Drain Flies: Where They Breed, and Why Bleach Doesn't Shift Them
Drain flies are small, fuzzy grey insects, a few millimetres long, that breed in the slimy film inside sink, shower and floor drains. Scrubbing that film out of the pipe is usually what gets rid of them. Benefit Pest Management works across Newcastle, Lake Macquarie and the Hunter. We can identify drain flies from a photo. Our head technician holds pest management technician licence 5083379, issued by the New South Wales EPA.
We get back to every enquiry within one business day.
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Pests we treat
Bleach, boiling water and fly spray
Getting rid of drain flies is usually a cleaning job, and bleach is the obvious thing to reach for. They come out of a grubby place, and bleach is made for grubby places. It is the wrong tool, though, for the slime that keeps them coming.
The flies on the wall are the adults. Their eggs and larvae are down in the pipe, in the slimy film that coats the inside of the drain. Bleach poured down the plughole does little to shift that film. The larvae in it are surprisingly resistant to bleach, so the next lot carries on growing and the wall fills up again.
Boiling water on its own has little effect on the eggs or the adults. Very hot water earns its place as a flush once a drain has been scrubbed.
Fly spray only reaches the adults on the wall, and the next generation is still down in the pipe. Pouring insecticide down the drain is worse still: it usually does not kill the larvae, and it can damage the sewer system it ends up in.
What does shift the film is a stiff drain brush, followed by very hot water and an enzyme drain cleaner, which breaks down whatever slime the brush leaves behind. Do all of that on the drain they are coming from. One safety note. Never mix bleach with any other drain cleaner, before or after it, because some combinations give off toxic gas.
Drain fly or fruit fly?
Up close, a drain fly is grey to brownish-black and fuzzy, with broad hairy wings folded over its back like a little roof. That furry look is why they also get called moth flies, and you will hear sink flies, filter flies and sewer flies as well. They all belong to one family of flies, the Psychodidae. The species common in houses has black and white flecks on its wings and turns up in warm and temperate places right around the world.
Most of the time they sit still on a wall or a tile, and they are at their busiest in the evening. As fliers they are weak, and as travellers they are worse, so the ones on your wall most likely came out of a drain close by.
A fruit fly is yellow-brown with red eyes. It breeds in rotting fruit and anything fermenting, and it hovers over the fruit bowl and the compost. A drain fly needs a wet, slimy film, which is why it turns up in the bathroom, the laundry and around the kitchen sink. Scrubbing a drain does nothing for fruit flies, so it pays to know which fly you have first.
Bigger grey flies around the bin are house flies, which we cover with the other everyday pests on our common pests page. If the same damp bathroom has small, silver, wingless insects running about, those are silverfish.
How a few drain flies become a lot
Each female lays dozens of eggs in the slimy film that builds up inside a drain, and they hatch within a couple of days. The larvae spend one to two weeks in that film, feeding on it, before they turn into adults. Each adult lives about two weeks. A new generation can be on the wall within a couple of weeks, and every one of them grew up in drain slime.
The film can build up in any drain in the house, the basin’s overflow hole and the bend in the pipe under the sink included. A drain that goes a while without a scrub, a shower that drains slowly or a pipe that leaks gives them more to work with.
If you can't get rid of drain flies
If flies are still turning up after a few weeks of properly scrubbing every drain, the source is probably somewhere you cannot easily reach or see. It could be a cracked pipe, a leaking shower base or damp hidden inside a wall, and that is the clearest case for ringing us. If it turns out to be a plumbing fault, it is a plumber’s job, and we will say so and tell you what to ask them to look at.
When we come out, we check every drain in the house and run our moisture meter over the walls around the wet areas. We also go up into the roof void to see how the exhaust fan is going. The treatment plan comes from what we find.
Drain flies need a pest controller less often than you might think, and we would rather tell you that on the phone than book a visit you do not need. At home, drain flies sit with the rest of our residential pest control work. Ring us on 0467 561 548 and tell us what you have tried so far.
Where we do this
We work from our base at Caves Beach across Newcastle, Lake Macquarie, the Hunter, Port Stephens and the Central Coast, with no call-out fee.
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FAQ
Drain fly questions
Do drain flies bite?
No, and they do not sting either. In a house they are a nuisance more than anything. They do breed in the muck inside a drain, though, so they are not something you want walking across the kitchen bench.
Will drain flies go away on their own?
Not while the film they breed in is still there. The adults die off, but a drain that keeps its slime keeps producing new ones. The numbers hold or climb until it is cleaned.
How long does it take to get rid of drain flies?
Usually two to three weeks after the drain is properly clean. Adults that have already hatched live about two weeks, and eggs laid before the clean can still come through. A few more flies after the first scrub is normal.
Can drain flies come from somewhere other than a drain?
Yes. They will breed in almost any wet, rotting film. Outside, their standards are low: blocked gutters, pot plant saucers left full of water, bird baths and soggy mulch all qualify. Indoors, a leaking shower base can do it too. The drains are still the first place to look.
Do drain flies mean I have a broken pipe?
Most of the time they mean a drain in normal use needs a scrub. A cracked or leaking pipe only becomes likely once every drain is clean and the flies keep coming, and finding it is a job for a plumber.
Are drain flies a problem for a food business?
Yes. Standard 3.2.2 of the Food Standards Code requires a food business to take all practicable measures to keep pests out. Floor drains in a commercial kitchen are a likely breeding spot, so in a kitchen like that, drain flies come under our commercial pest control work.
Next step
Try this tonight
Before bed, turn a clear plastic container upside down over the drain you suspect, with a sticky trap fixed to the inside. Start with the drain they cluster around, because they rest close to where they came out.
Flies on the trap by morning point to that drain. Pull the grate or the plug and scrape the slime off the inside of the pipe with a long, stiff drain brush, as far down as it will reach. Flush it with plenty of very hot water, then finish with the enzyme drain cleaner from the hardware shop. Repeat the scrub until they stop, then every so often after that.
If the container stays empty and the flies keep turning up anyway, the source is probably somewhere else, and that is when a call makes sense.
A quote costs nothing, and neither does the advice.
Photos come through the form on our contact page.
