What Are They?
Fungus gnats are common pests of potted plants. The adults are tiny, mosquito-like flies. They don’t bite, but can be nuisances flying about the house. Folks who keep potted plants near their computer or TV often notice them flying near the monitor.
The larvae feed on fungi and organic matter in potting media and they sometimes damage plants by feeding on root hairs and roots. African violets and cyclamen are especially susceptible to this type of root pruning, but other potted plants can also be injured. Heavy root pruning can cause stunted growth. Root feeding by fungus gnat larvae can also spread or predispose plants to certain diseases.
Gardeners who keep potted plants outside during the warm months may begin noticing these small flies after they have brought their plants in for the winter. Fungus gnats are common in greenhouses, where they can build to high numbers. They also occur outdoors, but they don’t pose a problem there.
Fungus gnat larvae live in the upper few inches of media in potted plants where they feed on fungi, decaying organic matter and root hairs.
What to Look For
Adults are long-legged, weak-flying, mosquito-like flies that are only about 1/10 of an inch long. They are usually seen flying near potted plants. You can use yellow sticky traps to check for adults, but don’t expect them to provide control. The long, slender larvae have black heads and translucent white, legless bodies. Mature larvae are about one-quarter inch long. Check for larvae by embedding a half-inch-thick slice of raw potato in the surface of the potting media, waiting several days and then looking to see if any larvae have accumulated underneath the potato slice. Look carefully; individual larvae can be hard to see.
What to Do
You can use pyrethrin-based indoor aerosol sprays to control the adults, but this won’t solve the problem. You have to control the larvae that are developing in your potted plants. The easiest and best way to manage fungus gnats is to allow the soil in the upper part of the pot to dry between waterings. If heavy infestations persist, drench pots with insecticides containing the biological insecticide, Bacillus thuringiensis israelensis, (Gnatrol or Knock Out Gnats are two common brand names) or the botanical insecticide, azadirachtin, (Azatrol is one brand name available to home gardeners).
Greenhouse gardeners may prefer to use parasitic nematodes (Steinernema spp.) or mites (Hypoaspis spp.) to control these pests. These can be purchased from biological control supply houses. It’s OK – these are the kind of nematodes and mites that attack insects, not plants.
Avoid overwatering. This is the main cause of fungus gnat problems. Use only well-composted organic matter in your potting mix. Incompletely composted organic matter is more favorable to fungus gnats. Keep drain saucers clean; larvae can also develop in accumulations of organic matter in dirty saucers.