Drain fly larvae are small, gray, legless worms about 3/8 inch (roughly 10 mm) long. They live in the slimy buildup inside drains, and if you’ve spotted tiny wriggling creatures in your shower, sink, or bathtub drain, there’s a good chance that’s what you’re looking at. Here’s how to identify them with confidence and what to do about them.
Size, Shape, and Color
A drain fly larva looks like a tiny translucent-gray worm with a slightly flattened body. Both ends are somewhat darker than the midsection, which can give them a faintly striped appearance. They have no legs, no obvious eyes, and no antennae visible to the naked eye. At full size they reach about 3/8 inch, so they’re small enough to miss unless you’re looking closely, but large enough to see without magnification.
Their bodies are segmented, and under close inspection you can see a short breathing tube (called a siphon) at the rear end. This tube lets the larvae breathe while submerged in the wet, organic gunk that lines your pipes. Some species also have tiny hooks near this tube that help them anchor to surfaces inside the drain.
Where You’ll Find Them
Drain fly larvae don’t just live near drains. They live inside the gelatinous biofilm that coats the interior walls of pipes, especially in drains that see infrequent or slow-moving water. This biofilm is a mix of bacteria, soap residue, skin cells, hair, grease, and other decomposing organic material. The larvae feed by scraping this film off hard surfaces with tiny blade-like teeth on their mouthparts, shredding it into fine particles before swallowing.
The most common spots include bathroom sink and shower drains, floor drains in basements or garages, and rarely used utility sinks. You might also find larvae in the overflow hole of a bathroom sink, in a condensation pan beneath a refrigerator, or around a leaking pipe joint where moisture and organic buildup collect. If you see the fuzzy, moth-like adult flies hovering near a wall or ceiling, the larvae are almost certainly living in the nearest drain.
How to Confirm an Infestation
The simplest detection method is the tape test. Before bed, place a strip of clear adhesive tape across the drain opening, sticky side down, leaving small gaps at the edges so air can still flow. In the morning, check the tape for tiny adult flies stuck to the underside. Repeat this for several consecutive nights, since not all adults emerge on the same day. You can also tape a clear plastic bag loosely over the drain opening overnight for the same purpose.
If you want to see the larvae directly, remove the drain cover and shine a flashlight into the pipe. Look for a dark, slimy coating on the inner walls. The larvae will be embedded in or crawling along this film. You can also scrape some of the buildup out with a stiff brush or knife and examine it on a white paper towel.
How Long the Larval Stage Lasts
Drain fly larvae spend 9 to 15 days in this stage at typical room temperature (around 70°F). In warmer conditions, closer to 85°F, the larval period can shrink to as few as 8 days. This is the longest phase of the drain fly’s life cycle, which means larvae are what you’re most likely to encounter inside the drain itself. After this period, the larvae pupate and then emerge as the small, fuzzy adult flies you see resting on walls.
Are They Harmful?
Drain fly larvae are not parasitic and don’t bite. They feed exclusively on decaying organic matter and bacteria, not living tissue. In extremely rare cases documented in medical literature, drain fly larvae have been associated with a condition called myiasis, where larvae are found in or on a person’s body. But the species involved are adapted to feeding on contaminated material, not healthy skin, and confirmed cases are unusual enough to be published as individual case reports. For the vast majority of people, drain fly larvae are a nuisance, not a health threat.
Getting Rid of Them
Killing the larvae you can see won’t solve the problem if the biofilm they feed on remains intact. The goal is to remove that slimy layer from your pipes so there’s nothing left for larvae to eat or new eggs to hatch in.
Start mechanically. Use a stiff drain brush to physically scrub the inside walls of the pipe as far down as you can reach. This does more than any chemical on its own because it breaks up the biofilm where larvae and eggs are embedded. After scrubbing, flush with boiling water. Pouring a full pot of boiling water down the drain once or twice a week helps loosen remaining organic buildup.
For a deeper clean, combine half a cup of salt, half a cup of baking soda, and one cup of vinegar. Pour the mixture down the drain, leave it overnight, and flush with boiling water in the morning. This helps break down grease and organic residue that brushing alone might miss.
Enzymatic drain cleaners are particularly effective because they contain proteins that actively digest the components of biofilm, including the fats, starches, and other organic compounds that hold the slime together. Look for products that list protease, lipase, or amylase among their active ingredients. These work more slowly than chemical drain cleaners but target exactly the material drain fly larvae depend on, and they’re safer for your pipes.
Avoid using bleach as your primary treatment. It can kill larvae on contact but doesn’t effectively penetrate or remove the biofilm layer, so new eggs laid in remaining slime will simply hatch and restart the cycle. The most reliable approach combines physical scrubbing, hot water flushing, and an enzyme-based cleaner repeated over one to two weeks to outlast the full larval development period.