Drain Flies in Your Bathroom: What They Are and How to Get Rid of Them
You’ve noticed them — tiny, fuzzy, moth-like flies hovering near your bathroom sink or perched on the wall beside your shower. They’re slow, they’re small, and they seem to appear out of nowhere. Welcome to the world of drain flies — one of the most common and most misunderstood household pests in BC bathrooms.
Drain flies aren’t dangerous, but they’re persistent. And if you don’t deal with the root cause, they’ll keep coming back. Here’s everything you need to know about identifying, eliminating, and preventing drain flies in your bathroom — whether you’re in Maple Ridge, Coquitlam, or anywhere in the Lower Mainland.
What Are Drain Flies?
Drain flies belong to the family Psychodidae, and they go by several common names: moth flies, sewer gnats, filter flies, or sewer flies. Whatever you call them, they’re distinctive enough to identify once you know what you’re looking at.
Appearance
Drain flies are small — about 2–5mm long, roughly the size of a fruit fly. Their most distinguishing feature is their wings: broad, leaf-shaped, and covered in tiny hairs that give them a fuzzy, moth-like appearance. When at rest, they hold their wings flat over their body like a roof, which is why “moth fly” is such an apt name. They’re typically grey or tan in colour.
Unlike house flies or fruit flies, drain flies are weak, erratic flyers. They tend to make short, hopping flights and often just sit on walls, mirrors, and ceilings near their breeding source. You’ll rarely see them flying more than a metre or two from the drain they emerged from.
Lifecycle
Understanding the drain fly lifecycle explains why they’re so persistent. Adult females lay 30–100 eggs in the gelatinous organic film (biofilm) that coats the inside of drain pipes. Eggs hatch in 32–48 hours. Larvae feed on the biofilm for 9–15 days before pupating. The entire lifecycle from egg to adult takes about 2–3 weeks — which means a new generation emerges from your drain roughly every two to three weeks if the breeding site isn’t eliminated.
Adults live for about two weeks. During that time, they don’t bite, don’t carry diseases in residential settings, and don’t eat your food. They’re a nuisance, not a health hazard. But they reproduce fast enough to feel like an infestation if the organic buildup in your drain goes unaddressed.
Why Drain Flies Appear in BC Homes During Summer
You might encounter drain flies year-round, but summer is peak season for a few reasons.
Warm Temperatures Accelerate Breeding
Drain fly development is temperature-dependent. In warm summer conditions, the lifecycle speeds up — meaning faster reproduction and larger populations. June through September is prime drain fly season across the Lower Mainland.
Organic Buildup in Drain Pipes
Every drain in your home accumulates organic matter over time: soap scum, hair, skin cells, toothpaste residue, shaving cream, and body oils. This buildup creates a thick biofilm coating the inner walls of your drain pipes — invisible from above but visible if you remove the drain cover and look inside with a flashlight. This biofilm is the breeding site. No biofilm, no drain flies.
Seldom-Used Drains
Guest bathrooms, basement sinks, secondary showers, and utility drains that don’t get regular use are the most common drain fly breeding sites. Without regular water flow to flush organic matter, biofilm builds up undisturbed. The P-trap (the curved section of pipe that holds water to block sewer gases) can also dry out in unused drains, creating a direct connection to the sewer system where drain flies can access your home.
This is particularly common in older homes throughout Maple Ridge and Pitt Meadows, where finished basements often have a bathroom or laundry sink that sees infrequent use. It’s also an issue in homes with secondary suites — when a suite is vacant between tenants, the drains become breeding sites.
Aging Plumbing
Homes built before the 1990s in the Fraser Valley often have cast iron or older ABS drain pipes that develop interior roughness over time. Rough pipe surfaces accumulate biofilm faster than smooth modern piping. Cracked or damaged pipes — common in older Coquitlam and Port Coquitlam homes — can leak into wall cavities, creating hidden breeding sites that are difficult to locate and treat.
The Drain Test: Confirming Drain Flies
Before you start treating drains randomly, confirm which drain is the source. The drain test is simple and reliable.
How to Do It
- In the evening, dry the area around the suspect drain thoroughly.
- Cover the drain opening with a piece of clear packing tape or plastic wrap. Press the edges down firmly but leave the centre loose enough that air can flow (so the tape doesn’t get pulled down by the drain’s air pressure).
- Leave it overnight.
- Check the tape in the morning. If drain flies are emerging from that drain, you’ll find adults stuck to the underside of the tape.
- Repeat for 3–4 nights to account for the sporadic nature of adult emergence.
Test every drain in the affected bathroom — sink, shower, bathtub, and floor drain. Test any other drains in nearby rooms too. It’s common for drain flies to emerge from a bathroom floor drain while homeowners assume the sink is the source.
Is It a Drain Fly or Something Else?
Three small flies are commonly confused with each other in BC homes:
- Drain flies: Fuzzy, moth-like wings, found near drains, slow-flying.
- Fruit flies: Smooth wings, red eyes, found near ripe fruit and compost, more active flyers.
- Fungus gnats: Slender, mosquito-like body, found near houseplants with overwatered soil.
The treatment for each is completely different, so correct identification matters. If the flies are concentrated near a drain and have the distinctive fuzzy-winged appearance, you’re dealing with drain flies.
DIY Drain Fly Removal: Step by Step
The key principle: you must remove the biofilm breeding site. Killing adult flies without removing the biofilm is pointless — new adults will emerge every few days.
Step 1: Mechanical Cleaning
This is the most important step and the one most people skip in favour of chemical shortcuts.
- Remove the drain cover and inspect the inside of the drain with a flashlight. You’ll likely see a dark, slimy coating on the pipe walls.
- Use a stiff drain brush (a long, narrow bottle brush works) to physically scrub the inside of the drain pipe. Push the brush as far into the pipe as you can reach, rotating it to remove the biofilm from the pipe walls. This is where the eggs and larvae live.
- Flush with hot water after brushing. Not boiling water — just the hottest your tap produces. Boiling water can damage PVC pipe joints.
Step 2: Enzymatic Drain Treatment
After mechanical cleaning, apply an enzymatic drain cleaner (not a chemical drain cleaner). Products like Bio-Clean, InVade Bio Drain, or similar enzyme-based formulations use beneficial bacteria to digest the organic matter that chemical scrubbing couldn’t reach. Follow the product directions — most require overnight application with no water flow.
Important: Don’t use bleach or chemical drain cleaners. Bleach kills bacteria on contact but doesn’t remove the biofilm structure. Once the bleach is flushed away, the biofilm regrows rapidly. Chemical drain cleaners (like Drano) can damage pipes and don’t address the organic layer effectively. Enzymatic cleaners work slower but address the root cause.
Step 3: Prevent Regrowth
- Run water regularly in every drain, including guest bathrooms and seldom-used sinks. A 30-second flush once a week prevents biofilm from accumulating and keeps P-traps full.
- Clean drain covers weekly. Hair and soap scum that collects on the drain cover falls into the pipe and feeds biofilm growth. A quick wipe every week makes a significant difference.
- Install drain screens to catch hair before it enters the pipe. A $5 mesh screen eliminates one of the primary organic inputs.
- Monthly enzyme maintenance. A monthly dose of enzymatic drain cleaner in problem drains prevents organic buildup from reaching levels that support drain fly breeding.
Step 4: Check for Broken Pipe Seals
If the drain test confirms drain flies but cleaning the drain doesn’t resolve the problem, the breeding site may be in a broken or offset pipe joint behind the wall. Biofilm accumulates at leak points, creating breeding sites you can’t reach with a brush. This is where professional help becomes necessary — a plumber or pest control technician can identify hidden breeding sites using camera inspection.
When DIY Isn’t Enough
Most drain fly infestations resolve with thorough drain cleaning and ongoing maintenance. But some situations require professional intervention:
Persistent Infestations Despite Clean Drains
If you’ve cleaned every drain in the bathroom and drain flies keep appearing, the source may be hidden: a broken pipe behind the wall, a floor drain you didn’t know about, biofilm in the overflow opening of a sink or tub, or even a sewer line issue. Professional inspection can identify breeding sites that aren’t accessible from above.
Multi-Drain or Multi-Room Infestations
When drain flies are emerging from multiple drains across different rooms, the issue may be systemic — a main sewer line problem, a septic system issue, or widespread pipe deterioration. This is more common in older homes in Maple Ridge and Pitt Meadows where original plumbing is aging.
Commercial Kitchens and Restaurants
Drain flies in commercial food service environments are a health inspection concern. Floor drains, grease traps, mop sinks, and commercial dishwasher drains all produce the organic buildup drain flies need. Professional treatment on a scheduled maintenance basis is essential for commercial compliance. Restaurants and cafés across Coquitlam, Port Coquitlam, and the Tri-Cities area are subject to Fraser Health inspection standards that include pest management requirements.
Preventing Drain Flies From Coming Back
Once you’ve eliminated an active infestation, prevention is straightforward:
- Weekly water flush through every drain in the house — 30 seconds of running water prevents P-traps from drying and flushes incipient biofilm.
- Monthly enzymatic treatment in bathrooms and kitchen drains.
- Prompt attention to slow drains. A slow drain indicates buildup in the pipe — exactly the conditions drain flies exploit.
- Fix leaks promptly. A dripping faucet or leaking pipe fitting creates a perpetually moist environment that supports biofilm growth.
- Good bathroom ventilation. Running the exhaust fan during and after showers reduces humidity that contributes to biofilm growth in drain pipes and on bathroom surfaces.
Get Rid of Drain Flies for Good
Drain flies are a nuisance, not a crisis. But they won’t go away on their own, and they’ll keep coming back until you eliminate their breeding site. The approach is mechanical first (clean the drain), biological second (enzymatic treatment), and preventive ongoing (regular flushing and maintenance).
If you’ve tried the DIY approach and the fuzzy little flies keep showing up, it’s time for a professional assessment.
Dealing with persistent drain flies in your BC home? Call Canadian Pest Control at (778) 598-7378 or visit cpestcontrol.ca to schedule a free inspection. We serve Maple Ridge, Coquitlam, Port Coquitlam, Pitt Meadows, and the surrounding Lower Mainland, and we’ll track down the source of your drain fly problem — even when it’s hidden.