Black flies abound in Iowa right now

Quick facts

  • There are many species of black flies with the blood feeding females preferring some hosts over others. 
  • Black flies are also commonly called buffalo gnats
  • In Iowa there is only one generation per year of the most problematic species with flies active for about 3 weeks from late May to early June.
  • Larvae live exclusively in flowing water
  • Standing water and spring flooding do not effect current population size as eggs for flies occurring now were laid last spring.

About 33 species of black fly in North America have been recorded to be a nuisance to humans either by biting or swarming around our faces when outdoors.  Bites are often not very painful, but some people react to the bites and will experience itching. 

I have not received any samples of black fly in order to determine what species is causing a problem for humans now, but it is likely Simulium venustum, a species that commonly occurs in large numbers and bites humans.  However, it is likely there are currently several species causing problems to humans, livestock and wildlife across Iowa.  In the Iowa black fly adults are generally present by late-May to early June depending on temperatures. The adult flies are usually present for about 3 weeks before they die.

Female S. venustum travel on average of 5-8 miles from where they developed as larvae in running water.  They fly within 2 days of emerging from their pupae and are capable of traveling further in search of food.

Only female black flies take blood meals.  They need the nutrients in blood to develop eggs.  Black flies only feed on warm blooded vertebrate animals. Some species prefer to feed on humans, some on other mammals, and others prefer birds.  There are no known human diseases carried by black flies in the United States.

A picture of a black fly on human skin
A black fly to scale on human skin.

Females locate hosts visually and by smell.  They are attracted to the odors of sweat and carbon dioxide we exhale.  Simulium venustum prefers to land on darker colored surfaces so avoid wearing darker colored clothing, particularly dark blue.  They are less attracted to yellows, whites and brighter colors. 

Black flies can cause death of livestock and poultry and are capable of transmitting disease.  Large enough black fly populations can cause deaths of chicken and other poultry, presumably due to blood loss and toxemic shock.  Check animals frequently during a black fly outbreak.

Products containing Bacillus thuringiensis israelensis (Bti) have been shown to control larval black fly, but treatment must  be done as part of an area-wide management program.  There are no practical treatments for adults.  Fogging, like is used against mosquitoes, has not been shown to be effective.  

Mosquito repellants can provide some relief to humans, but often do not work as well as they do against mosquitoes.  We receive numerous reports that various kinds of essential oils or vanilla extract help repel black flies; the effectiveness of these remedies varies greatly from individual to individual.

Fans can reduce black fly numbers in the limited area where the air is moving. 

Consult with your veterinarian about products available for livestock and poultry.

Immature Stages

Depending on the species of black fly females will lay eggs either while flying over flowing water or on wet rocks and other bits of substrate just above the water line where eggs are quickly washed into the water and settle into the sediment at the bottom..  Most black fly species are capable of producing 150-600 eggs.  If kept moist down in the mud eggs may be able to survive short term drying of the water source, but overall they need constant water.

Simulium venustum eggs overwinter and begin hatching in April.  Females of this species typically carry 200-500 eggs and lay eggs on the surface of flowing water by tapping their abdomen on the water surface

Black fly larvae live in shallow, clear, water in rivers, streams, swamp seepages, seepages and basically anywhere that water is flowing.   The spindle-shaped larvae live on the bottom attached to rocks and other submerged objects with silken pads the larvae weave.  Larvae are filter feeders and feed on organic matter flowing by them in the water.  Larvae can develop in just a few weeks depending on water temperature.

Larvae spin a cocoon in a suitable location under water.  There is not much research on what is a suitable location but it probably depends on water flow and other factors.  The larva cleans off a surface prior to spinning a cocoon.  Once development is complete the adult flies release all air in their respiratory system which breaks open the pupal case and the new adult fly rises to the water surface in a bubble of gas.

If you would like to know more about black flies I highly recommend the book ‘The Black Flies (Simuliidae) of North America by Peter H. Adler, Douglas C. Currie, and D. Monty Wood.  I have found it available for sale at very affordable prices. 


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