Identification
Adult Psychoda are tiny (2-5mm), dark grey to almost black, with a distinctly furry appearance — the body and wings are densely hairy, giving them a moth-like silhouette (hence the name 'moth fly'). They rest with wings held roof-like over the body. They're weak flyers and short-range; you'll typically find them near the drain rather than throughout the room. Larvae are small (4-6mm), worm-like, grey-cream with darker segments — they live in the gelatinous biofilm inside drain pipes and are rarely seen unless drain piping is dismantled.
Habitat in BC
Drain fly breeding sites: the gelatinous biofilm (a mixture of hair, soap residue, organic matter) that coats the interior of bathroom sink drains, shower drains, floor drains, especially in infrequently-used fixtures. Basement floor drains, laundry-sink drains, restaurant floor drains, and the drains of infrequently-cleaned guest bathrooms are prime sites. Commercial settings — restaurants, cafes, bars — have accelerated pressure due to food-waste biofilm accumulation in floor drains.
Signs you have drain fly (moth fly)
- Tiny dark fuzzy flies hovering near bathroom sink, shower, or floor drain.
- Flies appearing in rooms that have been unused — a spare bathroom, basement suite.
- Larvae visible in drain (only if you look inside — check with a flashlight).
- Recurrent flies that return within a week of killing visible adults (a breeding site is active in the drain).
Risk & damage
Minimal health risk. Drain flies don't bite, don't carry typical disease vectors, and don't feed on or contaminate food directly. They are a minor allergen for sensitive individuals (inhaled fragments in heavy infestations), and in commercial food-service settings they can transfer biofilm bacteria between fixtures via adult fly movement. The real risks are reputational: in restaurants, a drain-fly problem is a health-inspection red flag; in Airbnbs and hotels, guests notice immediately.
Seasonality in Metro Vancouver
Year-round in Metro Vancouver indoor environments — they breed inside plumbing regardless of outdoor temperature. Slight seasonal uptick in summer when ambient indoor humidity rises. Commercial pressure is tightly correlated with kitchen usage patterns; a restaurant closed for a week-long break can emerge to a drain-fly bloom.
Treatment approach
The entire treatment is drain cleaning, not insecticide. Retail foam drain cleaner (enzyme-based, like Bio-Clean or InVade Bio Foam) applied daily for 5-7 days dissolves the biofilm that supports the larvae. Bleach and standard drain openers (sodium hydroxide) don't work well — they run off the biofilm without penetrating it. For commercial settings with multiple drains, we use a professional enzyme treatment system and train staff on preventive monthly application. We never fog, bomb, or spray insecticide for drain flies — the adult flies die in 2-3 days regardless; eliminating the biofilm eliminates the population.
When to call a professional
For a single-drain residential issue, DIY enzyme foam for a week typically resolves it. Call for professional service if: the problem persists after 2 weeks of DIY enzyme treatment, you're a restaurant or commercial food-service operation where health-inspection documentation matters, or you suspect an undisclosed drain issue (broken trap, cracked pipe, long-sealed floor drain). Commercial treatment includes a documented sanitation protocol and staff training.
Prevention playbook
How to prevent drain fly (moth fly) in Metro Vancouver homes
- 1
Run infrequently-used drains weekly
Guest bathrooms, basement floor drains, utility-sink drains — any drain not used daily is at risk. Run hot water through each for 30 seconds once a week to disrupt biofilm accumulation.
- 2
Monthly enzyme drain treatment
Bio-Clean, InVade Bio Foam, or equivalent enzymatic drain foams. Apply monthly per manufacturer directions. These dissolve the biofilm that drain flies breed in. Cheap DIY prevention.
- 3
Never pour grease down drains
Grease is the primary biofilm-feeding input in residential kitchens. Scrape cooking grease into waste, not the sink. For commercial kitchens, this is already protocol — enforce it.
- 4
Cover floor drains with removable strainers
Allows water flow but prevents organic debris (hair, food scraps) from accumulating in the trap. Lift weekly and clean.
- 5
Address slow drains immediately
A slow drain = accumulating biofilm = drain fly harbourage. Fix slow drains promptly — mechanically (snake) first, then enzymatic. Aggressive chemical drain openers (Drano) damage pipes and don't reliably reach biofilm.
- 6
Commercial: post-close drain sanitation protocol
Restaurants and commercial kitchens: enforce a closing checklist that includes floor-drain rinse with hot water and weekly enzyme foam. Documented in the sanitation log satisfies VCH pest-control requirements.
The Wild Pest service
See our Drain Fly (Moth Fly) treatment page
Transparent pricing, 60-day return guarantee, same-day response across Metro Vancouver. Every treatment is documented with photos and service notes.
Frequently asked questions about drain fly (moth fly)
Will bleach kill drain flies?+
No, not effectively. Bleach runs off the gelatinous biofilm that larvae live in without penetrating it. Enzymatic drain foams (Bio-Clean, InVade) are specifically engineered to cling to and dissolve the biofilm. Bleach is the wrong tool for this specific pest.
Are they the same as fruit flies?+
No. Fruit flies (Drosophila) are tan-coloured, smaller-winged, and breed on rotting fruit and organic matter on surfaces. Drain flies (Psychoda) are darker, furrier, moth-like, and breed exclusively in plumbing biofilm. Treatment is completely different — fruit fly treatment is surface sanitation; drain fly treatment is in-pipe biofilm elimination.
How long does it take to eliminate a drain fly population?+
With consistent daily enzyme treatment, 5-7 days to see significant reduction, 2-3 weeks to complete elimination. Adult flies live only 3-5 days, so once breeding stops, the visible population collapses quickly. Persistence beyond 3 weeks indicates an undiscovered breeding site.
Can drain flies come up from the sewer?+
Not directly — they breed in P-trap biofilm above the sewer water, not in the sewer itself. A dry P-trap (in a rarely-used guest bathroom) can allow sewer-gas odor but not a fresh fly introduction from sewer. Run water in unused drains weekly to keep P-traps filled.
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