Drain Flies in Your St. Louis Home
You’ve noticed them hovering near the bathroom sink: tiny, fuzzy, moth-like flies that don’t seem to go away no matter how clean the bathroom is. They’re slow and easy to swat, but new ones keep appearing every morning.
These are drain flies, and they’re breeding in a place you probably haven’t thought to check: the organic film inside your drain pipes.
What Are Drain Flies?
Drain flies (also called moth flies or sewer gnats) are small, fuzzy-winged insects about 1/8 inch long. They look like tiny moths and tend to rest on walls and ceilings near sinks, showers, and floor drains. They’re weak fliers and are most active at night.
Despite their appearance, drain flies aren’t related to fruit flies or house flies. They belong to a different family entirely and have a completely different breeding habitat. The University of Maryland Extension's guide to drain flies explains that they breed exclusively in the gelatinous organic film that builds up inside drain pipes, sewage treatment beds, and any standing water with decaying organic matter.
Where They Breed in Your Home
Drain flies lay their eggs in the slimy biofilm that coats the inside of infrequently used or slow-draining pipes. Common breeding sites in St. Louis homes include bathroom sink drains (especially guest bathrooms that don’t get daily use), shower drains with slow drainage, basement floor drains, laundry tubs, and sump pump pits.
In older homes across Maplewood, Clayton, and Brentwood where plumbing may be original or aging, the buildup inside pipes tends to be heavier, which creates more breeding habitat.
How to Confirm the Source
If you’re not sure which drain is the source, place a piece of clear tape sticky-side-down over each suspect drain overnight. In the morning, check for adult flies stuck to the tape. Whichever drain catches flies is the breeding site.
This matters because treating the wrong drain won’t solve the problem. Drain flies don’t travel far from their breeding site, so the flies you see near the bathroom sink are almost certainly breeding in that sink’s drain.
How to Get Rid of Drain Flies
The solution is straightforward: remove the organic film inside the drain. Chemical drain cleaners and bleach don’t work well because they flow over the biofilm without fully removing it. Instead, use a stiff drain brush to physically scrub the inside of the pipe, then flush with hot water.
For floor drains and basement drains that can’t easily be brushed, an enzyme-based drain cleaner (available at most hardware stores) can break down the organic film over several applications. The Virginia Tech Cooperative Extension’s drain fly fact sheet recommends combining mechanical cleaning with enzyme treatments for persistent infestations and cautions against pouring insecticides down drains, which is ineffective and can damage sewage systems.
Run water through infrequently used drains at least once a week to prevent film from building up. If you have a guest bathroom or basement drain that sits unused for weeks at a time, that’s likely your most vulnerable spot.
If you’re in Richmond Heights or anywhere with a home built before the 1960s, consider having your plumbing inspected. Cracked or broken sewer lines beneath the slab can create breeding sites that no amount of drain cleaning will fix.
When to Call a Professional
Drain fly problems that persist after thorough cleaning may indicate a broken pipe, a hidden drain (some homes have capped floor drains behind walls or under appliances), or a sewer line issue. Our pest control services include inspection and treatment for drain flies, including identification of hidden breeding sites.
The MU Extension’s guide to selecting a pest control service recommends choosing a company that will identify the source before treating, which is exactly our approach.
How Pure Pest + Lawn Can Help
Schedule a free inspection. We identify every breeding site, including ones you can’t easily access.
We treat the source. Targeted applications that eliminate the biofilm and the larvae developing in it.
Enjoy a fly-free bathroom. No more fuzzy flies on the mirror every morning.
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Drain flies driving you crazy? Call 314-222-PEST (7378) or schedule online.
