly season, from a University of Minnesota Extension entomologist that there are 30 species of black fly in Minnesota. The Purdue Extension website page about black fly notes that Illinois has 12 species of black fly while Minnesota and Wisconsin have 30 species because they have better black fly habitat. Iowa likely has somewhere between 12 and 30 species of black fly.
Why does a black fly bite? The female black fly needs a blood meal. After that meal, the female lays between 200-500 eggs in one batch. Most species lay their eggs in a flowing water environment, with eggs on rocks, concrete stabilization structures, driftwood, and even aquatic plants. Black flies have a complete metamorphosis cycle with adult to egg to several stages of larvae to pupae that emerges as an adult fly to start the cycle over. Depending on the black fly species, there can be more than one generation per year. The male black fly does not bite animals. Both male and female black fly will rely on nectar for energy for flight. A black fly can travel upwards of 10 miles (further on air currents) from the hatch site.
When do they bite? Just like mosquitoes, black flies are most active during the dawn and dusk time. Black flies do not like windy places but are often found in low-lying areas, places with dense vegetation, and shaded areas. Black flies do not enter structures like a house. Black flies (and horse and deer flies) use their scissor-like mouth parts to cut the skin and then lap up the blood. The bites can cause swelling, itching, and the black fly inject an anticoagulant that can make animals shocky (the circulatory system cannot maintain adequate blood flow, limiting oxygen and nutrients being delivered to organs).
Why do black flies swarm the head? Black flies are attracted to carbon dioxide. Dark colors seem to be more attractive to black flies as well.
Why are black flies a problem in some areas, but not others?
Black fly populations are a springtime issue and populations will increase after heavy rain, the melting of a heavy snow pack, and sustained flooding. When questioning why black flies are here but not several counties away, you need to consider what happened up the watershed! The UMN Extension entomologist also noted that, “If streams experience prolonged flooding, especially later in the summer, I can count on large black fly populations the next spring if water levels are somewhat high.”
- In 2018, southern / southwest Minnesota and northwest Iowa had significant snow this past winter and north-central Iowa has received heavy rains in the spring… those weather events meant more available aquatic habitat for the life cycle of the black fly.
- SOAR admitted seven hatch-year 2018 eagles from Dubuque, Woodbury, Sioux, Tama, Marion (2), and Winneshiek counties. A total of 17 hatch-year 2018 (bald eagles, red-tailed hawks, barred owls, and broad-winged hawk) were admitted in 2018.
- In 2019, south-central / southeast Minnesota plus north-central and northeast Iowa had significant snowfall and rain.
- Through 15 June 2019, SOAR has admitted six hatch-year 2019 bald eagles and one red-tailed hawk all from northeast Iowa. SOAR also learned that a non-releasable bald eagle on display in Clayton County, Iowa was also impacted by black fly.
Can anything be done to control the black fly population?
We wonder if when there is a large outbreak of one thing (black fly in this case) that there is an imbalance in the ecosystem somewhere.
Black fly larvae and pupae predators include fish and dragonfly larvae. Dragonflies are a major predator on both the larval and adult stages. Maybe there are some plantings that could be done to attract dragonflies.
Are ecosystems missing healthy populations of insect predators. Many animals will gladly eat flying insects including bats, swallows, and purple martins. Would adding bat roosting houses help? What about a purple martin house?
Many ask if there is an insecticide that could be used to treat an area for black fly? Introducing anything new to the environment will have ripple effect throughout the food chain. Remember DDT? Enhancing and improving existing natural habitat may be a slow go but a better long-term help.