Phorid/Humpbacked/Scuttle Fly

The common name of humpbacked fly comes from their humpbacked profile when viewed from the side which is due to the small head and prominent pronotum, while that of scuttle fly refers to their habit of running about in an active erratic manner. Phorids are mainly nuisance pests but there are cases of larval infestation of wounds, intestines, and eyes of humans. Worldwide there are over 2,500 species known, while about 226 are currently recognized in the United States and Canada.

Recognition

Adults about 1/64-1/4" (0.5-5.5 mm) long; with a characteristic humpbacked appearance in lateral view. Color black, brown, or yellowish. Antenna very short, basal 2 segments very small, 3rd segment globular and bearing a long arista (bristle), arista rarely plumose/feathery (1 species). Wings with strong, heavily pigmented veins in costal (front) area, remaining veins (usually 3 or 4) weak, oblique, and without crossveins. Hind femur flattened. Mature larvae up to 3/8" (10 mm); form, spindle-shaped with inconspicuous projections on posterior/rear segments to shorter, broader, and somewhat flattened with conspicuous projections dorsally, laterally, and particularly on the rear/terminal segment; color whitish, yellowish white, or grayish.

Similar Groups

  1. Small fruit flies (Drosophila spp.) with eyes usually red, antenna with arista plumose, wing with 2 breaks in costal/front vein and remaining veins strong, not oblique, and with crossveins.
  2. Darkwinged fungus gnats (Sciaridae) with one of weaker rear veins forked/branching in outer half of wing, eyes meet bases of antennae.
  3. Fungus gnats (Mycetophilidae) with coxae elongated, wing with thickened portion of costal/front vein ending near wing tip and several rear veins forked/branching.
  4. Black files (Simulidae) with wing broad at base and narrowing towards tip, usually gray.

Biology

Females lay their eggs either onto/into or at least close to the larval food, with 1-100 being laid at one time and up to 749 in her lifetime. There are 3 larval instars. The 3rd instar larva crawls to a drier area to pupate. The developmental time (egg to adult) for 2 common phorid flies is as follows:

  1. Dohmiphora comata (Bigot). Found throughout the United States and Canada. Egg batch size is 30-40 eggs. AT 74F/23C, females lay 9-12 eggs/day for a lifetime average of 368 (range 77-535). Developmental time in days at 72-75F/22-24C, egg 1, larvae 5-8, pupa 13-15, for a total of 21-28 days.
  2. Megaselia scalaris (Loew). Found Indiana to Massachusetts, south to Texas and Florida, and in California. Egg batch size averages about 32 eggs at 74F/23C, and 39 eggs at 81F/27C, with a lifetime total average of 391 eggs (range 42-749) at 81F/27C. Developmental time in days at 72F/22C, egg 1.3+, larvae 5.5+, pupa 15.1+, for a total of 22+ days; at 85F/29C, egg 0.7+, larvae 3.5+, pupa 6.0+, for a total of 11.1+ days.

Habits

Adults can often be found at flowers or on larval food materials which consist of moist decaying organic matter. Because they frequent such unsanitary places, they may transport various disease-causing organisms to food materials. Several species breed in human corpses and are commonly referred to as coffin flies when they become problems in morgues, mortuaries, and mausoleums. Phorid flies are of great concern in health-care facilities because of their unsanitary habits, and because larvae have been found in the open wounds of patients. Larvae breed in a wide variety of moist decaying organic matter which includes dung, vertebrate and invertebrate carrion, fungi, and decaying plant material. Some are parasitic on a wide variety of invertebrates including many insects and other arthropods. In structures, breeding materials can include the moist organic film lining drain pipes, the moist residue in the bottom of trash receptacles, the moist material found in the cracks of and under kitchen equipment, in elevator pits, in garbage disposals, in rotting vegetables and meats, dirty moist mop heads, faulty septic systems, etc.

In offices, overwatered potted plants are often the source. In health-care facilities and mausoleums, fresh-cut flowers in vases are frequently the source. In homes, pet stores, and zoos, phorid flies can breed in the moist soiled bedding materials and excrement found in the bottom of the pet-animal cages. Unusual places include a tin of boot polish, a pot of glue paint, and a bucket of animal-based glue. A particularly difficult breeding source to locate and correct is when sewage pipes leak or break under concrete slabs and the flies breed in the released moist organic matter and saturated soil, and then enter the structure through cracks/openings in the slab. Phorids in mausoleums present a unique challenge because the breeding sources are typically the corpses which are legally protected, the hidden internal drainage system which transports and/or harbors the body fluids which drain from the corpses and coffins, and the many cut flowers brought in by friends and relatives. With corpses buried in the ground, phorids appear about one year after the time of burial.

Control

Follow the basic 5 steps of identification, inspection, sanitation, mechanical control/exclusions, and insecticide application if required. The key is finding and eliminating all of the breeding sources; don't stop until all potential sources have been inspected. Insect light traps (lLTs) are effective in harvesting adult flies and can be of particular value in mausoleums or as a temporary measure until the breeding source can be located and removed in commercial accounts. It should be noted that several commonly suggested control methods do NOT work. Among these are pouring bleach and/or boiling water down infested drains because it does not kill the larvae breeding in the film lining the pipe; the film must be removed either mechanically or with special drain cleaners. For leaking or broken sewage pipes under slabs, drilling the slab and injecting pesticide into the soil does not work. The slab must be opened, the pipe repaired, and all the contaminated soil must be removed and replaced. Residual pesticides are rarely if ever required for the control of phorid flies, but after breeding sources have been eliminated, an ULV application of non-residual pesticide can be used to kill the adults present.