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Insects

Drain Flies

Psychodidae family

Danger Level: 1/5Updated September 2026 · Boise, ID

IdentificationHow to Identify Drain Flies

Quick Answer

Drain flies (also called moth flies) are tiny, fuzzy gray flies about 2 to 5mm long with leaf-shaped wings that sit roof-like over the body. They look like miniature moths and are clumsy fliers that hop in short bursts rather than buzzing around the room.

Full identification notes

You'll usually find them sitting still on the wall, mirror, or ceiling right above a bathroom or basement drain.

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Drain Flies Behavior & Habits

Understanding how drain flies behave helps prevent infestations

Drain fly larvae live in the gelatinous biofilm that builds up on the inside of pipes, P-traps, and floor drains.

More about their habits

Females lay 30 to 100 eggs at a time in that gunk, and the full lifecycle runs about 8 to 24 days, which is why a few flies can turn into a steady cloud over a week or two. Adults don't bite, don't transmit disease, and rarely travel more than a few feet from where they hatched.

Drain Flies Risks & Dangers

What drain flies can do to your health and property

Health Risks

Drain flies don't bite or transmit diseases to humans.

Property Damage

No property damage.

Full health and property details

Health Risks

In large numbers, their body parts and feces can potentially cause allergic reactions in sensitive individuals.

Property Damage

They're purely a nuisance pest that indicates organic buildup in drains.

Signs of Drain Flies Infestation

Look for these indicators in your home

Signs You Have Drain Flies

What homeowners spot first

  1. Tiny gray moth-shaped flies sitting on walls near a sink, tub, or shower
  2. Flies hovering or hopping out of a floor drain when you walk past
  3. A few flies in the basement laundry sink or utility drain even when nothing is running
More signs to look for
  1. Steady presence in a bathroom that's wiped down daily but still produces flies
  2. Flies caught on a strip of tape laid sticky-side-down over a suspect drain overnight

Drain Flies in Boise & the Treasure Valley

Drain flies show up in Treasure Valley homes year-round because they breed indoors, but our call volume jumps in late winter and early spring when basement floor drains have sat unused all season.

More on what we see locally

The heaviest calls cluster in older Boise neighborhoods like the North End, the Boise Bench, and East End, plus older Garden City rentals where cast iron drains and unused laundry-room floor drains hold biofilm for years. Vacation rentals and second bathrooms anywhere in Meridian, Eagle, and Nampa are another common source because the P-traps dry out between guests. If you've poured bleach down the drain twice and still see flies, the breeding spot is almost always a floor drain or an upstairs shower P-trap that nobody thinks to check.

Our Solution

How We Handle Drain Flies

What our approach looks like

Drain fly control starts with finding the breeding drain, which is almost always biofilm built up inside a pipe or P-trap.

Our full approach

We use enzymatic drain foams that cling to the pipe walls and break down the gunk, mechanical brushing when a drain is badly fouled, and an insect growth regulator in the trap to stop the next cycle. Surface sprays alone never solve the problem because the larvae are sitting safely below the water line. New customers start service for $49, and a quarterly plan at $119 keeps the rest of the home covered too.

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How to Prevent Drain Flies

Steps you can take to reduce the risk of infestation

  1. Clean drains regularly with enzymatic cleaners
  2. Run water in rarely-used drains weekly
  3. Fix leaky pipes that create moisture
More prevention steps
  1. Clean out P-traps periodically
  2. Address slow drains promptly

Drain Flies Questions Answered

Common questions about identification, prevention, and treatment

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Why do I have drain flies if my drains aren't clogged?

Quick answerDrain flies breed in the biofilm on pipe walls, not in standing water, so a drain that flows fine can still grow them.

The gunk sits above the water line on the inside of the pipe where you can't see it. In our experience across Boise and Meridian homes, the worst breeding drains are usually basement floor drains and rarely-used guest-bath showers, not the kitchen sink most people suspect.

Will bleach kill drain flies?

Quick answerNot reliably.

Bleach kills a few adults and skims the surface gunk, but it slides past the biofilm too quickly to reach the larvae living inside it. Enzymatic drain foams that expand and cling to the pipe wall for 30 to 60 minutes work much better. If you've already tried bleach twice with no luck, that's the usual sign you need a foam treatment in the right drain.

How do I find which drain is the source?

Quick answerLay a strip of clear tape sticky-side-down over each suspect drain at night, leaving a small gap for airflow.

By morning the breeding drain will have flies stuck to it and the others won't. Check every drain in the house including basement floor drains, laundry-room utility drains, and second-bathroom showers, since those are the spots most Boise homeowners forget. If you'd rather skip the detective work, call us at (208) 297-7947 and we'll find and treat the source on the first visit.

Think You Have Drain Flies?

Call and we'll talk through your options. We usually get out within 24 to 48 hours.

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