Drain Flies and Fruit Flies in Food Facilities

Editor’s Note: This article was reprinted with permission from Pinto & Associates.

When you have tiny flies emerging from drains in a food facility, they are usually either drain (moth) flies, fruit flies, or phorid flies. The first two, especially, can be associated with drains that need a good, deep cleaning. They breed in the semiliquid scum and goo that accumulates inside the drain. Often, a good drain cleaning program will go far in eliminating the problem. Phorid flies (and sometimes drain flies), on the other hand, can originate from breaks in the sewer line, often beneath the slab. Simple drain cleaning won't solve this problem.

In food facilities, locate and map all floor drains. Check them monthly. Ask staff about floor drains that have been covered by equipment and those that have been prone to blockage. You can determine whether or not the drains are a part of the overall fly problem by either (1) taping a plastic bag over the drain opening, or (2) laying a sticky trap or strips of tape, sticky side down, over the drain opening. Don't completely cover the drain opening. Check the next day for flies that have emerged from the drain and become caught in the bag or on the tape. You can also use a knife or spatula to scrape the organic goo from under and around the lip of the drain. Spread it out on a flat surface and, using your flashlight, check for fly larvae wriggling in the goo.

Cleaning fly breeding sites is often part of a pest management program offered to restaurants and other food facilities. The gelatinous gunk in drains that breeds flies cannot be successfully removed with boiling water, bleach, or household drain cleaners. Specialty drain cleaning can be done with scrub bushes, pressure washers, steam cleaners, or bacterial gels, but more efficiently with handheld foam equipment using degreasers and bacterial and enzyme cleaning products (biocides).

A foaming system is effective at cleaning floor drains because it allows you to fill the void quickly without removing grates. A 90 degree void tip lets you put the mixture directly on the sides of the drain. A biocide cleaning agent with a thick foam like shaving cream stays on vertical surfaces for an extended time, long enough to suffocate the larvae and allow time for the bacteria in the product to digest the scum until the grease is released a dissolved. Do not wipe off the foam; let it do its work. 

Use a wet/dry vac if necessary to clean up any residue after foam application. After the initial drain "clean-out," foaming is usually repeated monthly but follow the manufacturer's directions for repeat applications. To keep little-used floor drains from drying out, add a tablespoon of cooking oil into the drain. Oil floats and covers the surface of the water in the drain, reducing water loss through evaporation.

When using bacterial cleaning products, do not mix them with other common cleaning agents. And make sure that restaurant cleaning staff do not pour bleach or mop water with other cleaning agents down drains that have been treated with biocides. These cleaning products will kill the bacteria and stop your treatment from working. It is an advantage, however, for cleaners to clean floors with a biocide mop product or hose end product. reducing water loss through evaporation.

Once drains are clean, look for other less obvious fly breeding sites too, especially for fruit flies. The main sources of fruit flies are usually not the drains but other small spaces where food and moisture accumulate. These flies need such a small amount of material in which to breed, that typical cleaning methods can't eliminate all of the breeding sites. In fact, standard cleaning usually just adds the moisture needed to make the sites more desirable for fruit flies. Check any place where organic material stays constantly damp such as overlooked gunk behind drink dispensers, inside equipment frames, in the bottom of recycling containers, refrigerator drip pans, and other areas that are rarely (if ever) cleaned. Use your foaming cleaner to hit these areas as well.

The authors are well-known industry consultants and co-owners of Pinto & Associates.