You flip on the bathroom light and there they are: a few tiny, fuzzy gray flies sitting on the wall next to the tub. Swat one and it smears into a powdery smudge. Those are drain flies, and they’re one of the most common “mystery bugs” we get asked about in Treasure Valley homes.
Here’s the honest truth about drain flies. They come from the drain, not from outside, and the fix is in the drain too. Sprays and traps can knock back the adults you see, but if the film they breed in is still there, a new batch will keep hatching. This guide covers how to identify them, how to clean the breeding source out of bathroom and kitchen drains, and how to keep the rest of your kitchen from feeding house flies and fruit flies.
How to Identify Drain Flies
- Size and look: about 1/16 to 1/8 inch long, gray to tan, with a fuzzy body and broad, hairy wings held like a little roof over the back.
- Behavior: weak, hopping flight. They tend to sit on walls, mirrors and the sides of tubs and sinks rather than buzz around the room.
- Where: near sinks, tubs, showers, floor drains, basement drains and sometimes the drip pan under a fridge or an evaporative cooler.
- When: often in the evening or at night, and often in a bathroom that doesn’t get used much.
They’re also called sewer flies or filter flies. University of Minnesota Extension has a good page on drain flies with photos that help with identification.
Drain flies, fruit flies or fungus gnats?
- Fruit flies are tan with red eyes and hang around ripe produce, recycling and sugary spills.
- Fungus gnats are dark and long-legged, and they come from damp houseplant soil.
- Drain flies are fuzzy, gray and wedge-shaped, and they stay near the drains and water.
Why They Show Up
Drain flies lay their eggs in the slimy, gelatin-like film that builds up inside drain pipes, overflow openings and floor drains. The larvae live in that film and feed on the bacteria and gunk in it. In a warm house, they can go from egg to adult in one to three weeks, so a small problem can grow quietly.
A few things make it more likely here in the Treasure Valley:
- Hard water. Much of the valley has hard water, and mineral scale inside pipes gives film something to grip.
- Guest baths and basements. Drains that sit unused can dry out or collect film without being flushed. A dry floor drain trap also lets sewer air and flies up into the room.
- Evaporative coolers and drip pans. Swamp coolers, dehumidifiers and fridge drip pans hold standing water through the summer.
- Cabins, RVs and vacation homes. Plumbing that sits for weeks, or gets opened back up after winterizing, often has drain flies waiting.
- Septic systems. Rural homes out toward Star, Middleton and Emmett on septic should use septic-safe cleaners and keep up with tank maintenance.
Step 1: Find the Breeding Drain
If you have more than one drain, figure out which one is the source before you start scrubbing. The tape test works well:
- At night, lay a strip of clear packing tape sticky side down over the drain opening. Leave a little gap so air can move.
- Check it in the morning. Flies stuck to the underside mean that drain is producing them.
- Repeat on each sink, tub, shower and floor drain for a few nights.
Don’t forget overflow holes in bathroom sinks and tubs, the garbage disposal and any floor drain in the laundry room, garage or basement.
Step 2: Clean Out the Film
This is the step that actually stops them, and it takes some elbow grease.
- Pull the stopper or drain cover and scrub it with hot water and a brush. You’ll usually find hair and slime caked on.
- Scrub the pipe. Run a stiff, long-handled pipe brush down the drain as far as it’ll go, working it up and down to break the film off the walls.
- Clean the trap. Under a sink, you can usually unscrew the P-trap, rinse it out and put it back. Put a bucket underneath first.
- Clear clogs. A drain snake pulls out the hair and buildup that hold film in place.
- Follow with an enzyme or bacterial drain cleaner. These digest the remaining organic gunk. Use them as the label directs, usually overnight when the drain won’t be used, and repeat for a week or two.
- Clean the overflow. Use a bottle brush or pipe cleaner in sink and tub overflow openings.
A couple of myths are worth clearing up. Bleach poured down a drain mostly runs straight past the film without removing it, and it’s hard on pipes. Boiling water helps with light upkeep, but on its own it won’t clear an established buildup. Baking soda and vinegar can loosen a little grime, but they’re no substitute for the brush.
If the flies keep coming back
If you’ve cleaned every drain and you’re still seeing them after a couple of weeks, the source may be somewhere you can’t reach: a cracked drain line under the slab, a leak inside a wall or a broken sewer pipe in a crawlspace. That’s a job for a plumber, who can run a camera down the line. Leaks like these draw other pests too, so it’s worth getting them fixed either way.
Step 3: Keep Drains From Building Up Again
- Run water in guest bathrooms and pour a bucket of water into floor drains every couple of weeks so the traps stay full.
- Install hair catchers in showers and tubs and empty them often.
- Keep grease, coffee grounds and food scraps out of sinks.
- Run hot water for a bit after doing dishes.
- Use an enzyme drain cleaner on a regular schedule in drains that have had problems.
- Empty and clean drip pans, and keep evaporative coolers cleaned out during the season.
- Fix dripping faucets and run the bathroom fan to cut humidity.
Keeping the Rest of the Kitchen Fly-Proof
Drains aren’t the only fly source in a kitchen. House flies and fruit flies come for food, trash and compost, and Treasure Valley summers bring a lot of them, along with the other summer pests common in Idaho. In Eagle, Star and other towns with big gardens and backyard compost, we see plenty of kitchen fly trouble that starts outside the back door.
Daily habits
- Wipe counters right after cooking and wash or rinse dishes before bed.
- Empty and rinse the sink strainer every day, and clean the garbage disposal by running it with cold water and a handful of ice.
- Keep ripe fruit in the fridge, and don’t leave onions or potatoes going soft in a dark cupboard.
- Rinse cans and bottles before they go in the recycling bin.
Trash and compost
- Use a kitchen trash can with a tight lid, and take the bag out before it gets smelly, especially in summer.
- Wash the inside of indoor and outdoor trash cans about once a week in hot weather. A diluted bleach solution or a household cleaner works, then let them dry.
- Keep outdoor carts and compost bins away from doors and the patio.
- Use a lidded countertop compost pail and empty it every day or two.
- Outside, turn the compost pile regularly and cover fresh food scraps with a layer of dry leaves or shredded paper. That keeps fruit flies and house flies from laying eggs on top.
Screens and gaps
- Keep window and door screens in good shape, and patch small tears before they get bigger.
- Self-closing hinges on screen doors, or a magnetic screen curtain on a patio door, help a lot with kids and dogs going in and out.
- Seal gaps around pipes and vents under the sink with caulk.
- Check door sweeps each season and replace worn ones.
Simple traps
- Vinegar trap for fruit flies: a small jar of apple cider vinegar with a drop of dish soap. The soap breaks the surface tension, so flies that land sink. Replace it about once a week.
- Sticky cards or strips: yellow sticky cards near the sink, a window or the trash catch adults, and they also show you where activity is heaviest. Moving them around every few days helps you find the hot spot.
Herbs like basil, mint and lavender smell nice on the windowsill, but they won’t hold back flies from a full trash can or a dirty drain. The University of California’s pest note on flies is a good reference if you’re not sure which kind you have. If house flies are the bigger problem, our guide to preventing house flies at home goes into the outdoor sources.
Where Traps Help, and Where They Don’t
Sticky cards and fly traps are useful for keeping an eye on how many adults are around and for bringing those numbers down. They don’t touch the larvae living in the drain. If you only trap, you’ll keep catching flies for as long as the film is there. Clean the drain, then use traps to catch the stragglers and confirm the problem is fading.
ProGuard can add fly traps to any regular service visit, installing them and disposing of the used ones, which helps most in garages, around trash areas and in commercial kitchens and businesses with heavy house fly pressure. For a business with floor drains and dumpsters, our post on fly traps for Boise homes and businesses covers the commercial side. Fly traps are an add-on to a regular visit, not a separate fly service. Flies aren’t a guaranteed pest, so they aren’t covered by the free return visits that come with our quarterly plans. Cleaning out drains is still a job for you or your plumber.
Common Questions
Do drain flies bite?
No. They’re a nuisance, and they’re a sign of buildup in your plumbing, but they don’t bite or sting.
How long does it take to get rid of them?
Once the breeding drain is clean, adults that already hatched die off within a week or two. If new ones keep showing up after that, there’s another source to find.
Why do I only see them in one bathroom?
That’s usually the drain that gets used least, or one with a hair clog holding film in place. Start the tape test there.
Help With Pests Around Your Home
Our residential pest control plans handle spiders, ants, wasps and hornets, mice, rats and fall invaders, with quarterly, bi-annual and one-time service, and fly traps can be added to any regular service visit, where we install them and dispose of the used ones.
ProGuard is a local, family and veteran owned company based in Eagle, serving homes and businesses in Eagle, Boise, Meridian, Garden City, Star, Nampa, Caldwell, Emmett and Middleton. Call or text (208) 352-2062 or contact us about a pest control plan, and ask about adding fly traps.