Nuisance provisions of the Public Health etc (Scotland) Act 2008: guidance

SECTION 4 - SEWAGE FLIES

Classification

Sewage filter flies belong to the family Psychodidae, commonly known as moth flies. The principle sewage filter flies are Psychoda albipennis, other species of Psychoda and Tinearia alternata.. Sewage filter flies have a relatively slow breeding cycle with about eight generations a-year.

Habitat

They like moist, organic or septic systems for egg laying, and are common in the vicinity of sewage works. The larvae are often considered beneficial as an essential part of the cycle that breaks down waste into water-soluble compounds

Health Impact

They do not bite or sting, but can be a nuisance, flying in the eyes, mouth and nostrils of people. Because of their points of origin, they can carry disease, although actual transmission is extremely unlikely. They do not pose a contamination risk to food.

Nuisance

One way to confirm an infestation is by using a stick trap. As a general guideline, they might cause an occupier distress if 50 or more 'flying' sewage filter flies are present in a room on three successive days, though obviously this indication will vary and depend on such factors as room size etc. Most infestations take place during the spring and summer months as the adults emerge.

Assessment

Location of larval breeding sites is necessary and one method to check potential individual breeding sites is to cover the entrance with plastic film taped to the floor or fixture. If sewage filter flies are breeding there, they will accumulate beneath the film within a day or two.

Controls

Once located the larval breeding sites should be eliminated. Cleaning of breeding places to remove any organic matter will help elimination. A slow-moving drain can be cleaned with a stiff brush or other tool. Drains that cannot be scrubbed can be rinsed with water under high pressure, sterilised with boiling water, or treated with a bacterial agent to biodegrade the organic matter.

Household insecticides can be used to control adult sewage filter flies, but the effects will be very temporary unless the source of the larvae is also removed. Operators of waste water treatment works should have systems in place for treating beds with a larvicide where there is a risk of nuisance and should routinely check for high concentrations of sewage filter flies. The timing and dosing of the filter beds is critical to effectiveness, and must be carefully managed to prevent the release of chemicals into waterways or an effect on the balance of organisms in the ecosystem. In some cases it may be best to limit treatment to knock down or surface treatments.