Fruit Flies, Drain Flies Or Gnats? Find What Is Breeding Them

A small fly in the kitchen is the end of a process, not the start of one. Fruit flies, drain flies, fungus gnats and phorid flies each develop in a different wet place in or under the house, and killing the adults in the air changes nothing about the place they came from. Find the breeding site and the flies stop on their own.

October is when the kitchen half of this peaks in Northeast Ohio. The end of the garden lands on the counter at once, tomatoes and peppers and squash, apples from the orchard, cider in the fridge, and the compost bucket fills faster than anyone empties it. All of it ferments, and fermenting is the signal these flies are built to find.

Which Small Fly Is It?

Four small flies cover most of what shows up indoors. They look alike at arm's length and they mean completely different things, so the identification is the whole job.

Why The Fruit Flies Outlast The Fruit

Because the fruit was never the only site. Ohio State University Extension lists what draws them indoors: overripe fruit and vegetables, baked goods containing yeast, garbage cans, and beverages including fruit juice, cider, soft drinks, beer, wine and vinegar. A female lays about 500 eggs, and up to 2,000, singly near the surface of moist fermenting material, and those eggs hatch in 24 to 30 hours. One forgotten potato behind the bin can carry a household population by itself.

The site people miss is not food at all. Iowa State University Extension names the most likely spot in a house as a slow moving or seldom used sink, bathtub, shower or floor drain in which a layer of slime has built up above the water line. Fermenting material does not have to look like food to a fly. It has to be wet and organic.

So the search list is longer than the fruit bowl:

• Under and behind things. A potato or onion rolled under a cabinet, a tomato at the back of a windowsill, fruit at the bottom of a bowl.
• The containers, not the contents. The recycling bin that holds rinsed cans and the kitchen trash can both grow a film in the bottom seam.
• Drains that sit idle. A basement floor drain, a spare bathroom, a laundry tub, a utility sink.
• Mop buckets, wet rags and the pet bowl. Anything standing wet with organic matter in it.
• Garbage disposals and dishwasher seals. The splash area under the rubber gasket is a classic.
• Cider, wine and beer. A sticky bottle in the recycling is a food source. So is a drip tray.

The Drain Test That Settles An Argument

You do not have to guess which drain. Iowa State University Extension describes the test: cover the drain with plastic film taped down at the edges, and if the flies are developing in that drain, the adults accumulate underneath the film within a day or two. Run it on every suspect drain at once and the building tells you where the problem is by morning.

Drain flies live their whole life in that film. Ohio State University Extension fact sheet ENT-41 puts the eggs, larvae and pupae in the muck and slime on the sides of drains and overflow pipes, with the eggs hatching in 32 to 48 hours at 70 degrees Fahrenheit, the larvae feeding on decaying organic matter, bacterial film, algae and sediment for 9 to 15 days, and new adults emerging from the pupae in 20 to 40 hours. The whole cycle runs one to three weeks and the adults live about two weeks, which is why a problem in a guest bathroom feels permanent. It is continuous rather than permanent, and it has one source.

The same fact sheet is direct about the fix: eliminating the breeding place is the most effective thing you can do, which means cleaning the pipe and the trap rather than treating the air above it. Iowa State is equally direct about what does not work, stating plainly that there is no benefit to treating drains with bleach or ammonia. Pouring something caustic down the middle of a pipe does not touch a film clinging to the sides of it.

• Scrub the inside wall of the drain and the trap with a stiff brush. That is the step that removes the breeding material.
• Where a brush will not reach, Iowa State suggests rinsing with water under high pressure, or pouring boiling water slowly down along the sides of the pipe.
• A bacterial drain product that digests the organic buildup is a reasonable follow up. Follow the label.
• Keep seldom used drains wet and in use. A dry trap also lets sewer odor and insects up the pipe.
• Treat every drain in the room, not just the one you saw a fly on.

If It Is The Houseplants, Nothing In The Kitchen Will Help

Fungus gnats are a soil problem that happens to fly. Ohio State University Extension describes adults emerging in numbers from potted plants and damp, humus rich soil, with larvae feeding mainly on fungi in organic rich soil. The same fact sheet notes the larvae can injure the roots of potted plants, and that damaged roots can open the door to root disease, so this is not only a nuisance for the plant.

The lever is water, not insecticide. The larvae need the top of the pot to stay moist, so letting that surface dry between waterings takes the habitat away. Watering from the bottom, emptying the saucer, and repotting a plant that has been soggy all summer do more than anything you can spray. Yellow sticky cards near the pots catch the adults and, more usefully, tell you whether the number is going down. Extension also notes that gnats sometimes come in from outside, in which case the work moves to the perimeter of the building rather than the windowsill.

When Small Flies Mean Something Bigger

One pattern is worth knowing, because the answer is a plumber before it is anything else. University of Maryland Extension states that extremely large numbers of phorid flies on the first floor of a building may indicate a broken drain or sewer line under the structure. Cornell University's integrated pest management program says the same thing and adds the hard part: adults can indicate breaks in a sewer pipe under a slab foundation, and confirming it takes specialized and often destructive investigation.

Cornell also describes how that investigation actually goes. With the utility and floor plans in hand, small openings are drilled in the floor and each one is covered with a clear plastic cup, so flies emerging from the space under the slab take the path toward the light and collect where they can be counted. That maps the source. Nothing else ends it, because the material feeding the larvae is soil under a slab, not a drain you can scrub.

The signals that belong in this category are a steady supply of flies downstairs with no kitchen source, flies that keep returning after the drains have genuinely been cleaned, a damp spot or a sewer smell near a floor, and numbers that are not in proportion to anything you can see. Everything else on this page is usually a bowl, a bin, a trap or a pot.

What A Professional Visit Actually Does Here

It finds and removes breeding sites, which is the opposite of fogging a room. Our flying insect control work is built that way: drain treatments where the film is the source, light traps to monitor and reduce adults, and exterior applications where the pressure is coming from outside. In a bar, a cafe or a commercial kitchen those sources are predictable enough to put on a schedule, which is the thinking behind our hospitality pest control program. Our walkthrough of a restaurant pest control inspection covers where an inspector looks for the same sources.

Worth saying plainly for homeowners: small flies are not on the State Shield Essential list, which covers ants, spiders, millipedes, centipedes, cockroaches, mice, rats, bees, wasps and seasonal invaders across four visits a year. A fly problem is handled as its own job with its own free estimate. If you are weighing a recurring plan for the rest of that list, State Shield Essential is the page to read.

One last identification note. Small flies and young cockroaches both show up around drains, and they are not remotely the same problem. If what you are seeing runs rather than flies, read our comparison of German and American cockroaches before you treat anything, or go straight to cockroach control.

State Termite has worked Stark, Summit, Portage, Wayne and Tuscarawas counties since 1933. If the flies have outlasted a week of honest cleaning, tell us which room and which fixture and we will tell you what is likely feeding them: get a free estimate or call 330-494-6199.