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For obvious reasons, pests that have been frolicking in filth are unwanted guests in any building that produces food products. Large flies — sometimes referred to as filth flies — are one of the major culprits of contamination that food safety professionals need to look out for.

“I mean, nobody wants to open up their bag of cereal and find a dead fly in ther,” said Chelle Hartzer, consulting entomologist, 360 Pest and Food Safety Consulting in Lawrenceville, Ga.

Large flies — which include species such as the house fly, bottle flies (known for their shiny, metallic blue-green thoraxes), flesh flies and blow flies — are called filth flies for a reason, said Jack Harris, vice president, Insect-O-Cutor in Stone Mountain, Ga., a company that provides insect light traps for commercial clients.

“They tend to reside in and seek out and breed within the areas of filth,” he said. “Whether it’s decaying vegetation, manure, rotting food, garbage or sewage, that’s basically their five-star resort.”

If a facility has a large fly problem, the pests are likely coming from outside the building, because that’s where their breeding source is, said Hartzer. These aren’t the annoying, itty-bitty gnats or fruit flies you’ll see gathering around a drain. Instead, think insects about the size of the common house fly — one of the No. 1 offenders in food processing facilities.

“Every single food processing facility has the potential worldwide to have house flies,” Hartzer said.

RISKS. Large flies are known to carry bacteria, fungi, parasites and disease. They may breed outdoors, but when they’re looking for additional food sources, they’re apt to enter your facility, where they will mechanically transfer whatever filth they’ve picked up along their journey, said Harris.

“The big three are typically Salmonella, E. coli and shigella,” he said. “All three of those impact a person’s digestive system. You wind up getting diarrhea, cramps and fever sometimes, anywhere plus or minus five days of that experience after being exposed to that bacteria that the large fly imparted onto the product.”

Large flies regurgitate their food, but they don’t always eat it all back up, said Harris, so that biomass can be deposited onto food products if a facility has an infestation.

A recent study showed that large flies are also capable of transmitting multi-drug-resistant bacteria, Hartzer said.

“So, bacteria that’s not just resistant to a drug, they’re resistant to multiple drugs,” she said. “Especially in situations with food, that’s not a good thing to have.”

IDENTIFICATION. If you’re noticing flies along your facility’s windowsills, that’s an indication of a large fly infestation. Large flies gravitate toward windows as they seek warmth or even escape, Harris said.

Food processors can also identify an infestation based on what these pests leave behind.

“There’s flyspeck, which is basically fly poop, which looks like tiny black pin drops along surface areas,” Harris said. “And that’s a telltale sign that you do have flies in the area.”

Consulting an in-house or third-party pest management professional is a sure way to identify the exact type of fly you’re dealing with, but many online resources exist that can be helpful to food professionals for initial identification.

“If you have a good eye and a good reference sheet, I think a lot of people can do it themselves,” said Hartzer.

Her favorite resource is the University of Florida’s Featured Creatures database, which offers in-depth information on a variety of pests ( Many universities offer similar identification resources via their entomology departments.

Harris also has found success working with Georgia’s land-grant universities and extension offices. “We’ve been fortunate to work with these folks, because they have the expertise and the resources to really help you out identifying what kind of pest and environmental concerns are happening on your property,” he said. “Whether you’re doing soil analysis or have odd insect activity — some strange beetles arrive or something — these people are available in every state and are a great resource.”

Food professionals shouldn’t be afraid to reach out to PMPs, Harris and Hartzer added.

“Look for PCOs that specialize in commercial or industrial businesses,” Harris said. “If I was a sanitarian, I wouldn’t hesitate to ask these service providers for references and use all the services available to protect our product quality and our brand integrity.”

PREVENTION. “When it comes to pest control and food safety, the three most important words tend to be sanitation, sanitation, sanitation,” said Harris.

Prevention measures are key when it comes to large flies, so a facility should do whatever it can to minimize attractants and safe harbors for pests. Outside the building, look out for messy dumpsters and critters leaving behind feces where flies congregate. Bruce Studer, general manager of Gardner Manufacturing Co. in Horicon, Wis., which manufactures fly control lights and commercial fly traps, recommended keeping all dumpsters at least 100 feet away from the building.

Exclusion measures are also important, such as implementing positive facility air pressure and installing screening and door and curtain strips. Seal any spaces where flies could attempt entry.

“Anything that you can seal — any door, any window, any vent, any pipe, any entrance that connects the inside of your site to the outside of your site — if you can seal that up or screen it, that will help immensely to keep those things out,” Hartzer said.

Another area that people tend to overlook is the inlets on the roof.

“Those need to be screened,” Studer said.

Harris has seen positive effects from facilities revamping their exterior lighting, switching from mercury vapor lamps to sodium vapor lamps.

“When your production facility’s out in the middle of a field somewhere and you have it lit up with very good color rendition mercury vapor lamps, which emit significant amounts of insect-attracting UV light, your building’s going to be bright and shiny, and it’s going to look fantastic from a distance,” Harris said. “Unfortunately, it’s also going to attract almost all the flying insects in the area over to it.”

Orange-tinted sodium vapor lamps emit significantly less ultraviolet light, which attracts the flies. The more ultraviolet light you can reduce, the better, Harris said.

“If you turn off the strobe light, they’re not going to come to the dance,” he said.

Facilities should also pay attention to irrigation and vegetation, Harris said, eliminating standing water around the premises, as well as any tall grasses growing against the building or near entrances.

“Ideally, you’d have an apron around your facility to reduce contact,” he said.

If your facility has insect light traps, monitor those and note if certain traps are filling with insects, Studer said.

“If you have a trap that, let’s say, is at the front of the building, and all of the sudden, you get a ton of insects in that trap, and then as you walk to the subsequent traps around the building, you’re starting to see less and less, well, you can focus your search by looking at the number of insects in an insect light trap on the glueboard or in the tray,” he explained.

CONTROL. Let’s say that despite your best prevention efforts, you’re seeing the signs of a large fly infestation in your facility. Flies are collecting in light traps, lining up on the windowsills or buzzing around the building. What to do now?

First, find out where the flies are breeding, said Hartzer.

“Maybe it’s a dumpster that overflowed and there’s gunk on the dumpster pad,” she said. “Or maybe there’s a dead animal. A lot of these things like to breed in all kinds of filth. So, where does your waste go once it’s outside the facility? Find those areas, make sure that they’re relatively clean and do those continual inspections on the outside as well as the inside.”

Food facilities should work alongside the pest control team, whether in-house or third-party, for best results, Hartzer said.

“Keep that partnership strong so that the pest control team is relaying the information on where the sanitation issues are or where some of those exclusion points might be,” she said.

Baits are another resource to control large flies, she said.

“You’re dealing with adults who are flying and entering the facility, but you’re also dealing with those larval populations that are in the messy, gunky sanitation issues,” said Hartzer. “So there’s good baits for the adults to start knocking those down, and they can be placed very carefully, very strategically for the best efficacy.”

For larval infestations, sanitation is usually the best treatment, Hartzer said.

“Sometimes you can do some liquid applications, but really, the larvae are all inside the gunky material,” she said. “So getting a pesticide or a treatment inside there doesn’t always work so well. You can just clean the mess, and that eliminates a lot of your problems.”

Photo courtesy of Stoy Hedges; Fly Photo: pzAxe | iStockPhoto

COMMUNICATION. Honesty is the best policy if a facility wants to rid itself of flies. Allow PMPs full site access, said Harris.

“Let the pest professional get in there with his or her tools, observations and experience and help fight the fight,” he said.

Documentation is essential, said Studer. When someone in the facility finds an area with an infestation, they should immediately document it and share that information with PMPs proactively.

“That’s where a lot of times the food facilities will fall down is they don’t document,” he said. “They rely on the PMP to document it. But the reality is the inspectors need to document it themselves and then let the PMP know.”

Most food facilities have a pest sighting log, Hartzer said, but many don’t use it properly or effectively. Quality assurance managers should make sure they are documenting to the best of their abilities the pest’s location and the cause of its presence before contacting a PMP.

“Communicate that information to them, and follow up with them and say, ‘What did you find? What do we need to do?’ ” Hartzer said. “Because again, there’s usually some sanitation issues that go along with this.”

Remember, eliminating pests requires partnership.

“Everybody has a role to play,” said Hartzer. “So however many employees are at that facility, at that plant, they can be keeping an eye out for these things. And when they notice a sanitation issue or when they notice flies are starting to circle around, that’s something that they can pass along to their pest control company so they can quickly go to those areas and address them.”

Added Harris, “As long as everyone is truthful with one another, there’s plenty of options to help keep a facility in good measure or to get it back to it.”

The author is a contributing editor to Quality Assurance & Food Safety magazine.