Do your potted plants have fungus gnats?

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Potted plant
Potted houseplant. Photo: Tiffany Woods
Last Updated: 
December 29, 2006

CORVALLIS - Ever notice those irritating little insects that fly out of the soil when you water your potted indoor plants? You are probably seeing fungus gnats.

Tiny, non-biting flies, fungus gnats feed and breed in the soil of house and greenhouse plants such as cyclamens, poinsettias, salvia, geraniums and ornamental peppers, explained Amy Dreves, an Oregon State University entomologist.

Fungus gnats thrive in soils rich in organic material and moisture, where the larvae feed and the adults reproduce. If conditions are especially moist and fungus gnats are abundant, you may notice tiny, narrow slime trails on soil surfaces that resemble slug or snail trails but are much smaller.

Found outdoors as well, fungus gnats breed in decaying plant matter, leaf mold, organic mulch, grass clippings, compost and fungi.

These persistent gnats live short, prolific lives. An adult fungus gnat lays up to 200 eggs during the one week it spends as an adult winged gnat. The adult then dies. Legless larvae hatch out in about four days. After two weeks of feeding, larvae spend about three to four days as pupae before emerging as the next generation of adults.

Most fungus gnats are merely annoying and cause relatively little damage to plants. But several species of fungus gnat larvae feed on roots, causing damage to African violets and other houseplants. Others burrow into stems and leaves. Fungus gnat-damaged plants may show signs of wilting. Gnat damage may weaken plants and make them susceptible to root rots.

But they aren't all bad. Fungus gnats are important pollinators, help decompose decaying organic matter and are used as food by other small animals including birds, reptiles and beneficial insect predators.

To discourage fungus gnat infestations in your potted plants, Dreves offered some suggestions for prevention, exclusion and suppression:

Do not over water your potted plants. Let soil dry thoroughly to destroy breeding habitat. Provide good drainage.

Eliminate natural habitat in your house and yard. Eliminate irrigation system leaks and piles of decomposing organic matter. These are favorite hangouts for fungus gnats.

Screen and caulk leaky windows and doors to prevent indoor entry. Do this before August, as this is the time when they seem to be drawn back indoors.

Place yellow sticky traps horizontally on soil surface to trap gnats. These are available through your local garden store or catalog.

Apply beneficial nematodes (Steinernematids or Neoaplectanids) to control larvae. See the "PNW Insect Control Handbook," biological control system for listing of suppliers, or call your local Master Gardener help desk at the closest county office of the OSU Extension Service for this information.

B.t.i., (Bacillus thuringiensis subspecies Israelensis) is a naturally occurring disease for the fungus gnats and other flies. B.t.i. can be purchased at some nurseries.

Pyrethrin insecticides can be applied to soil surface. Remember to read the label and take proper precautions for using an insecticide.

To see pictures of fungus gnats, Dreves suggests:

Author: Carol Savonen
Source: Amy Dreves