Check your flies: Winged insects can be indoor pests

Local Agrilife extension agent provides tips for fighting insect pests

Staff report
House fly adults are 1/4 to 5/16 inch long. The top part of the thorax is dusty gray and contains four equally broad stripes running from front to back. The fourth wing vein makes a sharp bend to the outside and almost meets the third at the wing tip. Adults have sponging and sucking mouthparts, while larvae have hooks in their mouthparts for tearing.

Most homes and businesses eventually experi­ence problems with indoor flies. Flies are one of the largest groups of insects. Un­like most other winged insects, which have four wings, flies have only two.

Smaller house-infesting flies include drain flies, fruit flies, phorid flies, and fungus gnats. Larger flies, such as blow flies and flesh flies, occasionally invade homes to lay eggs on a decaying carcass. These flies rarely infest homes for long.

House flies and mos­quitoes rarely breed inside structures; however, they readily take advantage of open doors or unscreened windows to get indoors for food or shelter. Insecticides alone are rarely successful in eliminat­ing indoor fly infestations. Fly control is most effective when breeding sites in the structure are identified and eliminated. Because each type of indoor fly prefers slightly different breeding sites, identifying the fly should be the first step in any control effort.

Small house-infesting flies are generally less than 1/8 inch (4 mm) long as adults. All of the following flies are capable of breeding and living entirely indoors. These flies, however, are also found outdoors and may enter through open windows or doors. Fruit flies’ infestations are most frequent during the summer.

Fruit flies are sometimes referred to as vinegar or pomace flies because they are attracted to vinegar and almost any decaying fruit.

These flies will breed in almost any fermenting fruit or vegetable. They are also commonly found in spilled syrups, wine, or beer—es­pecially in moist places, such as under commercial kitchen equipment, bars, soft drink dispensers, and in cracked tile or flooring. In homes, breeding is more common in overripe or damaged fruits (especially ba­nanas) and spoiling vegetables such as rotting onions or potatoes.

Phorid flies are another fly found in homes. They often run and stop repeat­edly before taking wing, giving them another common name, “scuttle fly.”

Phorid fly larvae feed on a variety of decaying plant and animal matter. They are found in any type of moist, decaying material including decomposing animal carcasses, garbage, drainpipes, flowers in vases, wet potted plant soil, garbage cans, broken garbage disposals, dung, feces, and fungi.

Drain flies are also called moth flies because of their fuzzy, moth-like appear­ance. Drain fly larvae and pupae live in the thin micro­bial films often found in drains, underground septic tank field lines, and on filter stones in wastewater treatment plants. Drain flies feed on algae, bacteria, fungi, and other microscopic organisms associated with the film. Indoors, drain flies can breed in floor or sink drains, air conditioner and refrigerator con­densate pans, bottoms of garbage cans, under broken or cracked tile, in tracks of sliding glass doors, in wet cracks in the floor, or around dirty floor drains.

Fungus gnats are mosquito like, with long slender legs and bead-like antennae—though much smaller than mosquitoes. Also, unlike mosquitoes, fungus gnats do not bite, though they can be a nuisance when flying around one’s face.

Fungus gnat larvae live in moist locations such as potting soil. Larvae feed on the roots of plants or on fungi growing in potting soil. These infestations are most common during the winter months.

Indoor insecticide sprays or fogs may provide temporary relief from fungus gnat infestations, but they will not solve the problem. The only way to get rid of fungus gnats is to find and eliminate the breeding sites.

Large indoor flies are larger flies found principally outdoors but can also become indoor pests. House flies rarely breed indoors but may enter through open windows or doors.

House flies are outdoor flies that do not bite. They are generally not an impor­tant indoor pest unless doors or windows are poorly screened or frequently left open.

House flies breed in decaying organic matter such as livestock feces and garbage. House flies are not only a nuisance; they are thought to carry more than 100 diseases.

Soldier flies are outdoor flies that occasionally enter homes and buildings. Control involves finding and eliminating the food source. Breeding sites include rotting organic matter, such as spoiled grain, dead birds or other animals in an attic or chimney, or a decaying bee nest in a wall. Indoor worm compost bins have been known to har­bor soldier flies. Carrion flies include flesh flies and blow fly’s presence of these larvae in a home usually indicates that a bird, squirrel, rat, etc., has died somewhere in the structure.

Blow flies and flesh flies indoors indicate a home that is poorly sealed against wildlife. The house should be inspected for any openings where birds or mam­mals might enter. These openings should be sealed with sturdy wire, sheet metal, or other pest-resistant material. Chimneys should be sealed with a chimney cap, and roof turbines and vents should be screened to exclude birds, bats, and other wildlife.

Control suggestions


The key any indoor fly problem is to find and eliminate the source, that is, anywhere excess moisture and organic debris may have accumulated.           

·      Find and clean up any spilled or spoiled food on floors or in pantries

·      Clean sink and bathtub/shower drains

·      Check under liners in garbage containers and, if necessary, use soap and water to remove all organic residues

·      Clean under and around the floor drain cov­ers—especially in commercial buildings

·      Clean under foam floor mats in commercial kitchens

·      Hang mops off the floor to encourage drying—do not store dirty mops

·      Inspect and clean under and around the feet of refrigerators or other kitchen          equipment where organic matter collects

·      Check and clean condensate lines and trays as­sociated with refrigerators and icemakers

·      Check for signs of rodents or other wildlife that might indicate the presence of a dead animal—seal any potential animal entry points

Drain and septic treatment

To check whether a drain is breeding site, place a length of clear packing tape across the drain without totally covering the opening.

Check the tape periodically. If you see flies stuck to the tape, you have found a source of infestation.

After locating the sources, drains must be cleaned to eliminate any bacterial film inside the plumbing. Chlorine bleach or drain cleaner is largely ineffective at removing such films. To eliminate these breeding sites, you must clean the pipes and traps with a stiff brush. After scrubbing, flush the lines with hot to boil­ing water to remove any material left in them.

An alternative to scrubbing is to use an antibac­terial drain product designed to biodegrade organic films. These drain treatments usually require repeat applications over several weeks to eliminate the bacte­rial films.


Light traps take advantage of a fly’s attraction to short wavelength light.

They are most effective for larger flies, like house flies, but may also trap small flies and other flying insects. Baited traps are frequently used for fruit flies and, occasionally, for other flying insects.

Insecticides for fly control

Once the fly-breeding areas are cleaned or eliminated, you should not need to use insecticides. However, insecticides are sometimes helpful to knock down remaining adult flies, or to help control flies that come in from other locations.

Pyrethrin aerosol sprays, or other “flying insect” sprays, can provide temporary control of adult flies indoors. For outdoor fly problems, a residual insecti­cide labeled for outdoor fly control can be applied to fly resting sites. Be sure to follow label directions carefully, as many insecticide labels now prohibit spraying pavement, surfaces exposed to rain, or surfaces that drain onto pavement. The goal of these precautions is to prevent insecticides from being washed into streams or storm drains.

In some cases, commercial baits are available for controlling house flies. Bait formulations include products that can be used in bait stations or applied as a liquid or slurry to areas where flies land. Some baits can be toxic to pets or livestock, so follow label safety precautions.

For further information, contact Mark Arnold, County Extension Agent-Agriculture/Natural Resources, 701 South I-35 E Service Road #3, Waxahachie, or call 972/825-5175 or email: .