Conquering the Challenges of Flies

Features - Pest Management

The what and why of fly management.

April 3, 2020

© adobestock

Some of the more difficult pest control challenges in food facilities involve flies, both large and small. There are no two situations that are exactly alike, and it is critical that the fly species be identified because the biology of each provides clues as to the source and best practices for prevention and elimination. But there are a number of commonalities in control for any pest fly. This article discusses non-chemical strategies and insecticidal controls that can be applied in food facilities, as adapted from the chapter “Strategies for Managing Fly Infestations” of the PCT Field Guide for the Management of Structure-Infesting Flies (2nd Edition) by Stoy A. Hedges (GIE Media, Spring 2020).


Sanitation. Good sanitation practices, both indoors and outdoors, comprise the foremost step in long-term management of most fly infestations. This is especially true for infestations of small flies, such as fruit flies and phorid flies, inside buildings. If inadequate cleaning practices allow the accumulation of moist organic debris, these flies can breed in large numbers throughout a facility. Regular cleaning and attention to areas where breeding sources are most likely to occur lead to prevention of future infestations of these flies.

Sanitation also plays a major role in managing filth flies such as the house fly and blow flies. Poor sanitation in outdoor areas as well as poor trash handling practices can attract these flies in large numbers to the vicinity of a building. The more flies that are near a building, the greater the probability that flies could enter. For this reason, exterior sanitation is an important factor that should be examined when dealing with filth fly infestations.

Bacterial Cleaning Products. Either mixed with water or applied directly as a ready-to-use pressurized foam, bacterial cleaning products consume the organic materials being used by flies for breeding. Depending on the product, the bacteria are applied in mopping activities, when cleaning floors with a hose, by pouring down the drain, or with a compressed air sprayer/foamer directly to problematic areas or dumpsters and trash receptacles where flies may be breeding.

It is important to note that chemical-based cleaners must not be used in the same sites that bacterial products are employed, as that will kill the bacteria, thus rendering the bacterial cleaner ineffective.

Exclusion. Even when good sanitation practices are followed outdoors, filth flies (and other flying insects) can enter buildings if doors are propped open, door and window screens are not in place, or cracks are present in the exterior of the building. Keeping flies out by excluding them is the most effective measure to control filth flies inside buildings. Doors and windows must be kept closed at all times unless proper screens are in place.

Depending on the fly involved, the screens installed in doors, windows, and over vents may need to be closely examined. For example, some commercial facilities located near ponds, lakes, rivers, and marshes can be plagued with tiny flies and midges which breed in these bodies of water. If the mesh size of the screening is too large, these tiny insects can enter structures with ease. The screens on all openings may need to be changed to a smaller mesh size. Flies can squeeze through some amazingly small cracks to enter a building.

Flies can squeeze through some amazingly small cracks to enter a building, and will then be attracted to all types of food — in both the office and processing areas.
© damrong | adobe stock

Once attracted to a building by food odors or warm/cool air currents, flies will enter cracks around doors and windows. As many cracks as possible in the exterior of the structure should be sealed with a shrink-resistant sealant.

Weatherstripping along all edges of doors should be in good condition and allow no space through which flies could enter. In commercial buildings with double entry doors, the vertical crack between the doors is often not excluded by weatherstripping, thereby allowing flies and other insects to enter.

Air doors are often installed in larger commercial facilities. To be effective, these must be installed so as to project a continuous current of air away from the entryway with such force that insects cannot enter.

Air doors are most effective where air pressure remains constant, thus an ideal location is a doorway which leads from a warehouse area into a production area of a food plant. The air door’s air current can be adjusted, providing reasonable assurance of excluding flying insects from these locations at all times.

Lighting. Although most pest flies are daytime fliers, outdoor lighting at night can be a factor in some situations. Many of the same problems caused by flies in the daytime are multiplied by nighttime-flying insects. For this reason, it is always a good idea to examine the specifications of exterior lighting on your building to determine if it might create problems for a particular facility.

Mercury vapor and metal halide bulbs emit white light that is in the nanometer (nm) range most attractive to insects. Facilities with this type of lighting generally have huge numbers of night-flying insects attracted to buildings, making entry more likely.

Trap placement is key when dealing with filth flies outdoors to avoid attracting more flies to the area than would ordinarily be present.
© Khotcharak | adobe stock

Additionally, attracting flying insects in larger numbers increases problems with spiders which feed on the insects. Switching lighting to sodium vapor bulbs or lamps attracts far fewer insects and yellow filters can be installed on the glass/plastic covers on exterior lights which helps to filter out most of the light wavelengths attractive to flying insects.


Traps. Many traps have been developed for managing flies over the years. The original fly trap was fly paper which was placed or hung in areas where flies frequent. Fly paper evolved into fly strips which are hung in windows and other areas of fly activity, and several products are available which employ a variance on this.


Other types of traps have been developed and are currently available, most of which use some type of food attractant to entice flies into a container where they cannot escape or will drown in liquid. Although scent and food-baited traps do catch large numbers of flies, placement is key when dealing with filth flies outdoors to avoid attracting more flies to the area than would ordinarily be present.

By far the most efficient traps for capturing flies are electric insect light traps (ILTs). Originally developed in the 1930s and 40s, ILTs have undergone extensive research and refinement in both design and size. (See Flying Insect Control with ILTs,, for more information.)


The application of insecticides is generally a last resort in fly management programs. In most situations, the breeding sources must be found and removed, sanitation improved, and steps to exclude flies taken before liquid residuals or fly baits are considered. Following are some insecticidal controls your pest management professional may implement.

Exterior Residual Liquid Treatments. Spot treatments with a residual water-based insecticide can be applied outdoors to surfaces where flies land to rest. This helps kill flies before they can enter the building. Particular attention should be paid to the walls along and around doorways and walls surrounding dumpsters. Label permitting, the residual may be applied to exterior or interior walls of the dumpster or to the underside of the lid or cover of the trash receptacle. Filth flies typically fly closer to the ground so applications can be made within six feet of the ground. Treatments can be made routinely when flies are active, usually during the warmest months, or should be made in response to an active fly infestation.


Interior Residual Liquid Treatments. While residual liquid insecticides can be made to some indoor areas, labels should be carefully read to determine where and how applications can be made in processing plants. In approved areas, application can be made to window sills using a paint brush or a light residual spray. A few residual aerosol insecticides also may permit spot treatments indoors to control flies. Small applications made to corners around windows are effective at killing flies attracted to the windows.


Fly Baits. A number of fly baits are available and can be helpful in reducing fly numbers outdoors. Some can be applied indoors to control filth, fruit, and phorid, if approved for food processing plants. Placed in areas where flies are active, bait attracts flies which feed on it and then die, sometimes very quickly. Fly baits come in several formulations—granular, spot fly bait (mixed in water), in stations or strips, or in aerosol type cans. Granular fly baits are generally labeled only for outdoor use and may be applied directly to the ground, inside dumpsters, or inside securable, enclosed fly-bait delivery systems/stations designed for protection from the weather. These typically need to be reapplied after rain or when the treatment area has been cleaned with water.


Fly Mist Machines. Automatic mist machines (dispensers) were once widely used in many commercial facilities and are still used today by some companies. These small battery-operated devices are mounted on walls near ceiling level in non-food areas where fly control is desired. A timer triggers the machine to periodically disperse a one-second burst of pyrethrins in the air. These same machines are also used to disperse deodorants in restrooms and other areas. Mist machines can be timed to release insecticide bursts on a regular interval (e.g., every 15 minutes) or be programmed to only apply during off hours at night or early morning when the facility is not operating. An ideal location for these machines is an entry vestibule where one must pass through two doors to enter a building. Another suitable location is a room where trash is stored. Also consider if persons passing near these machines might have concerns should the machine disperse insecticide when they are near it. If automatic mist machines are used in a fly management program, the pest control professional should be responsible for maintaining the machines.



ULV Treatments. A common insecticide treatment technique used to reduce the numbers of adult flies indoors is a ultra-low volumetric space (ULV) treatment, also just called a space treatment.


A space treatment is applied to kill remaining adults after breeding sources have been identified and removed. The cubic feet of space to be treated must be calculated and the correct amount of insecticide applied. In addition, the air handling system and smoke detector must be temporarily deactivated; standard personnel and food safety precautions followed; and all label directions read and followed. Typically, the facility will need to be unoccupied during application. Follow label directions for specific rates and application instructions.

A good fly management program will usually use two or more of the aforementioned control methods depending on the situation. Fly control is a true example of integrated pest management (IPM) where insecticides are used sparingly in the overall control program. The source of the infestation and conditions contributing to it are the keys to successful management of fly infestations.

The PCT Field Guide for the Management of Structure-Infesting Flies (2nd Edition), Stoy A. Hedges, is available at