Goodbye, Large Flies

Annual Ant Control Issue - Annual Fly Control Issue

Exclusion and sanitation measures can go a long way in ridding your customer’s food facility of disease-carrying large flies.

June 15, 2022


© sndr | iStockPHoto

Editor’s note: This article originally appeared in an issue of PCT’s sister publication, Quality Assurance & Food Safety (QA).

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 PMPs need to look out for when servicing food- processing accounts.

“I mean, nobody wants to open up their bag of cereal and find a dead fly in there,” 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 your customer’s 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 client’s 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 your client’s 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 your customer is noticing flies along the 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.

Your customer also can 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.”

For food processors, consulting an in-house or third-party pest management professional is a sure way to identify the exact type of fly a facility is dealing with, but many online resources exist that can be helpful to your customer 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.”

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 PMPs should let their clients know what they can do 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 also should 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 customer’s facility has insect light traps, advise them to 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 customer’s best prevention efforts, they are seeing the signs of a large fly infestation in their 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.”

PMPs should work alongside the food facility staff for the 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.”

photo courtesy of Stoy Hedges

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.”

COMMUNICATION. Honesty is the best policy if a facility wants to rid itself of flies. Encourage your customer to allow the technician 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. Communicate to your customer that when someone in the facility finds an area with an infestation, they should immediately document it and share that information with you 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. Ask quality assurance managers to document to the best of their abilities the pest’s location and the cause of its presence.

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 senior editor of PCT magazine.


Case in Point

Based on his prior experiences, Stoy Pest Consulting’s Stoy Hedges gives us the rundown on all the ways flying insects can bug food facilities.

Flying insects are a fact of life around buildings and a constant presence outside during warm months of the year. Usually, when food facility managers and pest professionals think of “flying insects” in buildings, they focus on flies. Any flying insect, however, can end up in buildings, from wasps and beetles to leafhoppers and aquatic insects. How many flying insects might enter a building depends on a number of factors discussed below.

A building’s location plays the primary role in its risk for flying insect issues. A food plant located inside a city surrounded by other buildings will typically have fewer flying insects than one located next to fields, a wooded area or next to a body of water. One facility in Tennessee located next to a river was constantly bombarded by various aquatic insects that emerged from the river, attracted by exterior lighting. A food warehouse surrounded by fields had issues not only with flies entering inside but also bees, wasps, beetles, moths and various other insects.

Attracting as few flying insects as possible to the building is highly important to limiting interior sightings, more so for buildings in high insect areas as described above. During the day, overhead doors left open for ventilation or during loading efforts attract flies and daytime insects by the cooler (or warmer) air currents emitting from the building. Shadows created by open doors also are attractive to horse flies and deer flies. Bees and wasps fly inside searching for prey or moisture. Once inside, these insects become trapped and often fly deeper into buildings.

Nighttime flying insects comprise the greatest potential variety of insects attracted to a building via exterior lighting. Bright white lighting, such as mercury vapor or metal halide, attracts the most insects, while lighting in the yellow spectrum (sodium vapor) attracts the fewest insects. New exterior lights using LEDs do attract insects but fewer than metal halide. One strategy used by some facilities is to install exterior lighting using metal halide bulbs elsewhere to attract insects away from the structure.

Other strategies include using uplighting directed toward the building from a short distance away or directing lighting downward over entryways where the bulbs cannot be directly seen by insects away from the building.

Top: Moths and daytime insects captured on glueboard. Right: Metal halide light attracted insects to an area with a vent.
Stoy Hedges

Despite best efforts, flying insects will be attracted to a building, so steps at excluding them from entering inside is the best preventive step. Doors required to be open for ventilation should have screens installed. Screen doors are available for overhead doors. Doors that are used often will benefit from plastic strip curtains installed. Use of air doors or strip curtains between inside areas, such as between packaging and the warehouse, can screen out most flying insects from making their way deeper into the building.

The last line of defense is carefully located insect light traps (ILTs) mounted along the paths flying insects might take as they move from doorways deeper into buildings. To capture flies and daytime flying insects, ILTs should be located within five feet of the floor. ILTs mounted higher will capture mostly moths and beetles (although lower ILTs also capture these).

In high-traffic areas — where they can be safely used — electrocutor-style ILTs can accommodate large numbers of flying insects. ILTs that use glueboards can become covered up quickly in high-traffic sites and are best used in low-traffic areas and elsewhere, such as deeper into the building.

In cases where insects are noticed landing on the walls of buildings, spot and broadcast applications of residual insecticide labeled for such uses can kill insects before they enter. When flying insects are originating from landscaping, trees and shrubs, those plants may be treated with an appropriately labeled residual product (and corresponding state ornamental license). Where plant-originating insects are a chronic issue, removal and replacement of offending plants should be considered.

As with any pest management issue affecting a food plant or similar facility, analysis of the building, its location and contributing conditions should be done. Steps to mitigate each contributing condition should be implemented to minimize the numbers of flying insects seen inside, beginning with attracting as few insects as possible and then excluding them from entering.

The author is owner of Stoy Pest Consulting, one of the country’s leading urban entomologists and an author of several books.