Fungus Gnats: Do You Have Them?
This article was submitted by Mary Jane Frogge, Extension Associate, University of Nebraska Cooperative Extension in Lancaster County. The article appeared in the December 1995 NEBLINE Newsletter and was updated October 1998.

Have you ever noticed tiny, dark, flying insects in your house, especially around your houseplants? They may be fungus gnats.  

Adult fungus gnats are delicate, gray, dark-gray, or black fly-like insects about 1/8 inch long. They are often seen running over the soil surface of houseplants, especially around wet areas. They also are seen as you water, when they swarm up out of the plant. Fungus gnats are attracted to light and, in a severe infestation, will swarm over the windows. Adult fungus gnats do not damage plant materials but are a nuisance to the homeowner. The immature fungus gnat lives in the soil and are white, translucent larvae with shiny black heads. The larvae feed on any organic matter and can attain a length of about 1/4 inch.  

Female fungus gnats lay up to 300 eggs on the soil surface which hatch in five to six days. The larvae will feed on any organic matter present in the soil for 10 to 14 days. The pupal stage occurs in a silky chamber in the soil. Three to four weeks are necessary to complete the life cycle of the fungus gnat. Soils containing large quantities of decaying vegetable matter are most likely to host fungus gnat larvae. Plants grown in a growing media containing a high percentage of organic matter such as peat will have more problems with fungus gnats.  

Fungus gnat larvae cause damage to the root systems of infested plants by burrowing in the soil and feeding on the roots and sometimes the crowns of plants. Seedlings, rooted cuttings and young plants can be severely damaged or killed by fungus gnat larvae feeding on root hairs or roots. Fungus gnat larvae infestations on older, established plants are characterized by little new growth and foliage that appears to be off color. A severely infested plant may also drop foliage. While damage from the larvae may not be extensive for the established plant, the presence of the adults is considered intolerable.  

Once fungus gnat adults and larvae are discovered on the plant, control and prevention come in several forms. For infested plants, allow the soil to thoroughly dry between waterings. This will kill the larvae through desiccation as well as help prevent future problems. If the plant is of a type that cannot be allowed to dry out, drench the soil with an insecticide registered for this type of use. Be careful to properly measure and apply insecticides according to label directions for use in a house and take care to provide adequate ventilation.  

The best way to prevent new or future infestations of fungus gnats is to follow proper watering practices for your plants. Houseplants in the winter normally do not require as much water as at other times of the year. Fungus gnats are more of a problem in the winter, most likely due to overwatering. Whenever possible, allow soil surfaces to dry completely between waterings. A wet environment is an open invitation to fungus gnats as well as a host of other problems such as root rot and stem rot.

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