Fungus Gnats

- Small, dark flies that develop in soil -

Summary: Fungus gnats are small, dark flies that can be important pests in greenhouses and indoors on potted houseplants. They are also pests of commercial mushroom production. Larvae damage plants by feeding on roots and the adult flies can be a nuisance.

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

Identification of fungus gnats

Fungus gnats (right) are small (~1/8"), dark, primitive* flies that often plague potted houseplants in homes, greenhouses, and commercial mushroom growers. Fungus gnat larvae live in soil where they feed on soil fungi and plant roots. Full grown fungus gnat larvae are about 1/4" long and have a very distinct, dark head capsule (lower right).

Fungus gnat larvae damage plant roots

Adult flies can be a nuisance when present in high numbers but the damaging stage is the larvae that feed on plant roots. Fungus gnat larvae can do significant damage to young plants that don't yet have a developed root system. They are most often a problem in greenhouses and potted house plants.

Fungus gnats may have many generations per year so populations can build quickly under favorable, warm conditions. The best, and safest, way to mange fungus gnat damage is to monitor for adult flies using yellow-colored sticky traps and if an infestation is found you can treat soil with entomopathogenic nematodes (see below for details).

* Primitive flies include gnats, midges and mosquitoes. These are generally small (except crane flies), delicate flies with long antennae. The "higher" flies, like house flies, have short antennae and tend to be larger and more robust.

fungus gnat
fungus gnat larva and pupa

Fungus gnat drawings (magnified). Adult fly (upper); larva (middle) and pupa (lower).

Get rid of fungus gnats with traps and nematodes

Fungus gnats can often be managed by simply lowering soil moisture. You should always allow the top inch (2.5cm) of soil in pots to dry out before watering. This will prevent root rot and help to manage many soil insect pests, including fungus gnats. If limiting soil moisture is not an option then use a combination of yellow sticky traps to trap adult flies and entomopathogenic nematodes to control larvae that live in the soil.

Using yellow sticky traps

Yellow sticky card traps (see Sticky Traps) can be used to trap for fungus gnats, aphids, thrips and whiteflies. Flying insects are attracted to the yellow color and become stuck on the glued surface.

When used alone, however, sticky traps probably won't eliminate an infestation so sticky traps should be combined with some type of soil treatment to control larvae. Plan to treat the soil when more than a few flies are trapped every day. Use 1-2 traps per 1000 square feet of greenhouse space or in homes. Replace traps when they become covered with fungus gnats, other insects or debris.

Using entomopathogenic nematodes

Entomopathogenic nematodes (also called "Beneficial Nematodes") are tiny worms that can be applied to soil that seek out and destroy the larvae of certain insect pests such as fungus gnats. They are generally mixed in water and poured directly on the soil surface. There are different types of nematodes that each target a different group of insect pests; for fungus gnats try Steinernema carpocapsae (see Using Beneficial Nematodes).

Fungus gnat "mass migration" behavior

Fungus gnats also occur outdoors in rich organic soils. Occasionally, usually following heavy rains, large numbers of larvae will be seen moving in clumps or "ropes" across sidewalks and driveways. These migrations are harmless and will disappear as the surrounding soils dry out.

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