Fungus Gnats: How to Identify and Get Rid of Them

Fungus gnats are small, dark, mosquito-like flies whose larvae live in moist potting mix, and the adults are a nuisance rather than a danger to you or your plants. The fix every source agrees on is to let the soil surface dry out between waterings, and only if that is not enough, to add a biological control such as Bti.

Where the sources disagree, this guide says so. Fungus gnats are tied closely to watering, so read it alongside our guide to how often to water houseplants.

How to identify fungus gnats

Adults. Wisconsin describes delicate, dark brown or black flies about 1/8 inch long. Colorado State agrees on 1/8 inch and adds long legs and antennae. UC IPM gives a range of 1/16 to 1/8 inch. Under a magnifying glass, Wisconsin says the wings show a distinct rounded Y-shaped set of veins near the tips. UC IPM says the segmented antennae are longer than the head, and that they look similar to mosquitoes.

Larvae. UC IPM describes an elongated, whitish-to-clear, legless body with a shiny black head. Sources disagree on how big they get. Wisconsin says 1/4 inch when mature, and Clemson says as large as 1/4 inch. Mississippi State says mature larvae are about a half inch long. Colorado State says they are usually in the top 2 to 3 inches of the growing medium.

Where you notice them. Wisconsin says people usually notice adults flying around windows and lamps, or flying around, walking across and resting on the soil of potted plants. UC IPM says that in very wet conditions larvae can leave slime trails on the soil surface that look like trails from small snails or slugs.

Fungus gnats, shore flies and fruit flies

• Shore flies. UC IPM says fungus gnats’ long antennae distinguish them from the more robust shore flies, which are associated with algae and decomposing organic matter and have short bristle-like antennae.
• Fruit flies. Wisconsin says fungus gnats are often mistaken for fruit flies but are two distinct insects, and that the wing vein pattern separates fungus gnats from other small flies. Our sources give no other fruit fly features.

The fungus gnat life cycle

Wisconsin describes the life cycle as egg, larva (four stages, called instars), pupa and adult.

Eggs typically hatch within 3 to 6 days. Larvae go through the four instars over one to two weeks, then pupate near the soil surface. Adults survive approximately one week.

How long a generation takes. Wisconsin says egg to adult can take three to four weeks under optimal conditions, and Colorado State gives the same at room temperatures of 65°F to 75°F. UC IPM says a generation, counted from female to female, can be produced in about 17 days at 75°F, depending on temperature. Plan for a few weeks per generation.

They overlap. Wisconsin says indoors, eggs, larvae, pupae and adults are all present at the same time. Colorado State says reproduction is continuous year-round on indoor plants. This is why one round of anything rarely ends an infestation.

Why you have fungus gnats

Every source ties them to moisture and organic matter:

• Overwatering. Mississippi State says larvae thrive in growing media kept excessively moist and that overwatering is the main reason for heavy infestations. Iowa State says fungus gnats prefer moist soils, which makes them good indicators of overwatering.
• Peat-rich mix. Clemson says indoors they are most often a problem when the potting soil is rich in organic matter such as peat moss. Colorado State says moist media with a lot of peat moss are particularly attractive to adult females.
• Aging mix. Colorado State says that as growing medium ages or degrades it holds more moisture, which also attracts adults.
• Fall and winter. Colorado State says plants use less water then, so unless watering changes the medium stays moist.
• Decaying matter. Colorado State says containers with decayed bulbs and roots are an excellent food source for larvae. UC IPM says incompletely composted organic matter in potting media is often infested unless pasteurized first.
• Time outdoors. Colorado State says plants kept outside in summer are commonly colonized, and populations can increase once they come indoors.

Do fungus gnats damage plants?

The adults do not. UC IPM says larvae, when present in large numbers, can damage roots and stunt growth, particularly in seedlings and young plants. Wisconsin says high numbers may damage roots by feeding on root hairs or tunneling into the roots. Clemson says a heavily infested plant may lose leaves. Even so, Wisconsin says that in most situations they are a cosmetic problem. UC IPM adds that a wilting houseplant may not lack water, and that other causes of unhealthy roots are more common than larval feeding.

How to confirm you have fungus gnats

• Yellow sticky traps. Wisconsin says to place them near a plant’s leaves, and Maryland says to put them in the containers.
• The potato test, for larvae. Wisconsin says to place potato slices on the soil surface and check for maggots after three to four days. Colorado State says to insert 1/4 inch slices or wedges into the medium and turn them over to look for larvae on the underside. UC IPM says chunks of raw potato, cut side down and not the peels, are sometimes used.
• A look at the pot. UC IPM says visual inspection for adults is usually enough to tell you there is a problem, and to check the pot for excessive moisture and organic debris where larvae feed.

How to get rid of fungus gnats

Our sources order the options the same way: change the watering and the soil first, use a biological control next, and use insecticides last.

1. Let the top of the soil dry between waterings. UC IPM says to allow the surface of container soil to dry between waterings. Wisconsin says to let the top inch dry before you water, and Colorado State says the top 1 to 2 inches.
2. Take away extra moisture and food. Clemson says not to let water stand in the saucer. Wisconsin says to cover the soil with a 1/2 to 1 inch layer of coarse sand or fine gravel, which keeps the surface drier and makes it less attractive for egg-laying, or to water from the bottom so the roots get moisture while the surface stays dry. Colorado State says to remove any containers with decayed bulbs or roots. UC IPM says not to move infested plants to new areas where flies can emerge and infest other pots.
3. Keep traps up and watch the numbers. Keep the yellow sticky traps in place to see whether adult numbers fall. Our sources describe them as monitoring, not control.
4. Add a biological control if the gnats persist. Bti (Bacillus thuringiensis subsp. israelensis) products go on as a soil drench with enough water to reach the larvae (Wisconsin). They do not affect eggs, pupae or adults. Steinernema feltiae nematodes are named by UC IPM and Colorado State as the most effective nematode species against larvae. Details below.
5. Use insecticides last, if at all. UC IPM says insecticides are rarely warranted in and around homes, and Wisconsin says they are not warranted unless an infestation is severe. See below.

Biological control, in detail

• Bti repeat interval. UC IPM says repeat applications at about five-day intervals. Wisconsin says several applications five to seven days apart to control newly hatched larvae until the infestation is under control.
• Bti availability. The sources conflict. UC IPM says Bti products for fungus gnats are readily available at retail. Colorado State says Bti formulations sold for fungus gnat control are generally unavailable through retail outlets.
• Caterpillar-labeled products. Mississippi State says not to use Bacillus thuringiensis products labeled for caterpillar control, because they will not work on fly larvae.
• Nematodes. UC IPM describes them as live, perishable products that need immediate application.
• Predatory mites. UC IPM says Hypoaspis mites probably will not perform well in individual houseplants and are probably not a good choice for homes.

Chemical control, in detail

The sources do not agree on how soon to reach for insecticides. UC IPM says they are rarely warranted, and Wisconsin says chemical controls are not warranted unless an infestation is severe. Colorado State says insecticides may be necessary if problems persist several weeks after watering practices have been adjusted.

If you do use one, Wisconsin says to choose a product labeled for indoor use on houseplants and to follow all label instructions. The classes the sources name:

• Pyrethrins or synthetic pyrethroids for temporary control of adults. Colorado State lists bifenthrin, cyfluthrin, permethrin and lambda-cyhalothrin as active ingredients of pyrethroid-based products for houseplants. UC IPM says they may give temporary, fast-acting control if Bti or nematodes are not available and high populations are intolerable.
• Imidacloprid, a systemic insecticide, which Colorado State says kills larvae when applied to the growing medium.

How to prevent fungus gnats

• Let the soil dry. Our how often to water houseplants guide gives the rule for each of ten plants.
• Improve drainage. UC IPM says to raise the proportion of perlite or sand in the mix. Our repotting houseplants guide covers mixes.
• Start with clean mix. UC IPM says to use only pasteurized potting mix and store it off the ground in closed containers so it is not infested before use. Clemson says not to let water stand in saucers.

Our peace lily and monstera pages list fungus gnats among possible pests.

What does not work

• Caterpillar-labeled Bt. Mississippi State says it will not work on fly larvae.
• Vinegar traps. Wisconsin says vinegar traps and other methods used for fruit flies do not work for monitoring fungus gnats.
• Spraying adults. Colorado State says larvae in the medium are not directly affected by insecticides applied to kill adults. It also says short-persisting contact insecticides containing pyrethrins, soaps, oils and neem do not give sufficient long-term control of adults.
• Fogging. UC IPM says not to aerially fog indoors or try to spray adult gnats in flight.