Structural and Public Health Pests: Flies (House Flies, Filth Flies)

Pest Management In and Around Structures October 13, 2009 Print Friendly and PDF

Many species of flies can become a problem in schools. Each fly species has a distinct breeding site inside or outside the school building. In order to control flies, it is necessary to know which species is causing the problem and where it is breeding. Drain flies, fruit flies and fungus gnats are addressed in the next section.

Flies such as house flies, little house flies, dump flies, blow flies, and blue and green bottle flies which breed in food wastes (garbage) and/or animal feces, are generally referred to as "filth flies." Other flies such as stable flies breed in decaying vegetable matter like grass cuttings. Flies that invade cafeterias and kitchens are not only a nuisance; they also present a health hazard because they can contaminate food, utensils, and surfaces. Biting flies, like stable flies, can inflict painful bites.

The key to solving persistent fly problems is proper identification of the species. After the problem fly has been identified, information on life cycle, breeding sites, and effective management options can be readily obtained from a number of sources.

Table 1
Flies most likely to be encountered in schools and other structures. Drain flies, fruit flies and fungus gnats are addressed in the next section.

Common and species name Geographic distribution
house fly, Musca domestica Throughout the US.
little house fly, Fannia canicularis Throughout the US.
dump fly, Hydrotaea aenescens Throughout the southeastern US.
blow fly, Calliphora sp. Throughout the US.
blue bottle fly, Phaenicia spp. Throughout the US.
green bottle fly, Phaenicia spp. Throughout the US.
face fly, Musca autumnalis Throughout the US.
stable fly, Stomoxys calcitrans Throughout the US.


Monitoring and inspection for flies

It is important to correctly identify problem flies and pinpoint their breeding sites. Some of their characteristics can help you with identification; alternatively specimens can be taken or sent to a county extension agent who should be able to assist in identification.

If they cannot identify the specimen they will be able to refer you to a specialist who can. To collect specimens inside, use sticky flypaper or gather dead specimens from window sills and light fixtures. Individual flies captured for identification purposes should be held in a small vial to preserve key identifying characters.

Inspection practices should include ensuring that trash cans or dumpsters are placed away from building entryways; proper use of plastic bag liners in trash cans; all trash disposed in dumpsters is enclosed in sealed bags; adequate clean up of spilled food and drinks; properly sealed openings throughout the building exterior; and tightly fitting doors, door sweeps and window seals.

Cultural and physical options for fly management

To manage flies, you must find and reduce breeding sites, install and maintain screens to keep flies out of buildings, kill those flies that do get inside with a fly swatter or flypaper, and reduce or eliminate the odors that attract flies.

In schools that have programs where wastes are removed frequently, it is unlikely that flies are breeding on the school property. It is more likely that odors from dumpsters, garbage cans, kitchens, and cafeterias are attracting flies to the school from the surrounding neighborhood. House flies and blow flies, the species that most commonly invade buildings, usually develop outside and follow odors into the building. They can also be nuisance pests when students or staff eat outside of the building. In schools where waste removal is infrequent, fly populations can breed at the waste collection site.

Cultural, physical, and mechanical management options are preferred methods for management of flies and include the proper management of waste, physical methods such as screens and flyswatters, and properly maintained and fitting doors and windows.

Flies found inside a school building enter from the outside in almost all cases. Therefore, barriers preventing access of flies to the building are the first line of defense. Cracks around windows and doors where flies may enter should be sealed. Well-fitted screens will also limit their access to buildings. Outdoors, regular removal (at least once a week) and disposal of organic waste, including dog feces and rotting fruit, reduces the attractiveness of the area to adult flies and limits their breeding sites. Garbage should not be allowed to accumulate and should be placed in sealed plastic bags and held in containers with tight-fitting lids. Garbage should also be placed as far from a building entrance as is practicable. In general, poor exclusion and lack of sanitation are the major contributors to fly problems.

Sticky fly papers or ribbons are effective at eliminating low numbers of flies in relatively confined areas, but are not effective enough to manage heavy infestations or to provide control in an outdoor setting. A number of fly traps for outdoor control are commercially available and can be effective for periodic fly populations when they are not competing with nearby garbage or animal wastes. Indoor fly traps are also available.

Manufacturer‘s directions must be followed for the placement and use of these traps. For control of just a few flies, the time-tested fly swatter is appropriate. If fly swatters are used near food preparation areas, all food must be removed from the area and all food-contact surfaces thoroughly cleaned to avoid contaminating food with insect body parts.

Cultural and physical strategies for flies

  • Remove individual flies using flyswatters, fly paper or appropriate indoor light traps. Do not place flypaper or sticky strips above or near food preparation areas.
  • All food waste from the kitchen, cafeteria and other areas should be separated from other garbage, drained so it will be as dry as possible and then stored in sealed plastic bags before discarding. Place containers with small amounts of food waste, such as milk or yogurt cartons, into sealed plastic bags before disposal.
  • Plastic bags used for waste disposal should be thick enough to avoid tearing or puncturing by insects such as yellow jackets.
  • Promptly fix drains or electric garbage disposal units that leak, or drains that allow food waste to accumulate under sinks or floors. Leaky drains can attract many species of flies. Remove any food waste that has accumulated under sinks or floors or in crawl spaces or basements at the site of the broken drain, and then clean the area thoroughly.
  • In food preparation areas, rinse all cans, bottles, and plastic containers before recycling or discarding.
  • Inform students, teachers, and staff of the importance of placing garbage inside the proper containers. Garbage should not be left lying on the ground.
  • Place exterior trash cans and dumpsters away from building entrances. To avoid attracting flies into the building, place dumpsters and recycling containers upwind from the outside doors of the school, particularly for the doors to the kitchen or cafeteria. When dumpsters are downwind, flies are attracted to the waste odors and then find the odor trails that the breeze blows down from the doorways. Following these odor trails, they find their way into the building.
  • Wastes should be collected and moved off site at least once a week. Because flies breed faster in warm weather, garbage collection may have to be scheduled twice a week to reduce breeding sites.
  • Make sure garbage can and dumpster lids seal tightly when closed and remain closed when not in use. Repair or replace garbage cans with holes or with lids that do not close tightly.
  • Regularly clean garbage cans and dumpsters to prevent the build-up of food waste, an ideal place for flies to lay eggs. Use a high pressure stream of water or a brush and soapy water, if necessary. A solution of borax and water will eliminate odors. If possible, dumpsters should be fitted with drains so they can be hosed or scrubbed out as needed. Another option is to require the refuse company to clean the dumpster or replace it with a clean one more frequently.
  • Replace dumpsters with self-contained, non-leak compactors specifically designed to prevent leaks.
  • Properly clean and maintain exterior drains in trash handling areas including loading docks to avoid accumulation of organic matter and liquid.
  • Flies can develop in soil soaked with water used to clean garbage cans and dumpsters. Check these areas regularly. If you see maggots, scrape them up along with the soil and dispose of everything in a plastic bag sealed tightly.
  • Inspect dumpsters and other outdoor trash receptacles daily and remove any wastes lying on the ground.
  • Garbage cans on the school grounds should have removable domed tops with self-closing, spring-loaded swinging doors. Cans should be lined with plastic bags that can be tightly sealed and removed daily.
  • Keeping adult flies out of sensitive areas is the most important control measure that can be undertaken. Install screens over windows, doors, and vent holes to prevent flies from entering buildings. Weather-stripping or silicone caulk can be used to insure a tight fit. Torn screens can be repaired with clear silicone caulk. Screen doors should be fitted with springs or automatic closing devices that close the screen door firmly after it is opened. External doors that cannot be screened should be fitted with automatic closing devices, and/or vertical strips of overlapping plastic that allow human access but prevent fly entry. "Air walls" that force air across openings are another alternative to screen doors.
  • Fly traps can be used to reduce adult fly populations, capture specimens for identification, and monitor the effectiveness of control programs. Fly traps are not toxic and are more selective than using insecticide. Traps need to be serviced regularly, appropriately placed, and repaired or replaced when damaged.
  • Remove animal droppings promptly and put them into plastic bags that are sealed before disposal.
  • Storing garbage in sealed plastic bags and having cans and dumpsters cleaned and emptied frequently to eliminate odors is very important.
  • Eliminate the access point where flies are entering by sealing cracks, installing door sweeps, repairing door and window seals, etc.
  • Clean up food and drink spills immediately.
  • Store food items in sealed containers.
  • Use heavy gauge liners for waste containers and empty containers at the end of the day so that food is not left in the building overnight.

Table 2 Commonly used products for physical, cultural or mechanical management of flies and uses.

Type Example Products Uses
sticky traps Catchmaster Gold Stick® Fly Trap Catchmaster Bug and Fly Bonide Fly Catcher Ribbons Tapes or traps mounted in areas where flies are entering or resting. Avoid placing over food preparation areas.
light traps with sticky capture surface Catchmaster Dynamite 911 Gilbert® 2002GT Flying Venus Fly Trap Mounted in entryways or other areas where flies are encountered. Mount so that light is not visible from outside to avoid drawing flies to entryways.
light traps with electrocuting grids Fly-Zapper 22/14 Electrocutor Gilbert® 220 Guerrilla Fly Electrocutor Trap Mounted in entryways or other areas where flies are encountered. Mount so that light is not visible from outside to avoid drawing flies to entryways. Not for use in food preparation areas where insect body parts may come into contact with food or food preparation surfaces.

Pesticide options for fly management

While chemical pesticides may be effective for suppressing adult fly populations in some situations, they are not a substitute for proper sanitation and aggressive elimination of nuisance-fly-development sites. Because flies can quickly develop resistance to insecticides, use them only as a last resort to obtain immediate control of severe adult fly infestations, after all possible nonchemical strategies have been employed.

In most school situations, pesticides are not needed or recommended for fly management. Sanitation along with exclusion to keep flies out should be sufficient. In rare cases where non-chemical methods are not possible or effective, a non-residual aerosol may be used to knock down flies. Outside, a residual insecticide may be applied to surfaces such as walls and overhangs that are being used by the flies as resting areas. Fly baits used in trash or other areas may be effective in reducing the number of adult flies if proper sanitation practices are followed. However, when flies have access to garbage, baits will not effectively control them.

Table 3 Commonly used insecticide products for flies and uses.

a. Insecticides carrying a CAUTION label or exempt from EPA registration, in formulations that reduce potential for exposure

Active Ingredient Example Products Uses
imidacloprid Maxforce® Granular Fly Bait 432-1359 Granular formulation. To reduce ingestion and exposure hazard, use in a pre-manufactured bait station or inaccessible areas.
imidacloprid Maxforce® Fly Spot Bait 432-1359 Liquid formulation. To reduce exposure hazard, apply to inaccessible non-human contact surfaces

b. CAUTION-label formulations with greater potential for exposure.

Active Ingredient Example Products Uses
2-phenethyl propionate, eugenol EcoEXEMPT KO (EPA Exempt) GNon-residual contact insecticides spray.

c. CAUTION-label formulations with greater potential for toxicity and/or exposure.

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.