Three flies commonly get called gnats indoors: fungus gnats from damp potting soil, fruit flies from ripe or fermenting food, and drain flies from the slime inside drains. This guide shows you how to tell them apart, find the source and clear it.
By The Homefield Journal Editors Reviewed
The word gnat covers several tiny flies, and the three you are most likely to find indoors breed in very different places, so identifying yours comes first. Fungus gnats are dark, delicate flies that look a little like mosquitoes. The University of California Statewide IPM Program puts most adults at about 1/16 to 1/8 inch long, with long segmented antennae and light gray to clear wings. They are attracted to light and often show up near windows first, and they tend to stay close to potted plants.
Fruit flies, also called vinegar or pomace flies, are about 1/8 inch long and usually have red eyes. University of Kentucky entomologist Michael F. Potter describes the front of the body as tan and the rear as black, and notes that small flies or gnats seen in a kitchen are probably fruit flies. They are especially common in late summer and fall, when ripe and fermenting produce draws them.
Drain flies are fuzzy, and University of Maryland Extension describes them as 1/16 to 1/4 inch long, pale brownish gray to black and found mostly in kitchens, bathrooms and basements near floor drains. Their larvae live in the gelatinous film inside drains and traps.
Each of these flies spends its early life as a larva in something moist, and UC IPM notes that the most effective control methods work on those immature stages. University of Maryland Extension suggests covering a suspect plant or drain with screen or cheesecloth and checking it over the next several days for flies. For drains and garbage disposals, the Kentucky fact sheet recommends taping a clear plastic food storage bag over the opening overnight, since any adults emerging from the drain will be caught inside. To check potting soil for fungus gnat larvae, UC IPM describes pressing chunks of raw potato into the soil with the cut side down. After a few days, lift the pieces and look for larvae.
Maryland Extension also lists sewer leaks, dirty garbage cans, saucers under potted plants and the condensate pans under refrigerators and air conditioners as places where drain flies can breed. Kentucky adds empty bottles and cans, trash containers, mops and cleaning rags for fruit flies, and notes that a single forgotten rotting potato or onion can breed thousands.
Fungus gnat larvae need constantly moist soil, and Maryland Extension notes that they die when the soil dries out. These steps follow UC IPM and Maryland Extension guidance:
When you repot, UC IPM recommends using only pasteurized potting mix and avoiding compost that has not fully broken down, because it is often infested with fungus gnats. At 75°F, UC IPM says fungus gnat eggs hatch in about 3 days, larvae take about 10 days to become pupae and adults emerge about 4 days later, so keep these habits up for a few weeks while the larvae already in the soil finish their cycle. For more care advice on indoor plants, browse our flowers and plants section.
The Kentucky fact sheet says a female can lay about 500 eggs and that the whole cycle from egg to adult can be completed in about a week. It also warns that spraying the adults leaves the problem in place as long as the breeding sites remain, so the work starts with the food:
The jar trap catches the adults that remain once the source is gone, and tight-fitting 16-mesh screens on windows and doors help keep more from coming in from outdoors.
Drain flies, and sometimes fungus gnats, breed in the slime that builds up inside drains and traps. Maryland Extension recommends cleaning traps and drains mechanically or with an over-the-counter drain cleaner followed by hot water. Mechanical cleaning means scrubbing the film out of the drain opening and the trap. If you use a drain cleaner, read the label and follow its directions exactly. Maryland Extension notes that adult drain flies may live up to 2 weeks and that development from egg to adult takes 7 to 28 days, so a few adults may keep appearing for a while after cleaning. Repeat the clear bag test afterward on each drain, including basement floor drains you rarely use, to confirm it has stopped producing flies.
UC IPM considers insecticides rarely warranted for fungus gnats in and around homes. If drying the soil and trapping are not enough, two biological options act on the larvae. Bacillus thuringiensis subspecies israelensis, usually shortened to Bti, is a naturally occurring bacterium sold in soil drench products, and UC IPM says it is toxic only to fly larvae. Bti does not reproduce or persist indoors, so potted plants may need repeated applications at about five-day intervals. The Bt sold for caterpillars is a different subspecies that is not effective against fly larvae, so check that the label names israelensis. Steinernema feltiae nematodes are the second option, and UC IPM notes that they work when conditions are moist and temperatures are between 60 and 90°F.
If gnat numbers are intolerable and those products are unavailable, UC IPM says pyrethrins or a pyrethroid insecticide can give temporary control when sprayed on the surface of the potting soil and on the plant parts where adults rest. It advises against fogging indoors or spraying gnats in flight, and suggests treating plants outdoors when possible. The Environmental Protection Agency explains that pesticide labels are legally enforceable and that every label states it is a violation of federal law to use the product in a manner inconsistent with its labeling. Choose a product labeled for your use, such as house plants, and follow its directions.
Prevention means keeping moisture and decaying material from building up where flies can breed. Water houseplants once the soil surface has dried, empty plant saucers and buy pasteurized potting mix. In the kitchen, refrigerate ripe produce, empty and clean trash and recycling containers, and wipe up juice spills. Clean sink and floor drains regularly and check appliance condensate pans. UC IPM also recommends screening and caulking leaky windows and doors to help keep pests from coming indoors. Adding these tasks to a seasonal home maintenance checklist keeps them on your calendar, and you can find more household routines in our cleaning and maintenance section.
When to call a pro
Gnats that keep returning after you have dried the soil, cleaned the drains and removed overripe food may be breeding somewhere you cannot see or reach, and that is the point to bring in a plumber, a licensed pest control company or your county Cooperative Extension office.
Remove the place where they breed, because the adults keep coming as long as larvae are developing. Dry out the potting soil for fungus gnats, discard or refrigerate ripe produce for fruit flies and clean the drain and trap for drain flies. Yellow sticky traps and a cider vinegar jar trap then catch the adults that remain.
Adult fungus gnats do not damage plants, according to UC IPM, and University of Maryland Extension calls them annoying but harmless indoors. Their larvae can damage roots and stunt growth when they are present in large numbers, particularly in seedlings and young plants. UC IPM adds that overwatering, root decay fungi and poor drainage are much more common causes of wilting.
The cider vinegar jar described by University of Kentucky Entomology is a trap for fruit flies, and it catches the adults left after you remove the produce or spill they were breeding in. For fungus gnats around houseplants, UC IPM and Maryland Extension use yellow sticky traps placed in the pots, together with drier soil.
Plan on at least one full life cycle after you remove the source. UC IPM puts a fungus gnat generation at about 17 days at 75°F, the Kentucky fact sheet says fruit flies can go from egg to adult in about a week, and Maryland Extension gives 7 to 28 days for drain flies.
Sources checked on November 2, 2026.
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