The Pests

Flies in a Kitchen: House Flies, Drain Flies, and the Film in the Drain

Written for: Restaurant owners and managers, bar managers, dish-area leads, maintenance staff

Parts of this page marked Professional synthesis, not law are a Board Certified Entomologist's reading of published records. They are not legal rules, they are not legal advice, and they are written to read the same for an owner, a customer, or either side's counsel.

“Flies present” is written on 78 routine inspections in the two years measured on this site, and it is critical 94.9 percent of the time. Flies are also the one pest with a real season on the Wasatch Front: the count peaks in August and September (seasonality). Two different flies, two different fixes.

House flies and blow flies: the door and the dumpster

The large flies in a dining room came in through an opening and are breeding somewhere outside, usually at the dumpster, the grease bin, or a bag that missed the pickup. The program is exclusion and sanitation, in that order: a self-closing back door, screens on anything propped open, air curtains where a door must stay open, a dumpster lid that closes and a pickup schedule that keeps it from overflowing in August. All of that is in the Food Code’s outer-openings section and in the Exclusion Walk.

Insect light traps help, placed right. They are supplemental, and the Food Code is specific about where they may not go:

Insect electrocution devices are considered supplemental to good sanitation practices in meeting the Code requirement for controlling the presence of flies and other insects in a food establishment. — FDA Food Code 2022, Annex 3, Public Health Reasons, 6-202.13

(B) Insect control devices shall be installed so that: (1) The devices are not located over a FOOD preparation area; and (2) Dead insects and insect fragments are prevented from being impelled onto or falling on exposed FOOD; clean EQUIPMENT, UTENSILS, and LINENS; and unwrapped SINGLE-SERVICE and SINGLE-USE ARTICLES. — FDA Food Code 2022, § 6-202.13(B), Chapter 6 p. 4

A fly strip over the drink station or a zapper over the line is cited on about 93 inspections in the window, almost always as a non-critical line, and it is one of the few pest lines that is corrected on site with any frequency: take it down (what inspectors cite).

Drain flies: the film in the drain

The small, fuzzy, moth-like flies at the dish machine and the floor drains are drain flies (moth flies). They do not come from outside. They breed in the gelatinous organic film that lines the inside of a drain, and the adults you see are the sign of larvae you don’t.

In commercial facilities and restaurants common developmental sites include sink and floor drains in food preparation areas, grease traps, mop drying tubs, and evaporation pans placed beneath appliances. — UC IPM, Pest Notes: Moth or Drain Flies

The eggs, larvae and pupae can be found in the bacterial muck, slime or gelatinous film often accumulating on the sides of drains and overflow pipes in homes, sewage disposal beds, septic tanks and wet compost. — Ohio State University Extension, ENT-41, Drain Flies

The only thing that removes them is removing the film. Mechanically, with a brush.

To remove the organic film within the drain, use a hard bristle brush with an industrial drain cleaner. — UC IPM, Pest Notes: Moth or Drain Flies

Remove larval habitat by using a hard bristle brush to remove the scum film from inside the drain. — Utah State University Extension, Drain Fly

This requires cleaning sinks, drain pipes, and traps with a stiff wire brush or by steam. — Penn State Extension, Moth Flies in the Home

Boiling water alone will not do it, and neither will bleach. This is the most common thing a kitchen tries, and the extension literature is unanimous that it is not a control:

A common misconception is that pouring boiling water and/or bleach down a drain will control these flies. — UC IPM, Pest Notes: Moth or Drain Flies

However, these treatments do not remove the organic film within the drain; and they provide at best short-term control of flies. — UC IPM, Pest Notes: Moth or Drain Flies

larvae hide behind scum making attempted treatments with boiling water, pesticides or other chemicals ineffective — Utah State University Extension, Drain Fly

Where hot water has a place is after the scrubbing, as a flush:

If using mechanical means, flush lines with boiling water or caustic drain cleaners to remove any material left behind by the cleaning process. — Ohio State University Extension, ENT-41, Drain Flies

So the routine is: pull the drain cover, scrub the inside of the drain body and the first run of pipe with a stiff brush and a drain cleaner meant for organic film, flush, and put it on the weekly list (self-inspection checklist). A kitchen with drain flies is a kitchen whose drains have not been scrubbed; nothing sprayed in the air changes that. And do not use a vinegar trap for drain flies; they are not drawn to vinegar the way fruit flies are, and the trap tells you nothing.

What I do about the adults

Professional synthesis, not law. While the drains are being scrubbed, the adults on the walls can be knocked down with a dilute soap solution in a spray bottle; in my own practice that is about 2 to 3 percent ordinary dish soap in water, applied to the insects, which wets and drowns them. I label this as field practice, not a published recommendation: I did not find an extension or peer-reviewed source that states it, so it is offered as what I do rather than as something a source says. It is not a pesticide, it does not touch the larvae, and it is no substitute for the brush.

What this site will not say

Nothing here claims a pesticide is “safe.” Products labeled for use in food-handling establishments exist for flying insects, with their own restrictions on where and how; the label and the licensed applicator decide that (who may apply pesticide). For drain flies, the honest answer is that no product on the market replaces the brush.