We’re all familiar with the term ‘drain flies’ but what species are we referring to, exactly?


Small, slow flying insects found in kitchens and bathrooms are often identified as ‘drain’ flies. Whereas this is not particularly helpful in terms of identification, it is understandable how it has come about, as many of the small pest flies found in kitchens will be found breeding in drains at some point. So what species are we talking about and does it matter from a control point of view?

The first group of ‘drain’ flies belong to the family Psychodidae, which also include the medically important, biting sand flies (sub-family Phlebotominae). These flies are most accurately called drain flies and belong to the sub-family Psychodinae; they are also commonly called sink flies, filter flies, sewer flies or sewer gnats. They are true flies although they sometimes get incorrectly called ‘drain moths’, due to their superficial moth-like appearance.

There are around 4,000 species of Psychodinae worldwide, more common in humid tropical areas. Like all drain flies their life-cycle is rapid, which means populations can get quickly out of control. The eggs hatch within 48 hours and the resulting larvae moves through four instars in under two weeks, followed by one to two days in a pupae, before emerging as an adult.

Drain flies live around aquatic/sludge like environments, in which the larvae live and feed. The larvae, which are 4-5 mm long, are largely transparent and therefore difficult to see. It is more likely that the customer will spot the adult flies. The adults are generally nocturnal, although they are attracted to lights. During the day they may be seen resting on walls and ceilings. As they are not strong fliers, the adults do not disperse far from their breeding sites. Clogmia albipunctata, pictured above, is a common drain fly found in bathrooms and kitchens.

The other group of flies which are sometimes called ‘drain’ flies are Drosophila, the key species being Drosophila melanogaster. Although it is also called the common fruit fly, this does cause some confusion, as the true fruit flies belong to the family Tephritidae. It is probably best practice to call these flies vinegar or fermentation flies, due their habit of living around and breeding in rotting fruit.

Drosophila melanogaster, the vinegar or fermentation fly
Drosophila melanogaster, the vinegar or fermentation fly


D. melanogaster is one of the most studied insects in the world. With its rapid life cycle – they can progress from egg to adult in seven days at 28ºC – and large numbers of offspring per generation, it is widely used to study genetics and how traits are passed down through the generations. These same attributes also mean they can create a pest problem that can quickly get out of control.

Although there are variations in colour, the standard ‘wild’ form is a mid-brown fly, about 2.5-3 mm long, with obvious red eyes. The adults lay their eggs in rotting fruit or other decaying, sweet material. They are also attracted to wine and beer, which gives rise to one of their other common names, the bar y. If your customer has a problem with D. melanogaster, it means there must be rotten or fermenting fruit nearby. Checking kitchens for rotten fruit, especially under and behind cupboards and appliances, to eliminate potential breeding sites, is a good first step in any control program.

The other group of flies which are sometimes called ‘drain’ flies are the Phoridae, the phorid flies. However, they are more commonly called scuttle flies, due to their nature of scurrying across counter surfaces.

A common scuttle fly (Phoridae),
A common scuttle fly (Phoridae), Megaselia scalaris (Image credit: Charles Schurch Lewallen –, CC BY 3.0)


Phorid flies feed on rotting food – any decaying plant or animal material, and indeed their presence may indicate a dead animal nearby. This habit gives rise to one of their other common names, the confine fly. The various species vary in size from 0.5-6 mm and are typically brown to black in colour. They also have an obvious hump to the thorax. The source of infestation of phorid flies can be more difficult to locate as they are happy to breed in any decaying organic matter and will readily spread away from the breeding site.

Whatever the ‘drain’ fly causing the problem, the basic control process is the same; firstly, eliminate the breeding sites and secondly remove any adult flies. Locating the source of the infestation – the breeding site – is the challenge. It is likely to be different for the different groups of fly – for Psychodinae it is likely to be in a drain, typically a bathroom; for Drosophila melangogaster it is likely to be rotting fruit in a kitchen or food storage area; for phorid flies it could be anywhere.

Hygiene is key to eliminating and preventing a ‘drain’ fly infestation – ensuring any food spills are cleared up and there are no hidden decaying fruits or vegetables in cupboards is a good start. When investigating a potential infestation, remember even seemingly insignificant spills can support a significant population of larvae. For example, food debris in a small crevice can provide enough food for a significant number of Drosophila larvae. Ensuring drains are cleaned regularly to prevent the build up of food waste in kitchen drains, and prevent hair and other organic matter blocking bathroom drains is also vital to eliminate breeding sites.

Once the breeding sites have been removed, any adults also need to be eliminated. It is possible to treat rooms and surfaces with insecticide, but this can sometimes be problematic, as the areas of infestation are often food preparation areas, so always review the label. Pyrethrum based sprays can be a good option for sensitive areas. Trapping techniques can also be used – light traps are an option for Psychodinae and Phoridae, and a DIY trap consisting of a beaker with paper funnel, using cider vinegar as an attractant, can be quite effective for Drosophila.

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