Question

For the past week, we have been finding tiny, dark flies on the walls in the master bathroom. They’re sluggish and I’m pretty sure they’re not fruit flies. Do you know where they could be coming from?

Answer

I believe what you are seeing are drain flies. They’re sometimes called moth flies because of their fuzzy-looking wings. They sit on walls, or fly in short hops, or hover around drains. Drain flies have a significant “yuck!” factor because the female drain fly lays her eggs in the dark, gelatinous scum that builds up on the inside of drain pipes. You know, that mass of slimy hair and goo that we clean out periodically when the drain clogs. The tiny, whitish drain fly larvae are down in there feeding on that drain goo. Adult flies emerge later from the drain and tend to stay in the general vicinity.

Drain Cleaners Won’t Do It

Everyone’s first thought is to dump some drain cleaner down the drains. That almost never works. Drain fly larvae are very resistant. Hot water will not kill the larvae, and neither will bleach, soaps, and most household drain cleaners. For a serious infestation, you need professional help. The only effective control is to remove all of the scum on the inside of the pipes and that usually requires a specialty product or equipment.

We can’t say for sure that the flies are coming from the drains, though, without a professional inspection. Drain flies can come from sewer line breaks (a much, more serious problem), and sometimes they are not developing in drains at all. In your case, we would start our inspection with the bathroom, but other drains in the home could be infested as well.

What’s the Cure?

If drains are the source of the flies, a professional treatment usually involves a combination of a long-handled stiff brush and a specialty drain cleaning product. Pest control companies use a thick gel or foam bacterial or enzyme cleaner that clings to pipes and traps and slowly “digests” the scum and suffocates the drain fly larvae. Sometimes a pressure washer, steam cleaner, or a drain snake is also used as part of the process. Usually all drains in the house must be cleaned at the same time because drain flies can fly to other drains to lay eggs.

Rarely, drain flies can be an indication of a sewer line break. They may be coming up out of drains, but breeding and developing in sewage-contaminated soil beneath the slab. If the drain fly problem can’t be resolved with drain treatment, you might require the services of a plumber to run a camera down the line looking for breaks.

Drain flies are sometimes found breeding in trapped stagnant water in mop buckets, drain pans, sump pumps, even garbage cans or recycling bins. It’s even possible that the flies are coming in through windows from a breeding site outside. Call us for a professional inspection.

Photo credit: Libby A. Baker, styled Creazativity / Foter.com / CC BY-SA