How to Get Rid of Fungus Gnats in Houseplants for Good

The Short Version

To get rid of fungus gnats in houseplants, dry out the place where they breed and kill the larvae in the potting mix. Let the top 1 to 2 inches of mix dry between waterings, set yellow sticky traps to catch the adults, and drench the mix with a product containing Bacillus thuringiensis subspecies israelensis, known as Bti, several times at five to seven day intervals. Keep going for a full life cycle, which extension sources put at three to four weeks.

Swatting the flies does little on its own, because the flies you see are a small part of the population. University of Wisconsin Extension reports that a female lays up to 200 eggs in cracks and crevices on the surface of moist soil, and that adults survive only about one week. The eggs, larvae and pupae stay in the pot and produce the next wave.

Make Sure They Are Fungus Gnats

Adults are delicate, dark brown or black flies about 1/8 inch long, with long legs and long antennae. Colorado State University Extension points to a distinct Y-shaped pattern on the forewings. UC IPM gives the common size range as 1/16 to 1/8 inch and says the adults stay near potted plants, running across or resting on the growing media and foliage. Because they are attracted to light, you may first notice them flying near windows.

The larvae live in the mix. UC IPM describes them as having a shiny black head and an elongated, whitish to clear, legless body. Wisconsin gives their length as 1/4 inch when mature.

The adults are a nuisance and nothing more. They do not damage plants or bite people. The larvae feed in the potting mix, and UC IPM notes that when present in large numbers they can damage roots and stunt plant growth, particularly in seedlings and young plants. Colorado State rates the larvae as minor pests of established houseplants, so a few gnats around a large, healthy plant call for a change in watering before anything stronger.

The Life Cycle and Why One Treatment Fails

Fungus gnats pass through four stages, and three of them are hidden in the pot. The timing depends on temperature, which is why the published figures differ a little.

Stage	UC IPM, at 75°F	University of Wisconsin Extension
Egg	Hatches in about 3 days	Hatches within three to six days
Larva	About 10 days	Four growth stages over one to two weeks
Pupa	About 4 days	Four to five days
Adult	Not given	Lives about one week
Egg to adult	About 17 days	Three to four weeks

Colorado State’s figures run a little longer for the larva, two to three weeks, with an adult life span of seven to 10 days and the same three to four week total.

UC IPM notes that fungus gnats have many overlapping generations each year, so a pot can hold eggs, larvae, pupae and adults at once. Bti acts on the larvae, and UC IPM adds that it does not reproduce or persist indoors. Eggs that hatch after a drench meet no treatment, which is the reason the sources recommend repeating it.

The Routine That Ends an Infestation

The routine has five parts. The first two cost almost nothing and should start at once.

Let the surface dry. Colorado State advises allowing the growing medium to dry between waterings, especially the top 1 to 2 inches. Wisconsin’s version is to let the top inch dry out before you water, or to water from the bottom so the roots get moisture while the surface stays dry. The method is covered in how to bottom water houseplants. UC IPM adds two related points: provide good drainage, and clean up standing water around the plants.

Trap the adults. Yellow sticky traps catch adults and show you whether numbers are falling. UC IPM suggests cutting a trap into smaller squares and attaching them to wooden skewers pushed into the pots. Colorado State places the cards underneath the plant canopy or on the edge of the container.

Test for larvae. Raw potato draws larvae out of the mix. Colorado State says to insert 1/4 inch slices or wedges of potato into the growing medium, and Wisconsin says to check the slices for maggots after three to four days. UC IPM notes that the potato also works as a trap: after a few days, remove the infested pieces, dispose of them and replace them with fresh ones. This test tells you which pots are the source.

Drench with Bti. Products containing Bti control the larvae when watered into the mix. Wisconsin recommends several applications spaced five to seven days apart. UC IPM explains why repeats are needed: Bti does not reproduce or persist indoors, so infestations in potting media might require repeated applications at about five-day intervals. Buy a product whose label lists fungus gnats and follow its mixing rate.

Cover the surface. Wisconsin suggests covering the soil with a 1/2 to 1 inch layer of coarse sand or fine gravel.

Other Larva Treatments: Nematodes and Predatory Mites

Bti is the easiest product to find, and it has two biological alternatives. The nematode Steinernema feltiae is described by Colorado State as particularly effective against fungus gnat larvae. UC IPM says it works when temperatures are between 60 and 90°F, a range that covers a normal living room. The nematodes are mixed with water and applied as a soil drench according to the label.

UC IPM also lists Hypoaspis, a light brownish predatory mite adapted to feeding in the upper layers of moist soil.

If Bti or nematodes are not available and the numbers are intolerable, UC IPM notes that pyrethrins or a pyrethroid insecticide may provide temporary, fast-acting control. Temporary is the important word, so use a spray only alongside the routine above. UC IPM’s instruction is to make sure the product is labeled for your use, such as house plants, and to read and follow its directions.

Keep Them From Coming Back

Prevention comes down to moisture and to what goes into the pot. UC IPM’s first instruction is to avoid overwatering and provide good drainage. Check the mix with a finger before watering, and empty saucers so pots do not stand in water. The gnats breed in moist mix, so a pot that keeps producing them is staying wet at the surface, and the steps for saving an overwatered houseplant cover what to check below the surface.

Be careful with organic matter. UC IPM warns against using incompletely composted organic matter in potting media unless it is pasteurized first, because it will often be infested with fungus gnats. It also advises storing pasteurized potting soil off the ground and in closed containers to prevent it from becoming infested. Fold over and clip an opened bag, or tip the rest into a lidded bin. If you save mix from year to year, the steps for reusing old potting soil apply indoors as well.

Leave a sticky trap in place near new plants and near any pot that has had gnats before. When a trap starts catching adults again, restart the routine promptly, since at 75°F a new generation can follow in about 17 days.