Fungus Gnat (Sciaridae)

What are fungus gnats?

Fungus gnats (right) are small (~1/8"), dark, primitive* flies that often plague potted houseplants, interior plantscapes, greenhouses, and commercial mushroom growers (sciarid flies). 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 houseplants.

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. If an infestation is found you can treat soil with a low-toxicity microbial insecticide (see below for details).

* Primitive flies are the 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.

Get rid of fungus gnats with traps and microbial insecticides

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 and microbial insecticide to control larvae that live in the soil.

Using yellow sticky traps

Yellow sticky card traps can be used to monitor for fungus gnats, aphids, thrips and whiteflies. Adult flies are attracted to the yellow color and become stuck on the glued surface.