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Thousands of true fly species belonging to the Diptera order of insects have been documented worldwide, many of which are common pests of homes. Fly species that are categorized as pests of homes are divided into two groups known as small flies and large flies. In addition to being relatively small in size, small fly pests breed in sludgy sources of organic matter like sewage and drain scum, while large fly pests breed on more solid and isolated sources of organic matter like carrion, excrement and garbage. While large flies can infest homes from either indoor or outdoor breeding sites, small flies tend to become abundant in homes that also contain their favored breeding substrate.

The most commonly managed small fly pests within homes include drain flies, phorid flies, fruit flies and fungus gnats. Small fly infestations are often associated with defective or clogged pipes, an abundance of drain sludge, and/or sewage backup. Pest control professionals often encounter small fly infestations within restaurants where multiple drains frequently become clogged with large amounts of food, but in homes, small flies often originate from sewers and emerge from indoor drains. Small fly infestations are common in homes that contain at least one toilet that is seldom used.

Toilets that remain unused long enough for water to evaporate out of the bowl provide sewer-dwelling small flies with direct access into homes. In order to prevent foul sewer odors from traveling into homes through pipes, all drains are connected to a special pipe that bends downward into a U-shape. This U-shape is known as a “P-trap,” and it holds a small amount of water that acts as an odor barrier. When drains are regularly used, P-trap water is constantly replenished, but if drains remain unused for a long period of time, P-trap water evaporates. Once this occurs, sewer flies can travel up sewage pipes and enter homes through dry toilet bowls. Simply flushing dry toilets will suffice to prevent more sewer flies from traveling into homes through toilet drains, and when small flies are found within a home that also contains a foul smelling bathroom, the flies are almost certainly originating from the sewer. When a small fly infestation cannot be eliminated by flushing a long unused toilet, a defective septic tank or a broken sewer pipe are the most likely causes.

Have you ever experienced toilet-related pest issues within your home?