Small black flies in your house are almost always breeding somewhere indoors, which means killing the adults alone won’t solve the problem. The key is finding and eliminating their breeding site, then trapping the remaining adults while they die off. Most infestations clear up within one to two weeks once you cut off the source.
The term “black flies” indoors usually refers to one of three insects: drain flies (tiny, fuzzy-winged flies that hover near sinks and showers), fungus gnats (slender flies that swarm around potted plants), or small house flies. Each breeds in a different spot, but the approach is similar: remove the wet, organic material where larvae develop, trap the adults, and seal entry points so new flies can’t get in.
Identify Where They’re Breeding
Black flies don’t just wander into your house in large numbers. If you’re seeing a steady stream of them, they’re reproducing inside your home. The most common indoor breeding sites are drains and potted plant soil, but any spot where moisture and organic matter sit undisturbed can become a nursery.
Drain flies lay eggs in the slimy organic buildup inside pipes. They favor drains that don’t get much use, like a guest bathroom shower or a basement floor drain, where water collects in the trap and sits for long periods. Larvae feed on the soap scum, algae, fungi, and bacteria coating the inside of those pipes. Sink drains, toilets, tub drains, and leaky shower pans are all potential sites.
Fungus gnats breed in damp potting soil. If you overwater your houseplants or your pots lack good drainage, the top layer of soil stays moist enough for larvae to thrive. You’ll typically notice these flies hovering right around your plants or landing on the soil surface.
Other overlooked breeding spots include garbage cans with standing water at the bottom, wet compost bins, water collecting beneath leaking pipes, and refrigerator drip pans that haven’t been cleaned. Check all of these before you start treating.
A Simple Test for Drains
Place a strip of clear tape over a suspect drain at night, sticky side down, leaving enough gap for air to flow. If flies are emerging from inside the pipe, you’ll find them stuck to the tape in the morning. Test each drain in your house separately so you know exactly which ones to treat.
Eliminate Larvae at the Source
For drain flies, the goal is removing the organic sludge inside the pipe where larvae live. A stiff drain brush pushed down into the pipe and rotated does more than any chemical pour. Scrub the inside walls of the drain, then flush with boiling water. Enzyme-based drain cleaners (not chemical drain openers) can help break down remaining buildup over several days. Run water through rarely used drains at least once a week going forward to keep the trap full and the pipe flushed.
For fungus gnats in potted plants, let the top inch or two of soil dry out completely between waterings. The larvae can’t survive in dry soil. For a faster fix, a naturally occurring soil bacterium called Bti is toxic to fly larvae (including fungus gnats and black flies) but harmless to plants, pets, and people. Mix it with water and drench the soil, following the product label. Because Bti doesn’t persist indoors, you may need to reapply every five days until the infestation clears. A layer of coarse sand or decorative gravel on top of the soil also discourages adult flies from laying new eggs.
For garbage cans and other standing water sources, the fix is straightforward: dump the water, clean the container, and keep it dry. Fix any leaking pipes that create puddles beneath cabinets or in crawl spaces.
Trap the Remaining Adults
Even after you eliminate the breeding site, adult flies already in your house will keep buzzing around for a week or more. Traps speed up the process and help you gauge whether the population is shrinking.
The simplest DIY trap takes about 30 seconds to make. Pour a quarter cup of apple cider vinegar into a jar, add a few drops of liquid dish soap, and cover the top with plastic wrap secured by a rubber band. Poke several small holes in the plastic with a fork. Flies enter through the holes, and the soap breaks the surface tension of the vinegar so they sink and drown. A paper cone inserted into the jar opening works the same way. Place traps near the areas where you see the most flies.
Sticky traps are another reliable option. Yellow sticky cards placed near potted plants catch fungus gnats effectively because the color attracts them. Window-mounted sticky strips catch flies drawn to light. These are non-toxic and safe around kids and pets, which makes them a better fit for kitchens and living spaces than chemical sprays.
Keep New Flies Out
Standard window screens have 18 openings per inch, which is fine for house flies but lets smaller insects like gnats and midges pass right through. If tiny flies are entering from outside, upgrading to a 20×20 or finer mesh screen blocks them. You can find DIY screen kits at most hardware stores and cut them to fit your window frames, sealing gaps with silicone.
Beyond screens, check for gaps around doors, especially sliding doors and garage entries. Weather stripping that’s worn or peeling creates easy access points. Make sure your bathroom and kitchen exhaust vents have intact screens on the exterior side as well.
Using Sprays Safely Indoors
Pyrethrin-based aerosol sprays will knock down adult flies on contact, but they don’t address larvae and they introduce chemicals into your living space. If you use one, take precautions: remove pets, pet bedding, food bowls, and children’s toys from the area before spraying. Keep pets out of treated rooms until the spray has dried completely or for whatever time the label specifies, whichever is longer. Cover fish tanks, since aerosol mist can enter the water through the tank pump. Turn off central heating or air conditioning so the system doesn’t circulate pesticide through the rest of the house, and ventilate the room well while the product dries.
Toxic residues can remain on treated surfaces even after drying, so wipe down counters and surfaces in food preparation areas before using them again. For most home infestations of small black flies, traps and source elimination work well enough that sprays aren’t necessary.
Timeline for Results
Once you remove the breeding source, you should see a noticeable drop in fly numbers within three to five days. The full lifecycle of most small indoor flies is about two to three weeks, so expect stragglers for up to that long. If the population isn’t declining after a week, you likely haven’t found the main breeding site. Go back and recheck drains, plant soil, and hidden moisture sources. Persistent infestations that don’t respond to these steps sometimes trace back to a broken sewer line or moisture problem inside a wall, which may need a plumber or pest professional to resolve.