Fungus Gnat

Need to know

  • Fungus gnat larvae feed on fungi and organic matter usually in soil.
  • Fungus gnat adults do not bite humans. 
  • Fungus gnats can breed (lay eggs) anywhere that has moisture and decaying vegetable matter.
  • In homes, houseplants are the most common breeding area.
  • Managment required locating the breeding area and reducing the moisture.

Description of fungus gnats

Fungus gnat adult
Fungus gnat adult.

Fungus gnats are very common, small, non-biting flies that normally go unnoticed. They are completely harmless, except they are an annoyance by their presence. Fungus gnats are frequently quite plentiful outdoors in fungi, damp soil and decayed vegetable matter. Though fungus gnats occasionally wander in from outdoors, a persistent problem with this nuisance in the house indicates an indoor breeding site. The immature stage of the fungus gnat is a small white maggot that lives in very moist areas high in decaying organic matter.

Life-cylce of fungus gnats

Fugus gnats are able to breed indoors given the right conditions.  What does breeding indoors mean? It means means that adult fungus gnatsare mating; the females are finding a good place to lay their eggs with moisture and fungi; the eggs are hatching into larvae are feeding on tiny fungi and decaying organic matter and they grow. Once larvae are full grown they transform into pupa. The pupal stage is where the larvae transforms into the adult. Adult fungus gnats have wings and fly around the house, so this is usually what we human inhabitants of the house notice.

Damage caused by fungus gnats

Wing veination of fungus gnat on sticky card.
Fungus gnat stuck to sticku card.  Note the long antennae and the characteristic wing vein pattern.

Fungus gnats in homes do not cause damage.  The adults do not bite and are just a nuisance.  Larvae feeding on in the soil of houseplants do not harm the plant unless they are present in very large numbers.  In high numbers they can feed on the roots of seedlings and young plants and stunt their growth.  

Management of fungus gnats

The key to management is to find what the larvae are eating. It is no use trying to kill adult insects unless you can kill all females before they lay eggs. So, although the adults are irritating and what you actually see, you have to do some detective work to find the breeding area.  Things like sticky cards can alert you to the presence of fungus gnats, but only kill adults which will not solve the problem.

The breeding area must be constantly moist and have decaying organic matter.  This habitat may occur indoors with houseplants or in slow-running drains, moisture-accumulating cracks and crevices, refrigerator drain pans, and other places where fungi and slime accumulate.  

Houseplants are by far the most common source of fungus gnats.  Reduce watering and allow the soil to dry between waterings will reduce fungus gnat problems.  Repotting with fresh soil can also help eliminate fungus gnat issues. If you have seedlings in a tray or drainage trays beneath plants this can be another location where fungus gnat larvae can survive. 

Insecticides are not effective at managing fungus gnat infestations in household situations.  This includes sprays, granular insecticides that are watered in or dissolved and poured over the soil. 

Do you live in Iowa and have an insect you would like identified?

Fungus gnat larvae
Fungus gnat larvae. Photo by Adam Sisson.

The Iowa State University Plant & Insect Diagnostic Clinic will identify your insect, provide information on what it eats, life cycle, and if it is a pest the best ways to manage them.  Please see our website for current forms, fees, and instructions on preserving and mailing insects.   

Contact information for each states diagnostic laboratory for U.S. residents.  If you live outside of Iowa please do not submit a sample without contacting the Plant & Insect Diagnostic Clinic.   

Last reviewed:
January 2023

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on September 12, 2016. The information contained within may not be the most current and accurate depending on when it is accessed.