Phorid flies, often called scuttle flies or humpback flies, are small pests that quickly infest homes and commercial buildings. They are recognized as “filth flies” because they breed in unsanitary conditions, posing a risk of transferring pathogens to food surfaces. Managing them is challenging because their breeding sites are frequently hidden and difficult to access. Understanding their behavior, particularly their attraction to light, is key to effective control.
Identifying Phorid Flies
Phorid flies are small insects, typically tan or dark brown, measuring less than 5 millimeters in length. They are often mistaken for fruit flies, but they possess a pronounced, arched thorax, earning them the nickname “humpbacked fly.” Unlike fruit flies, which have bright red eyes, phorid flies have smaller, darker eyes. When disturbed, they exhibit a key behavioral trait: a “scuttling” movement where they run rapidly and erratically across surfaces rather than flying immediately. This combination of a humped back and a quick, jerky running style confirms their presence.
Phorid Phototaxis and Light Attraction
The movement of an organism toward light is known as positive phototaxis. Phorid flies exhibit this behavior, meaning they are attracted to light, including specific ultraviolet (UV) wavelengths. This is why they may sometimes congregate near windows or light sources, particularly at night. However, this attraction is weak compared to the powerful olfactory cues guiding them to breeding sites. While light traps can capture adult phorid flies and serve as a useful monitoring tool, relying solely on them for control is ineffective because they do not eliminate the source where the larvae develop.
What Truly Attracts Phorid Flies
The primary drive for Phorid flies is the smell of moist, decaying organic material, which serves as the breeding and feeding medium for their larvae. Females seek out liquid or semi-liquid organic matter to lay their eggs. Common breeding sites are often hidden from view, making source location challenging. These sites include the organic sludge film inside drains, broken sewer lines leaking under concrete slabs, or decaying vegetables forgotten in basements. Any location with persistent moisture and decomposing material, such as drip pans, overwatered potted plants, or food residue beneath appliances, can become a breeding ground.
Practical Steps for Source Elimination and Control
Inspection and Identification
Effective control relies entirely on locating and removing the breeding source, as treating adult flies provides only temporary relief. The first step is a meticulous inspection of all areas where moisture and organic debris might accumulate, including plumbing areas, behind appliances, and trash receptacles. Sticky traps placed near suspected sources can help pinpoint the exact location of the infestation.
Treatment and Removal
For drains, physical removal of the organic film is necessary because hot water or bleach will not eliminate the larvae. Use a stiff brush to scrub the drain pipes, followed by applying a biological drain cleaner or gel containing microbes that digest the organic matter. Addressing plumbing issues is also essential, which means repairing leaky pipes or fixing broken sewer lines contaminating the soil beneath a structure. If the source is contaminated soil under a concrete slab, the infested material must be removed and replaced, often requiring professional intervention.