House Flies and Other Filth Flies

The house fly and other types of “filth flies” can become nuisance pests, but also are important for their potential to harm humans and animals. House flies, for example, can spread diseases such as food poisoning and dysentery. Flies, including stable flies and mosquitoes (which are also classified as flies, or Diptera), can inflict painful bites while feeding on the blood of humans and other animals, and some species transmit disease.

The habits of filth flies favor the spread of bacteria and other disease-causing organisms. Filth flies often feed and lay eggs on garbage, manure and carrion before contaminating human foods and food preparation surfaces by landing on them. When feeding, house flies regurgitate their stomach contents onto food to liquefy it before ingesting it. They also may contaminate food and surfaces by defecating on them.

The order Diptera is composed of the “true flies,” and is one of the largest groups of insects. Diptera means “two wings.” True flies have only two wings (one pair), instead of four wings (two pair) found in most other types of winged insects. All flies are attracted to moist organic material upon which they lay their eggs. This habit makes filth flies valuable as scavengers, but also brings them in contact with humans.

Filth flies can be divided into two groups, determined by their appearance and food preferences. Filth flies, such as the house fly, blow flies and flesh flies, are relatively small, soft-bodied insects with large eyes. They are strong fliers. Other filth flies, e.g., drain flies, fruit flies and phorid flies, are smaller with more delicate bodies and legs.

The common house fly is a dull gray fly, ¼-inch long with four dark stripes on the middle section (thorax) of its body. House flies typically lay eggs on animal feces and garbage. White, legless maggots (the larval stage) hatch from the eggs and grow to about ½ inch. When fully grown, maggots crawl away from their food source to undergo the pupal stage. They form a dark brown cocoon, known as a puparium, and later emerge as adult house flies that can fly one or two miles in search of suitable egg-laying sites.

Blow flies are so-called because the larvae develop inside the bodies of dead animals, causing the carrion to have a bloated appearance. They also are attracted to garbage. Blow flies are about the size of house flies or slightly larger. They have been called “bottle flies” because their shiny blue and green color resemble colored glass bottles, though some species are shiny black or bronze. Large numbers of these flies indoors usually indicates the presence of a dead animal such as a mouse or bird inside the structure.

Appropriately named, flesh flies usually seek carrion or scraps of meat on which to lay their eggs. Like house flies, adult flesh flies are dark-colored (gray or black). Common species have three dark stripes on the thorax. They are slightly larger than house flies and have a checkerboard pattern on the abdomen.

While not always found in filthy situations, stable flies deserve mention because both sexes feed on the blood of animals, including humans, often biting around the ankles. The bites are painful but are not known to transmit disease to humans. Females lay eggs in rotting straw and manure, moist piles of animal feed and yard waste.

Not associated with filth, cluster flies are mentioned here because they are a common household pest. They also resemble house flies, but hold their wings parallel to the body, not in a triangular configuration as house flies do. Cluster flies are covered with fine golden hairs and have no stripes on the thorax. These flies are unusual in that they lay eggs on earthworms. The larvae consume the worms. In the fall, adult cluster flies often invade homes, especially attics, for a warm, sheltered spot in which to spend the winter. Unlike other filth flies, the principal means of controlling cluster flies is exclusion, that is, sealing buildings to prevent their entry.

Also known as vinegar flies, fruit flies are attracted to sweet or fermented liquids such as liquor, syrup, soda pop and vinegar, in addition to ripening/rotting fruit. Females lay eggs in and around these materials upon which their tiny larvae feed. The gnat-sized adults typically have tan-colored bodies and red eyes.

Also known as humpbacked flies because of their arched thorax, phorids are tiny, dark-colored flies. The larvae feed on a wide variety of decomposing organic (of plant or animal origin) debris. If suitable materials are present, huge populations of phorid flies can build up quickly. Sources of infestation include liquefied garbage, sewage and carrion, often hidden in places difficult to inspect and access.

About 1/8-inch long, adult drain flies are slightly larger than other small filth flies. Their broad, hairy wings have given rise to another name: moth fly. They also have been called sewer flies, because they infest raw sewage. Drain fly adults are often noticed resting on bathroom walls. The larvae survive submerged in the gelatinous muck that accumulates in floor, sink and toilet drains, by extending their breathing tubes to the surface for air. To eliminate infestations, drains and traps should be cleaned with a wire brush and/or drain cleaner.

Fungus gnats are relatively delicate, long-legged flies that look like tiny mosquitoes.

The larvae of fungus gnats live in moist places where their food, fungus, grows. Indoor infestations can be associated with pigeon droppings and with over-watered potted plants where fungi develop. When removing pigeon or bat droppings, take care to wet the area first with disinfectant to kill disease-causing organisms that can be present in the droppings.

The key to managing all filth flies is sanitation. Eliminating fly breeding sites, i.e., the material to which they are attracted to and on which they lay eggs, is usually sufficient to eliminate and prevent fly infestations. Conversely, without thorough sanitation, other control methods are largely ineffective. Therefore, trash should be kept in sealed containers (in trash bags and/or cans with tight-fitting lids). Dumpsters should be kept as clean as possible, emptied regularly and kept as far away from buildings as is practical. Manure and other decaying plant and animal material should be promptly removed. Also, eliminate areas of excessive moisture.

Just as sanitation is the key to successful filth fly management, inspection is the key to sanitation. To eliminate fly breeding sites, one must first locate the attracting material. Often this can only be accomplished by conducting a thorough inspection of the premises, and by knowing what to look for and where to look. First, identify the flies involved, inspect for material that attracts that species and then eliminate the material.

Another important step in fly management is to exclude them from the premises. This is done by keeping doors, windows and vents closed as much is practical, and by screening and sealing around these and other fly entry points. Automatic door closing devices and air curtains that blow air away from doorways also can be installed to supplement an integrated fly management program.

In addition to fly swatting, mechanical fly control includes trapping. Sticky fly paper is one type of fly trap. Ultraviolet light traps are another, often used to supplement fly control in commercial buildings. To be effective light traps must be properly placed. This type of trap should be placed where it cannot be seen from outside the building, no more than 5 feet above the floor (where most flies fly), and away from competing light sources and food preparation areas. Bulbs should be changed at least once per year.

While the use of pesticides is usually not the best means of managing filth fly problems, sometimes chemical control can be a valuable component of an integrated fly management program. Pesticide-releasing fly strips can be placed in attics and smaller, unoccupied enclosed rooms where filth flies are a problem. Contact (non-residual) pesticides labeled for fly control can be applied as a space treatment (“fogged”) to kill adult flies. This type of control provides only temporary relief, however, and cannot be relied upon to eliminate the problem. Residual pesticides – those that remain active for some time – can be applied to outdoor surfaces where flies rest, such as the outside surfaces of barns, stables, restaurants and houses. Some pesticide bait formulations are also available for outdoor fly control, including use around dumpsters.

NOTE: When pesticides are used, it is the applicator’s legal responsibility to read and follow directions on the product label. Not following label directions, even if they conflict with information provided herein, is a violation of federal law.

This document was published with the assistance of the University of Illinois Extension. It is available on the Illinois Department of Public Health Web site at: www.dph.illinois.gov. For more information, contact the Illinois Department of Public Health, Division of Environmental Health, 525 W. Jefferson St., Springfield, IL 62761, 217-782-5830, TTY (hearing-impaired use only) 800-547-0466.