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Where do drain flies come from in a New Zealand bathroom?

From the slimy film inside your drains. The larvae live there and the adults flutter out — usually near the very drain they came from.

Will bleach alone kill drain flies?

Bleach kills adults and eggs but does not remove the biofilm the larvae live in. You need to scrub the drain with an enzymatic cleaner or drain brush and flush with boiling water to break the cycle.

Do drain flies bite people or pets?

No. They do not bite people or pets. They are a nuisance by their numbers and their presence, which signals a drain that needs cleaning.

How fast do drain flies reproduce in a Kiwi home?

The cycle from egg to adult can be as short as two to three weeks in a warm, sludgy drain — so a few flies can become a steady stream if the drain is not cleaned.

Can drain flies come from a floor drain I never use?

Yes — infrequently used floor drains are a classic source, because the water trap dries out and the biofilm builds up undisturbed. Pour water down every drain weekly to keep the trap wet and the drain clear.

Why do drain flies keep coming back after I clean the sink?

You may be cleaning only the plug, not the overflow hole or the pipe wall further down. Scrub the overflow, the strainer and as far into the pipe as a brush reaches, then flush with boiling water daily for a week.

Drain Fly

Tiny moth-like flies that live in your sinks and drains.

NZ Pest Guide

Drain flies — also called moth flies or sewage flies — are the tiny, fuzzy-winged insects you sometimes see sitting in a dark, damp spot near a sink, shower or floor drain. They look more like tiny moths than flies, with wings held tent-like over the body.

They breed in the slimy organic film that builds up inside drains, where larvae feed on the sludge. They do not bite and are not a major disease vector, but an untreated drain can produce steady numbers of them. The fix is cleaning the drain, not spraying the adults.

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How to identify a drain fly

Very small, grey or tan, with a fuzzy, moth-like appearance and wings held roof-like over the body at rest. They are weak, fluttering fliers and tend to stay close to the drain they emerged from.

•Adult about 2 mm, covered in fine hairs — moth-like.
•Wings held tent-like over the body when at rest.
•Weak, fluttery flight near sinks, showers and floor drains.
•A dark, slimy film inside the drain is the breeding ground.

Are drain flies dangerous?

Not in the way house flies are. They do not bite and are not significant disease carriers, but their presence points to a build-up of organic sludge in your drains, and large numbers can be a real nuisance in bathrooms and kitchens.

Life cycle & fun facts

Eggs are laid in the gelatinous film lining drains; larvae feed on the organic sludge, then pupate at the surface. The cycle can run in two to three weeks, so a drain left uncleaned will keep producing flies.

Fun fact: Drain flies are covered in water-resistant hairs — they can survive being flushed and simply climb back up the slimy drain wall.

What drain flies feed on

Larvae feed on the organic sludge and bacteria in drains, sewers and stagnant water. Adults barely feed — they exist to mate and lay eggs back into the slime.