How to Get Rid of Drain Flies (And Keep Them From Coming Back)

Short answer: Drain flies breed in the biofilm — the slimy microbial layer — that coats the inside of drains and pipes. Killing adult flies with sprays or flushing drains with boiling water does not remove that biofilm, so new flies hatch within days. The durable fix is to dissolve the biofilm itself with a biological surfactant so there is no food source left for larvae. One ounce of Pure B.S. Bio Surfactant per drain, once per week, eliminates the breeding environment.

What are drain flies?

Drain flies (Psychodidae, also called moth flies, filter flies, sink flies, or sewer flies) are small dark winged insects, 1.5–5 mm long, that look like tiny fuzzy moths. They are weak fliers and usually stay in the same room where they hatched — most often kitchens, bathrooms, basements, utility sinks, floor drains, or commercial food-prep and restroom drains.

A female drain fly can lay up to 300 eggs in 48 hours. Larvae hatch in 32–48 hours and develop into adults within 8–24 days. Because the generation time is so short, a single unnoticed drain can produce a visible swarm within two weeks.

Why are drain flies in my house?

Drain flies come for one reason: a food source. They feed on organic matter in standing water and damp organic buildup inside drains. The most common breeding sites:

• Kitchen sink drains (grease, soap scum, food residue)
• Bathroom sink and shower drains (hair, soap, skin cells)
• Floor drains in basements and garages
• Rarely-used guest-bath drains where the P-trap has dried out
• Commercial floor drains, mop sinks, and grease traps
• Garbage disposals
• Septic vents and sump pits

If you only see flies in one room, the source is almost certainly a drain in that room — not an outdoor entry point.

Are drain flies harmful?

Drain flies are not vectors for human disease and do not bite. They are primarily a nuisance and sanitation signal. Because they live in biofilm — which contains mixed bacterial populations — they can mechanically transfer bacteria onto surfaces. In a food-service or healthcare environment, that is a failed health inspection waiting to happen. In a home, the reputational damage is the bigger issue: guests notice.

Why do home remedies usually fail?

The common advice — boiling water, bleach, baking-soda-and-vinegar, or insecticidal drain sprays — works on the symptom (adult flies) but not the cause (biofilm). Here is what actually happens with each:

• Boiling water flushes some loose debris but cools rapidly once it contacts cold pipes. It does not dissolve biofilm, which is a structured matrix of bacteria, polysaccharides, and lipids that has adhered to the pipe wall.
• Bleach kills surface bacteria but rinses out before penetrating the biofilm matrix. It can also corrode metal drains and neutralize septic biology downstream — if you are on a septic system, this trades a fly problem for a tank problem, because your tank runs on the anaerobic bacteria bleach destroys.
• Baking soda + vinegar produces a short foaming reaction that has almost no mechanical scrubbing effect.
• Drain-fly sprays and foggers kill the adults you can see but do nothing to the 300 eggs laid in the last 48 hours. Adults re-emerge on a 2–3 week cycle.

The only durable fix is to remove the biofilm itself.

What actually works: remove the biofilm

Biofilm is the glue that holds a drain-fly infestation in place. Larvae eat it. Adults lay eggs in it. Remove the biofilm and the drain becomes uninhabitable.

The most effective way to dissolve biofilm without corroding pipes or killing septic biology is a biological surfactant — a compound produced by Bacillus bacteria that lowers surface tension between the biofilm and the pipe wall, allowing the biofilm to detach and wash away with normal water flow.

How Pure B.S. works

Pure B.S. (Bio Surfactant) is a biosurfactant produced by beneficial Bacillus bacteria. It is non-corrosive, non-hazardous, and safe for septic systems, grease traps, and metal pipes. The active biosurfactant loosens biofilm at the molecular level and the beneficial Bacillus bacteria then outcompete the harmful microbial populations that produced the biofilm in the first place. Unlike bleach, it does not create downstream problems — it is compatible with anaerobic digestion in septic tanks.

Application

• Dose: 1 oz per drain
• Method: Pour into drain, let soak overnight (8+ hours), rinse with hot water in the morning
• Frequency: Weekly for the first month, then monthly for maintenance. Repeat treatment matters more than any single heavy dose — in drain studies, it is the repeated cadence that produces a progressive drop in microbial load, not one intense application.
• Time to clear an infestation: Visible reduction in 3–7 days; full elimination in 2–3 weeks

For commercial kitchens, floor drains, or grease traps, increase dose to 2–4 oz per drain weekly. Bulk packaging is available for facility use.

Do I need a foaming applicator?

For a normal household drain, no. Here’s why the overnight pour works, and where its limit is.

Your sink has a P-trap — the U-bend under the basin that holds a permanent plug of water to block sewer gas. When you pour an ounce of biosurfactant in at night, it settles into that trap and stays there. The trap and low points stay wet continuously, which is why treatment placed there keeps working overnight. Eight hours of undisturbed contact is the whole reason the protocol says “overnight” instead of “pour and run the tap.”

What a pour cannot do is keep the vertical pipe above the trap wet. The tailpiece — the straight section between the drain basket and the U-bend — gets rinsed as the liquid passes through, then drains dry. Same for the sink overflow, that small hole near the rim with a channel behind it. If your buildup is concentrated in those above-water surfaces, an overnight soak reaches them for seconds, not hours.

That’s the gap a foaming applicator fills. Foam is gas dispersed in liquid, so it expands to fill a space and clings to vertical wall instead of running past it. Virginia Tech Extension recommends a microbial foam for drain fly larvae for exactly this reason, noting that foam “can fill fly breeding spaces that brushes and gels cannot access,” left in place several hours before rinsing.

We don’t sell a foaming applicator and we won’t point you at a random one. Useful foam depends on both the chemistry and the equipment generating it — shaking the bottle, spraying it, or improvising with a squeeze applicator does not produce foam that holds long enough to matter. A device that makes weak foam is worse than a good pour, because it feels like you did more.

Here’s what actually closes that gap, and it’s free: after you pour, brush what you can reach. Pull the strainer or stopper, run a stiff bottle brush down the tailpiece as far as it goes, and scrub the overflow opening. Virginia Tech recommends the same follow-up — a stiff brush and hot water to remove remaining biofilm after treatment. The trap gets the overnight soak, the tailpiece gets the brush, and between them you’ve covered both zones without buying anything.

If you manage a commercial kitchen, a multi-unit building, or long vertical stacks where brushing isn’t practical, foam delivery is a legitimate professional method and worth asking a pest or plumbing contractor about. That’s the setting where the apartment-complex project below used it.

Does it actually work at scale?

An apartment building came to us with chronic drain and odor problems across the whole stack — the kind of biofilm load a single-family drain never reaches. The beneficial bacteria liquified the accumulated solids and stripped the biofilm out of the lines, which resolved the plumbing issue and the smell together. The full write-up is here: Apartment Building Plumbing Problems: A Case Study.

How do I prevent drain flies from coming back?

Biofilm re-forms anywhere organic matter and moisture meet. Prevention means routine maintenance, not one-time treatment:

1. Apply a biosurfactant monthly to every drain in high-humidity or high-use areas.
2. Fix slow drains within a week. A slow drain means organic material is accumulating — perfect biofilm conditions.
3. Run water in unused drains weekly. Dry P-traps let sewer gases and flies into the building.
4. Clean garbage disposals monthly with hot water + biosurfactant.
5. Keep standing water out of basement floor drains and mop buckets overnight.

Do I need to call an exterminator?

For a residential infestation sourced from indoor drains, no — commercial pest-control sprays target the adults, not the breeding site, and cost 10–20× more than a bottle of biosurfactant. Call a professional only when:

• The source is outside your building (broken sewer line, cracked septic tank, standing water on the property)
• Infestation persists after 4 weeks of weekly biosurfactant treatment of every drain
• You are in a regulated facility (hospital, food plant, day care) with compliance documentation requirements
• The problem is a physical blockage, not a film — hair mass, wipes, mineral scale, roots, or a sagged or collapsed pipe. A biosurfactant dissolves organic buildup; it is not a drain opener and will not clear an obstruction.

Quick troubleshooting

I treated all my drains and flies are still coming. Check less-obvious sources: washing machine drain pan, dishwasher air gap, refrigerator drip tray, HVAC condensate line, basement sump pit.

Flies in a rarely-used bathroom. The P-trap has dried out. Run water for 30 seconds and pour 1 oz of biosurfactant into the drain.

Flies in an apartment — treatment not holding. The source may be in a shared vent stack or a neighbor’s unit. Treat your drains, then notify building maintenance.

Flies persist after a plumbing repair or waste line replacement. Replacing a section of pipe removes the biofilm in that section only. The breeding site is frequently in a part that was retained: the strainer body, tailpiece, P-trap, sink overflow, disposal connection, or the branch line upstream or downstream of the new work. Start by finding which drain is actually producing flies — tape clear packing tape over each suspect drain overnight, leaving a small air gap, and check in the morning. Then inspect and scrub the parts that weren’t replaced. If nothing turns up, widen the search to sources outside the line that was worked on: HVAC condensate pan and line, sump or ejector pit, floor drains, refrigerator drip tray, and any leak that has kept an area damp. A new pipe can also re-colonize if a fixture upstream keeps feeding organic material into it.

Commercial kitchen — health inspector flagged flies. Treat every floor drain, mop sink, and grease trap with 2–4 oz of biosurfactant overnight. Document weekly applications for the re-inspection.

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Questions or bulk-order interest? Email success@southlandorganics.com or call 800-608-3755. We help commercial facilities and multi-site operations build recurring biosurfactant programs.