Drain flies in BC bathrooms and kitchens: finding the breeding site and clearing it

Small flies in BC bathrooms — field ID

Species	Size	Appearance	Location	Source
Drain fly / moth fly (Psychoda)	2–4 mm	Very hairy, moth-like, tent wings at rest	Walls near drains	Drain pipe biofilm
Fungus gnat (Bradysia)	2–4 mm	Dark, slender, long legs	Near houseplants	Overwatered plant soil
Phorid fly (Megaselia)	1–6 mm	Humpbacked, runs fast	Drain area, refuse	Organic matter in drain or soil
Fruit fly (Drosophila)	3–4 mm	Red eyes, tan body, slow	Near fruit, vinegar	Fermenting organic matter

What drain flies are and how to identify them

Drain flies (moth flies, family Psychodidae) are 2–4 mm, extremely hairy, and hold their wings tent-like over the body at rest — giving them a moth-like appearance. They're slow, weak fliers. They don't bite and aren't associated with food contamination. They're strictly associated with water and organic matter. In BC homes, drain flies are found near: bathroom floor drains (especially slow drains), sink drains in rarely-used bathrooms, shower drains with accumulated hair and soap scum, washing machine overflow drains, sump pit edges, and occasionally in HVAC condensate drain lines. Adults rest on walls near the drain, particularly in morning hours when they're most active. The key diagnostic: place sticky tape over the suspect drain at night leaving a small air gap. Check in the morning — drain flies caught on the underside of the tape confirm that drain as the source. Test multiple drains if the source is unclear.

How to

Drain fly elimination protocol

Addresses the biofilm breeding site directly. Chemical treatments without biofilm removal fail consistently.

1
Confirm the breeding drain with sticky tape test
Place sticky tape over suspect drain at night with a small gap for air flow. Check in the morning — drain flies caught on the tape confirm that drain. Test multiple drains.
2
Mechanical removal: drain brush
Use a long-handled flexible drain brush. Scrub the inside of the drain pipe walls vigorously, working 30–45 cm down the pipe. The goal is to physically dislodge the gelatinous biofilm layer where larvae are breeding. This step is the most critical in the protocol.
3
Apply enzymatic drain cleaner — not bleach
Follow brushing with an enzymatic drain cleaner. Enzymatic cleaners digest the organic matter biofilm. Pour the recommended dose and allow to sit overnight without running water. Repeat on 3 consecutive nights. Bleach kills surface adults but doesn't remove biofilm — the population rebounds within days.
4
Address slow drain issues
Drain flies establish preferentially in slow drains where biofilm accumulates in the reduced-flow zone. Clear any mechanical blockage (hair, soap scum) to restore full flow. Biofilm accumulates much slower in fast-flowing drains.
5
Prevent recurrence in rarely-used drains
Guest bathroom floor drains and other infrequently used drains dry out and allow biofilm to build. Run water down these drains weekly and add a small amount of mineral oil to maintain the water trap seal and inhibit fly access.

Frequently asked questions

Can drain flies breed anywhere other than drains?+

Yes: condensate pan under HVAC with algae accumulation, sump pit walls with biofilm, washing machine door seal on front-loaders, poorly drained potted plant trays with organic debris, and standing water in basements. If drain treatment doesn't solve the problem, check these secondary sources.

Are drain flies seasonal in BC?+

Drain flies are year-round in heated indoor environments. As long as drain temperature exceeds 10°C and biofilm is present, breeding continues. No cold-dormancy period indoors.

How long does drain fly elimination take with proper treatment?+

With proper biofilm removal, adult populations crash within 7–10 days as the current generation dies without replacement. If adults persist past 2 weeks, there's an untreated breeding source. Re-investigate with the sticky tape test across all drains in the home.