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Fungus gnats in homes

Jeff Hahn

fungus gnat larvae

Photo credit: Mike McDonough

Fungus gnat larvae

Identifying the fungus gnat

The darkwinged fungus gnat (family Sciaridae) is a common indoor fly. Usually referred to simply as fungus gnat, this mosquito-like insect is a very small, about 1/16th inch long, slender dark-colored fly with very long legs. If you examine a specimen with a dissecting microscope, look for the tuning fork-like veins in the wing which help identify it. Adult fungus gnats are weak fliers and are usually found fairly close to where the larvae are developing.

The larva is a very small worm-like insect, growing no more than 1/4 inch long. It is a pale white almost translucent color. It has a black head but has no legs or other appendages. The larvae live in very damp conditions where they feed on decaying plant material, moist organic matter, and fungi.

Where to find fungus gnats

Fungus gnats in buildings are nearly always found with houseplants, particularly overwatered ones, where they feed primarily on decaying or damaged roots. They also can be found in houseplants growing in potting soil high in organic matter such as peat.

Fungus gnats rarely feed on healthy roots. They do not usually injure houseplants and are considered to be only nuisances. These flies may indicate certain plant diseases, such as Pythium aphanidermatum, the organism that causes damping off. This is typically a problem for commercial greenhouses, although homeowners who grow bedding plants could also have issues.

In rare situations, fungus gnats may develop inside the walls of buildings where the moisture is high enough to allow fungus to grow. This is most likely in new construction where the green wood still possesses a high amount of moisture. As the wood dries, the fungus gnats go away on their own.

It sometimes can be difficult to tell where the flies are coming from. One homeowner described flies buzzing around several rooms and flying into people's faces. A sample revealed the flies to be fungus gnats. The homeowner did have houseplants in their home giving them the likely source of the problem.

fungus gnat adult

Adult fungus gnat

In another case a homeowner found a fairly large number of flies around a sliding glass door. The homeowner believed the flies were coming from the outside during November. It is common for fungus gnats to be found around windows. Again, a sample from the home contained fungus gnats and the source of the problem turned out to be houseplants.

Controlling fungus gnats

The first step to control fungus gnats is to reduce the soil moisture of your house plants. Allow the soil surface to dry between watering, but do not allow plants to wilt. You can tell when your plant needs more water by picking it up and feeling how heavy it is. With a little practice you can get a sense of the weight of the plant when it needs water. Also, watch the color of the leaves. When most plants are ready to be watered, the foliage will be a bit dull and less lively.

The only effective product for treating fungus gnat larvae in the soil is a bacterial insecticide, Bacillus thuringiensis var. israelensis, (e.g. Knock-Out Gnats). Also known as B.t. H-14, this insecticide is specific to fly larvae. However this product does not kill adult fungus gnats. Look for this insecticide in garden centers or order environmentally friendly products online.

You can place sticky paper traps in the pots to help tell you which plants are infested with fungus gnats. The sticky cards can help reduce the number of adults, but you cannot completely eliminate the problem just with these traps.

There are insecticides that are effective against adult fungus gnats, such as pyrethrins. However, they are only a short-term solution. As long as there is a favorable site for the fungus gnat larvae to develop, adult flies will continue to be present despite treating the adults repeatedly.

An untested home remedy is to place a 1-inch layer of sand on top of the soil to prevent egg laying or larvae from reaching the soil. However, professional growers do not use this method and it is not likely to be very effective.

Originally published in Yard & Garden Line News, Volume 7, Number 1, January 1, 2005

Revised July 2016

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