Three Small Flies

Features - Pest Control

One Line Up. No Coincidences.

August 10, 2011

A food plant has been invaded, products are at risk of being contaminated, and the key to this mystery lies with the identification of the seemingly random line-up of suspects and what the villainy of each reveals about the plant. 

The Usual Suspects

  • Small Fruit Fly, aka Drosophila
    Identifying Marks: Although some have dark eyes, the most common distinguishing factor of the small fruit fly is its highly prominent red eyes.
  • Moth Fly, aka Drain Fly, aka Psychodidae
    Identifying Marks: “It looks like a very, very small Stealth Bomber,” said Jim Sargent, director of technical support and regulatory compliance for Copesan, referring to its overall shape. Its name is derived from the scales on its wings that make it appear moth-like, and it has “flattened thighs like willow leaves,” Sargent said. “Once you see one, you can always recognize it. There is nothing else like it.”
  • Phorid Fly, aka Humpback Fly, aka Phoridae
    Identifying Marks: While its most distinguishing factor is its name-producing humped back, the characteristic most easily identified with the naked eye is its tendency to run rapidly when disturbed, instead of immediately taking flight.

The Criminal Records
Small Fruit Fly. Attracted by decaying fruit and other decaying organic matter, the small fruit fly is the most common of the small flies. One source of such attractants can be created in the spray-washing of plant floors, said Ted Snyder, training and technical services manager for Batzner Services. In doing so, you force a lot of organic matter down the drain, he said. If you aren’t regularly cleaning the drain—as well as floor cracks, crevices, and corners where the organic matter can lodge, you could be creating feeding, breeding sites for the small fruit flies. 

The problem could also result from unclean recycling areas, or “It could be your product,” Snyder said, explaining that infestations could arise from not monitoring incoming and stored product for spoilage; or not processing fresh produce quickly enough.  These flies provide a real challenge, as the source can be difficult to find or even hidden, he said.

Moth Fly. “Moth flies (also called drain flies) do come up from drains, but they are not exclusive to drains,” Snyder said. They can come from, or be attracted to, the same sites as the small fruit fly, with a preference for even more highly decayed matter, such as that of sewage treatment areas and lines. If you have a nearby sewage facility, the moth flies can come in from outdoors; if the source of the infestation is within the plant, inspection should begin where most flies are seen, as they tend to stay close to the source.

Phorid Fly. “This is probably the most dangerous, most expensive, most troublesome small fly you can have,” Sargent said. “When you have an infestation, four times out of five, it means there’s a sewer break under the building.” And the only real option for correcting that is to tear up the floor, fix the break, clear the contaminated soil—“and hope that you got it all; that there isn’t another break.”

In addition, Snyder said, finding a small fly with a humped back “normally tells you that you have something at a high level of decay in your facility.” Sewer lines can break without anyone knowing, so the first sign is often the sighting of phorid flies, he said. They can come up through the drain or through a crack in the flooring near the break. “Hone in on where the phorid flies gather; they are probably coming up through a nearby expansion crack.”

The Crimes
“Small flies are filthier than large filth flies because they are most often associated with sewage, and they appear in higher numbers,” Sargent said. When you see flying insects, it will generally be two or three large flies, or thousands of small flies, he said.

Because they feed, breed, and live in sewage, small flies can transmit pathogens from those areas. In addition, according to the Food, Drug & Cosmetic Act, any insect found in a food production area is considered an adulterant. So, although it is rarely enforced, the presence of small flies could cause closure of the plant. In addition, he said, the preventive focus of the new Food Safety Modernization Act and the increased latitude it gives to FDA inspectors could mean stricter enforcement than in the past, including more plant closures.

“Generally small flies are an indictment of the sanitation and cleaning programs of food facilities. Spraying isn’t going to solve your cleaning problem,” Sargent said. “You can spray every day and knock down emerging flies, but that won’t get rid of your problem.”

The only exception to the sanitation solution is a plumbing break, he added. “You can have the best facility in the world, but if a sewer pipe is seeping under the building, you will have problems.” 


The Clean Up
For all suspects, clean up of the crime scene is a matter of finding and clearing out the source.

  1. As in any crime investigation, the key step is identifying the criminal, and in this case, it is the mandatory first step.
  2. Insect light traps (ILTs) can provide monitoring and capture of flies for identification and source detection; but “I would never rely on them for control of small flies,” Snyder said, noting it as more of a temporary fix. “It’s like the aspirin you get when you have a broken arm and are waiting for the cast to be put on.” But, he added, “I would expect facilities to have an ILT program for filth flies, so those will provide monitoring for phorid and moth flies; and a little for the small fruit fly,” which is less attracted to light.
  3. Once the suspect has been identified, the next step is using the knowledge of the suspect to find its source. For example, “Moth flies tell you that its source is pretty close to where it’s at,” Snyder said. “So if there is a fair number, you need to inspect right around there.

    No matter what it looks like,” Snyder reiterated, “You have to inspect there. I have seen bad infestations in otherwise clean facilities where a spot has been overlooked.

    “You should expect that level of inspection from your pest management provider,” Snyder added. “If not, you need to do it yourself—or the person who finds it will be an auditor and you will lose points.”
  4. “It’s not the fly that’s the problem, it’s the maggot,” Sargent said. “When you see a fly, it’s actually a grown-up maggot, and you have to find where that maggot came from.”
  5. To prevent future infestations:
    “Number one is: Don’t have liquid or organic waste,” Sargent said, adding, that means scrubbing drains with a brush, not just putting something into them.

    The flies generally originate outdoors, so proper screening of doors and windows and ensuring doors close securely are key to keeping them out. “They occur in the wild, but when they come inside, it’s flies gone wild,” Sargent said.

    “The best form of control is also the best form of prevention—sanitation,” Snyder said. The pest control operator (PCO) can help identify what is needed and the areas that need to be cleaned, but it is up to the plant from there. In fact, once a site is cleaned, there is often no need for chemicals to be applied. “Normally the sanitation alone will take care of it,” he said.

    And sanitation, Sargent re-affirmed, is up to the plant. “It is the responsibility of the facility to clean up and remove breeding sites. It is not the responsibility of a PCO. For small flies, the PCO is an important consultant, not a controller.
    “PCOs can point out sites and make recommendations, but they can’t do the sanitation or fix a sewer break,” Sargent said. “When it comes right down to whether you have small flies or not, it is up to plant management.”


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