Controlling Small Flies

Phorid flies, fruit flies and moth flies, oh my! If you’ve been in the pest control industry for a while, the most frustrating hours — maybe even days — have likely been spent attempting to solve a problem with one of these small flies. I know I have. I once spent nearly three days wandering a hospital in Houston looking for the source of the fruit flies that were appearing in a cardiac catheter unit. To make a long story short, the source turned out to be no more than 20 feet from the unit, in a forgotten trash can hidden behind a column in a corner. Have you ever had a day like that?

IDENTIFYING THE INSECTS. I’m asked a lot about small flies. The first question I always ask is, "What kind of fly is it?" Too many times, the answer I get is, "I don’t know." Well, I have to know if I’m going to be able to provide useful advice to the customer.

To narrow the possible identity of the fly, a series of questions needs to be asked:

1. What color is the fly? If it’s light brown it is probably a fruit fly or phorid fly.

2. Does the fly have red eyes? If so it’s a fruit fly.

3. If the eyes are not red, is the fly’s thorax severely arched and the head small? If so, it is likely a phorid fly.

4. If it’s a brown fly without red eyes or a severely arched thorax, are the fly’s body and wings covered with hairs, looking like a small moth? If so, then a moth fly is involved.

5. If the fly is black, is it thin bodied and fairly tiny? If so, it’s probably a fungus gnat.

6. Does the black fly resemble a small moth? If so, it’s a moth fly.

FINDING THE SOURCE. Of course, the offending fly could be another species or type of fly, such as a cheese skipper or a sphaerocerid fly. It is always best to consult a reference like the PCT Field Guide to the Management of Structure-Infesting Flies to be sure. The identity of the fly involved is critical to finding the source(s) because the identity provides the conditions in which the fly prefers to breed. Fruit flies, phorid flies and moth flies all breed in moist, decaying organic matter.

Fruit flies are partial to freshly decaying organic matter. They are found in fresh fruits and vegetables, garbage containers, in containers holding recyclable cans and bottles and in moist food trapped under equipment, in corners or in cracks.

Phorid flies can be found in similar areas to fruit flies but typically are discovered breeding in organic materials in a more advanced state of decay. Phorid flies can breed in more types of organic matter than any other type of fly. Although most often detected in drains, debris trapped in cracks and under equipment, phorid flies will breed in dead animals, organic glues, even organically based paints! This fly is the most difficult to control because of its varied breeding habits.

Moth flies are known commonly as drain flies because they usually are found in drains. They can survive in wetter environments than other types of flies, including where water stands for an extended period of time. The key to surviving in such an environment is the presence of organic matter.

Fungus gnats may be the easiest to pinpoint — in most situations. The larvae feed on fungi and molds, usually found in overly wet soil of potted plants and atriums. In the absence of plants, the fungi providing the food source will be a water leak of some type. A good question to ask is where any leaks have been repaired in the past two years. Often, the leak is repaired but the area is not dried out, thus permitting mold growth and providing a base for fungus gnat development. Occasionally, the gnats are breeding outside and entering through doorways or cracks around windows.

As with any pest, experience brings a number of lessons.

Lesson #1: With small flies, you’ll usually find more than one breeding source.

Take for example, a phorid fly infestation in a bakery. The professional assumed the flies were breeding in the drains and suggested the owner clean them. He did so using bleach and hot water which, if the flies had been breeding there anyway, would not solve the problem. Bleach doesn’t remove organic matter where the larvae live from the sides of the drain.

A close inspection of the drains by scraping with a screwdriver confirmed no fly larvae in the drains. By taking a putty knife, however, and scraping inside cracks under kitchen equipment where they met the floor, innumerable fly larvae were discovered. When the kitchen was cleaned using a water hose, flour would be washed into floor-level cracks. In fact, nearly every such crack in this bakery contained phorid fly larvae. Flies were even breeding underneath pallets of flour bags lying on the floor. The solution involved cleaning every crack, drying it and then sealing it. In this example, more than three dozen "sources" of fly larvae were discovered. Although not common, such cases will be encountered. Only a few breeding sites will be discovered for most infestations, but usually if the flies have been present for any length of time, more than one source is present.

Lesson #2: Flies fly, so don’t look for the source to necessarily be near where they’re seen.

Because most flies are active fliers, they can be found anywhere in an infested building. In one case, fruit flies being seen in an office building were traced to a dumpster in a sub-level loading dock. The flies were accessing the building by riding on the shipping elevator. In another situation, fruit flies were attracted by nighttime lights to the entry door of a hospital from their breeding site in a trash compactor/dumpster more than 50 feet from the doorway. The exception to this rule is moth flies — they are poor fliers and are usually present near the breeding source.

Lesson #3: Focus on areas where moisture remains constant.

All small fly species require constant moisture for larvae to survive. Investigate all such areas where organic matter is also present. Moth flies can survive in wet environments.

Lesson #4: A knife or small, flat spatula will be the best inspection tool when searching for breeding sites.

The breeding sites for most small flies will usually involve a comparatively small amount of organic matter typically trapped inside a crack, under equipment or the sides of a drain. A knife or spatula is needed to scrape out this debris to look for fly larvae — and that will confirm the area as a breeding site. Any moist or wet crack at floor level is a potential source.

Lesson #5: You can narrow the search area using traps.

A number of fly traps can be spaced throughout the infested rooms/building to pinpoint the area where the breeding sources are most likely to be found. Typically, the more flies found in a trap, the closer the trap is to the breeding site. This technique is used when the breeding sources have been difficult to locate. For fruit flies, a few ounces of fruit juice or vinegar will work well, as will a piece of banana with water.

Lesson #6: To confirm that flies are breeding in a drain or under a slab, use a clear plastic cup.

A clear plastic cup taped over a drain can trap emerging adult flies. With this technique, one can leave for a period of time, e.g., overnight, and then check for flies. When you suspect that phorid or moth flies are breeding beneath a slab, holes must be drilled through the slab and a cup taped over each hole. A clear cup is needed because the flies can be attracted to the light above the slab or the drain.

Lesson #7: Unless all breeding sites are found and removed, the infestation will continue.

The key to small fly infestation elimination is sanitation and repair of conducive conditions — both of which are the customer’s responsibility. The professional’s job is to point out areas requiring attention. It is common for a professional to identify a number of sites, have the customer clean them but the problem persists. This means breeding sources have been overlooked and additional investigation is required. It will be impossible to solve the infestation without the customer’s help.

Lesson #8: Bacterial drain products, regularly applied, can provide long-term relief from flies breeding in drains.

Several bacterial drain products are available to apply into drains to control phorid, fruit and moth flies. Properly used and regularly applied, such products have demonstrated success in eliminating flies breeding in the drains and in preventing infestations. Pest control companies are adding drain services to their commercial services and acquiring additional revenue from restaurants, schools and similar facilities that have experienced fly problems.

Some bacterial products can be applied into areas where moisture and organic matter collect, such as under dishwashing machines and ice makers. Regular applications can help control breeding until the areas can be repaired to eliminate them as potential breeding sites.

CONCLUSION. Controlling small flies takes persistence, time and attention to detail. The amount of organic matter needed to sustain a light infestation can be remarkably little, so it is easy to overlook breeding sites. The key is finding and eliminating these sites. If necessary, a space treatment can be used to kill any remaining adult flies. Use a process of elimination to narrow your search and pinpoint breeding sources. Start with the most obvious sites. In most situations, you’ll solve the infestation quickly, but don’t be surprised if the solution takes a little time. These little flies can be frustrating!