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Drain Flies

- Tiny flies that breed in organic gunk in sewers and drains -

Summary: Drain flies are tiny dark flies that may suddenly appear in kitchens and bathrooms. Drain fly larvae develop in the gelatinous gunk that forms inside drains and or other stagnant water sources. Cleaning away this gunk from household drains will eliminate the flies. no other treatment is needed.

Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)

Drain flies/moth flies

These tiny (~1/10") dark colored flies are sometimes common in homes near bathroom or kitchen drains. The flies are harmless but can be nuisance if present in large numbers. Drain fly larvae develop in the gelatinous organic slime ("black gunk") that often accumulates in drains. The flies can also be common around cold-composted compost bins.

Drain flies, also called moth flies because of the moth-like scales that cover their wings (see photo right), belong to the fly family Psychodidae. A related fly in this same family, called the sand fly, is responsible for transmitting the disease Leishmaniasis. Leishmaniasis not occur in the US.
drain fly or moth fly

Drain or moth fly (1/10"-1/5"). Notice long antennae which are characteristic of primitive flies like mosquitoes and midges.

Does bleach control drain flies?

The best and safest way to control drain flies is to remove the gelatinous, often black, "gunk" that builds up in drains. This is the material on which drain fly larvae develop.

Drains can be cleaned with a brush or one of the new enzyme/bacterial drain cleaners. It is not necessary to apply insecticides or bleach to control flies, in fact bleach and insecticides don't work very well. Once the drains are cleaned of the gunk layer the flies will go away.

Drain flies can also develop in the slimy, organic material that often forms around leaky pipes or, outdoors around sewage ponds. If these flies are found indoors after all drains have been cleaned it may indicate a leaky pipe somewhere, perhaps under the house.

Drain cleaners and drain fly control

Clogged drains should be cleared with a chemical drain cleaner such as the old standby Drano (tm) because the new enzyme/bacterial drain cleaners won't work on clogs. Instead, the new enzyme-based/biodegradable products are intended to remove only the gelatinous film that the flies develop on. Use these cleaners several times a year in all drains to keep ahead of the gunk buildup. Drain cleaner gels and other supplies for managing drain flies can be found here (DoMyOwnPestControl.com).

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Jack DeAngelis, PhD,  , email:  [email protected]

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