Control of Small Flies

Features - Pest Control

Seek and Remove Attractants

March 31, 2015

The presence of small flies in a processing plant can be a major public health concern. The flies will feed, breed, and live in contaminated areas then fly to land on food or food surfaces, transporting pathogens and increasing the risk of foodborne illness in humans. Two of the most common small flies are the fruit fly and the drain fly. To find out why these pests are such a problem and what can be done to prevent and eliminate them, QA questioned experts from the pest product industry. Following are their responses.

1. Why are small flies a problem in plants?

“Fruit flies and drain flies need certain conditions to thrive, including an available food supply and moist, warm conditions for breeding,” said BASF Global PSS Product Education Manager Thomas Nishimura. “Most food processing plants have these conditions readily available.”

Facilities that process and store liquid foods are very susceptible to fruit fly and drain fly infestations, said Syngenta Professional Pest Management Technical Services Representative Nicky Gallagher. “Food facilities can provide all the resources that these flies need, whether it is accumulated moist food, drains in need of sanitation, garbage receptacles, unmaintained floors with cracks and crevices, broken food containers, or even employee break rooms.”

Fruit flies have many attractants, said Aunt Fannie’s CEO Mat Franken, including color, scent, yeast content, fermentation, sugar, and others. They most frequently come into a plant by their “hitchhiking” on the foodstuffs brought into a facility, but they also can come in from the outdoors, he said.

Additionally, said Rockwell Labs Founder and CEO Cisse Spragins, fruit flies feed and breed in fermenting material, even very small amounts. So when food and moisture collects in a crack or crevice and remains undisturbed, fermentation can occur and the area can become a breeding site.

Drain flies, on the other hand, tend to originate from filthy sources that contain sewage, rotting food, or other organic materials, Gallagher said.

This is because, Spragins said, drain flies lay their eggs in organic gunk that builds up in drains, particularly drains that are actively used and kept moist. Such drains are prevalent in food processing areas, and food particles contribute to the organic build-up, creating the ideal environment. Additionally, she said, “the wash-down cleaning used in many food plants can make matters worse, regularly pushing moisture and food particles into cracks and crevices.”

Small flies—and their eggs—hide very well in shipping cartons and in drain pipes, said Sea Hawk Systems Consultant Robert Benson. “Drain flies are particularly difficult to eliminate because they are protected by the biofilm that resides on the walls of the drain pipe.” Ordinary spraying of insecticides only attacks the exposed surfaces that are sprayed. Covered areas and drains are unaffected, and this is where the insects and their eggs survive.

Additionally, Benson said, insect larvae leave residues that putrefy food and leave unsightly contamination, and insects leave dead insect bodies and body parts. This can be a major problem because both FDA and USDA have very specific rules on how much insect contamination may be in food. 

One of the problems of small fly control is that there are a variety of species, so, as with any pest, proper identification is critical. Frequently, Baumann said “small flies are categorized as fruit flies or drain flies, and further investigation shows that they are not related to fruit or drain flies at all.” Because they all have distinct biology and habits, effective control is dependent on proper identification.

2. Why are fruit/drain flies attracted to food plants?

Different fly species have different favorite attractants. Some like fermenting sugars, decaying vegetative material, or decaying proteins; all like fats, oils and grease; and moisture and carbohydrates of any sort can be strong attractants, said MicroSpring International Entomologist Jerry Hatch. Understanding these attractants enables a good inspector to look for where the flies could go and eliminate those areas before the flies get there—so you always stay ahead of the fly. “Most people look for where the flies are, so they focus on areas the flies have already been. But the key to control is to look for the attractants,” he said.

Both fruit flies and drain flies use decaying organic matter for food and breeding purposes, Nishimura said. Drain flies, as their name implies, can be found in floor and sink drains, as well as water traps of plumbing fixtures, sewage filters, and disposals. Such sites are often out of reach and difficult to keep clean, enabling them to become excellent, undisturbed harborage sites.

Fruit flies are attracted by ripened fruit, fermenting products, and sugary substances such as are found in many basic manufacturing ingredients, soda, and recyclable containers, he said. Additionally, “their small size makes them difficult to locate and they are often transported inside through product deliveries or through exterior entry areas near outside breeding sites.”

3. What can plants do to prevent small flies?

“Prevention is really the key to any successful small fly elimination in food manufacturing facilities,” Franken said. “It only takes two fruit flies to mate, and fertilized females can lay up to 500 eggs per fertilization.” As a result, fruit fly problems can get out of hand quickly. Fortunately, he added, new technologies allow plant management and personnel to efficiently and cost effectively prevent small fly blooms from occurring. Additionally, pest management professionals (PMPs) should have standard prevention protocols in place that utilize technological advances in fruit fly-prevention products. “Prevention is the only way to adequately inhibit fruit fly reproduction in your facility.”

Fruit and drain flies are both found in moist environments, but there are differences between these two types of flies, Gallagher said. Fruit flies feed and breed in yeast found in rotting and fermenting fruit, vegetables, and other organic materials. Drain flies develop in moist organic environments or polluted shallow water. They are typically found in the gelatinous film of drains, sewage disposal beds, septic tanks, and moist compost. So controlling these can help prevent small fly infestations.

Plant personnel are the eyes of the PMP as they are there each day, said Nisus Vice President of Technical Services and Regulatory Affairs Greg Baumann. “Any pests should be reported and noted in the Pest Sighting Log. If possible, samples should be collected for the PMP to examine on the next service visit.” Additionally, he said, sanitation is vital for prevention: Keep things clean and dry; look in hidden areas for spilled food including sweet materials; and pay close attention to the mop area. “Many times floors are mopped, but the wet mop with donut filling is left propped against a wall where it ferments and leads to infestation,” he said.

“Sanitation is paramount, to remove the organic build up,” Spragins said. Bio-sanitation (cleaning products that contain microbes that digest organic matter) are most effective in the moist, high organic-load environments encountered in food plants.

Because insects eat microscopic bacteria and mold, good housekeeping, cleaning with disinfection products, and vigilance are the key elements to good insect control, Benson said. However, he added, two distinctly different protocols are required for the prevention and control of fruit flies and drain flies. For fruit flies:

  • Upon receipt of fresh fruit, empty the shipping cartons, and remove them from the premises.
  • Wash the fruit and store it in a manner to prevent contamination. 
  • Have the premises regularly sprayed and or fogged in a manner appropriate for flying insects.
  • Focus daily cleaning on the elimination of potential food sources for the insects. 

For drain flies:

  • Clean all drains with heavy-duty cleaner that will cut through biofilm.
  • Scour the inside of drain pipes with a heavy-duty brush above the water trap. 
  • Apply an approved foam insecticide so that it clings to the pipe walls to give it sufficient contact time to penetrate fly eggs.

Following GMPs and SSOPs also helps in small fly prevention. Following are a few particular practices that Hatch recommends, but the most important, he said, is “to have someone responsible for making sure the cleaning is done at the end of each shift.”

  • If something is spilled, clean it up, and keep all wet or used wipes in a covered bin.
  • Use only self-closing garbage can lids.
  • Wash out cans and bottles that will be recycled.
  • Don’t allow windows or doors to be propped open.
  • Keep exterior garbage cans at least 25 feet from any entrance.
  • Reduce lighting above exterior doors.
  • Don’t stack, store, or plant anything next to an opening to the facility.


4. What should be done if an infestation is detected?

“Jump on the problem immediately,” Benson said. It is critical that the source of the infestation be found. 

Additionally, Franken said, it is important that plant personnel notify management immediately if small flies are seen anywhere in the plant. 

The first control measure should then be to remove/correct the breeding site(s), Nishimura said. “For drain flies, this would mean the physical removal of the organic detritus blocking the drain, and treating drains with effective products labeled for the prevention and control of drain flies,” he said. “Drain cleaners used alone in this instance would be of little use.”

For fruit flies, thoroughly inspect areas used for food storage/disposal of fruits, vegetables, and sugary food sources, including their empty containers, that may attract fruit flies and provide breeding areas. “If contaminated products are detected, remove them from the premises and dispose of them properly,” he said.

“Personnel should focus on thorough sanitation, paying particular attention to cracks, crevices, and voids where moist food can collect and be undisturbed, and drain areas,” Spragins said.

When working with a pest control provider, you should ensure that they install monitors and regularly check them, Hatch said. Following each service, any concerns should be brought to your attention, and non-chemical corrective actions should be performed as the first choice of action.

“Look to reduce the attractants by using bio-remediation materials,” he said, explaining, “Bacterial products can quickly reduce the breeding media and fly populations, if applied correctly, by removing the gelatinous breeding matter. This doesn’t mean pest products can’t be applied, but judicious and careful choices should always be made before applying a pesticide.” Although inspections can be conducted by facility management, professional training and knowledge really is needed to know what, where, and how to apply control measures, he said.

To accomplish all this, Benson recommends that plants have a written insect-treatment protocol in place along with a written protocol focused on cleanliness and control of trash and clutter. It is also helpful to use disinfecting cleaners and approved water-based, non-odor insecticide; train staff; regularly audit plant procedures; and bring in an outside expert initially and once a year thereafter.

5. How can small flies be eliminated?

Fly management can be complex and challenging, Gallagher said. Flies can fly or be carried along wind currents, so the location of the breeding source may not be close to where adults are found. In other situations, breeding sources may be so inconspicuous that only the eyes of a trained professional can find them. “An effective fly management program should first involve the development of a plan based on the unique requirements of the plant, as it may be organic certified or use complex equipment,” he said. “However, regardless of the type of plant, the potential for pest conditions should be understood and a plan implemented that involves inspection, sanitation, non-chemical and chemical control methods.”

Pest management professionals can apply IGRs and/or boric acid powder as a dust, a liquid or as a foam application to breeding sites, Spragins said, adding, “These products can be blended with a bio-sanitation product for application as well.” Traps and fogging also can provide relief from existing adult flies.

The most effective means of maintaining control is through Integrated Pest Management (IPM), Nishimura said. These include:

  • Inspection. For both an initial response to a pest infestation and follow up, ongoing inspections should be conducted. This is crucial for evaluating the results and effectiveness of a program, as well as revealing infestation/breeding sites that go unnoticed in routine pest control inspections.
  • Exclusion. Keep doors and windows closed, and conduct scheduled maintenance for repairs. On the exterior, look for breaks in caulk sealants, screens, weather stripping, and mortar joints that may allow pest entry.
  • Sanitation. Ensure that food/breeding areas are removed and breeding conditions corrected. These areas can be as small as food debris collected in wheel castors, table supports, etc., or as large as garbage can contents, dumpsters, and large drains.
  • Mechanical. Mechanical measures can include use of insect light traps (ILTs), glue traps, and air curtains. ILTs must be placed away from competitive light sources and window and door sight-lines to avoid attracting flying insects from the outside. Some flies are not highly attracted to UV light traps, so pheromone and other types of traps can be helpful.
  • Biological. After physically scouring drains with a stiff brush, use microbial drain cleaners to reduce the organic wastes which are food and breeding sources.
  • Chemical. As a last resort, a minimal amount of pesticide can be directed at the target species at specific locations within the plant. Areas such as trash dumpsters near building exteriors are sites where judicious chemical use can help with fly control. Use only when necessary and strictly according to label directions and precautions; ensure the label is in alignment with the use site; and always read and follow the label.

The key to small fly management in food and beverage plants is going back to the basics, Baumann said. “Find the source; control the pest; and take steps to prevent further infestation.”


The author is Editor of QA magazine. She can be reached at