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Drain Fly (Moth Fly, Psychodidae)
Identify it, prevent it, and know when to call. A South Florida guide from Pro-Spray.
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What Is a Drain Fly?
Drain flies, also called moth flies or sewer flies, are weak fliers that breed in the gelatinous film (biofilm) that coats drains, sewer lines and wet organic debris. The larvae live inside that slime, so pouring water down the drain doesn't reach them. They're common in guest bathrooms, floor drains in garages and restaurants, and homes with a cracked pipe under the slab.
How to Identify Them
Look for a small fuzzy fly sitting flat on the wall near a sink or shower, wings folded like a little tent. They hop short distances rather than flying far. If you swipe one, it smears into a powdery smudge.
Quick drain test: put clear tape over the drain opening (sticky side down, leaving a small gap) overnight. Flies stuck to it in the morning confirm that drain.
Life Cycle
Eggs are laid in the slimy film inside drains, sewer lines and other wet organic buildup. The larvae feed on that film and breathe through a small tube at the rear, so they can live in very wet places. They pupate in the same material, and the adults rest nearby on walls. The cycle keeps going as long as the slime stays in place.
Look-alikes and How to Tell Them Apart
- Fruit flies: tan with red eyes and clear wings, hovering around food.
- Fungus gnats: thin and dark, near potted plants.
- Phorid flies: small humpbacked flies that run in jerky bursts; they also come from drains and broken pipes but are not fuzzy.
- Small moths: pantry moths are larger, with patterned wings, and found near stored food.
Where They Hide in South Florida Homes
Guest bathrooms that sit unused, shower and tub overflows, laundry and garage floor drains, AC condensate drains and pans, and cracks in slabs over damaged pipes. Seasonal homes that sat empty over the summer often have a burst of drain flies when people return.
When to Call a Pro
Call if cleaning the drain doesn't stop them within a couple of weeks, if you can't tell which drain is the source, or if flies seem to come from the floor or a wall, which can mean a broken line below.
Signs of Drain Flies
- Fuzzy flies resting on bathroom walls, tubs and mirrors
- Flies most noticeable after a shower runs
- Gray slime in the drain or overflow
- Flies near a floor drain, condensate line or slab crack
Is it dangerous?
A nuisance, but they point to slime buildup in a drain or a broken pipe.
Prevention Tips
- Scrub drains and overflows with a stiff drain brush, then flush with hot water.
- Run water in guest bathrooms and floor drains every week.
- Keep AC condensate lines and pans clear.
- Have a plumber check for a broken line if flies keep coming from a floor or slab.
How Pro-Spray Helps With Drain Flies
We find which drains are producing flies, then use drain treatments made to break down the organic film where larvae live, plus targeted treatment for resting adults. If the source turns out to be a broken pipe under the floor, we'll tell you so you can get a plumber, since no treatment fixes a leak. Our Bug Free Guarantee: If pests come back between visits, so do we, at no extra cost.
New customer offer
$50 OFF your first service
+ FREE inspection
Then stay protected with our $125 quarterly pest control plan, about $42/month.
New customers only. No code needed and no expiration. Mention this offer when you call or text.
Frequently Asked Questions
Does bleach kill drain flies?
Are drain flies harmful?
Why do I only see them in one bathroom?
Reviewed October 2026 by Pro-Spray's family-run team
Pests don't wait.
Protect your home today.
Get $50 OFF your first pest service and a FREE inspection & estimate. Ongoing protection is $125 quarterly (about $42/month), with no contracts and no cancellation fees.
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