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Black Flies (Simuliidae)

- Tiny biting flies that occur near flowing water -

Summary: Black flies are small biting flies that develop in streams, rivers or other flowing water sources. They can be extremely abundant and annoying.

Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)

What are black flies?

Black flies are small (1-3 mm), usually dark colored, biting flies with a distinctive "humpbacked" profile (right). Immature, or larval, black flies develop in fast flowing streams, rivers and creeks where they cling to the undersides of rocks or debris. A few species breed in relatively slow moving water.

Black flies are very persistant and fierce daytime biting flies. The bite site often bleeds profusely because of a strong anticoagulant in the saliva of the fly. An itchy skin lesion usually develops as well. Like mosquitoes, black fly females require a blood meal in order to lay healthy eggs.

black fly
tiny black fly adult ~ 1-3 mm
common names - buffalo gnats, humpbacked flies; misspellings - black flys, blackfly

How can I avoid black fly bites?

Black flies can be present in enormous swarms that attack people and livestock near flowing streams and rivers.

It is impractical for individuals to control black fly larvae in the fresh water streams where larvae live. The best approach is to use insect repellents when your activity takes you near their habitat. Repellents are not always effective but are worth trying.

Black flies are normally encountered near flowing water. However, some species are able to travel on wind currents and thus occur some distance from a suitable breeding habitat.

Propane-powered mosquito traps may be useful for black fly control especially if your home is near a fresh water stream where black flies breed.

Black flies are not carriers of human diseases in the US but in some tropical regions of the world a very serious disease called onchocerciasis (river blindness) is transmitted by certain species. Black flies can be found almost anywhere you have permanent flowing water. Even small seeps and springs can support black fly breeding. We have found black flies in Death Valley National Park, one of the driest places on earth.


Mission: To provide accurate, up-to-date and unbiased information for solving common insect and mite problems around your home, business and landscape using least-toxic methods.

Jack DeAngelis, Ph.D.

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