The Pests

Fruit Flies: Find the Fermenting Thing

Written for: Bar managers, kitchen managers, produce receivers

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.

Fruit flies are a sanitation signal with wings. They are small, they are fast to breed, and they are always within a few yards of something fermenting. In a bar that is the mat, the drip tray, the bottle well and the recycling; in a kitchen it is produce and the floor drain. The whole program is finding the thing.

What they breed in

Drosophilid or vinegar flies, such as the common Drosophila melanogaster, as larvae feed in damaged or fermenting, overripe fruit of all kinds. — UC IPM, Fruit Flies

Fruit flies are strongly attracted to, and breed in, fermenting fruits or liquids. — Texas A&M AgriLife Extension, IPM Action Plan for Small Flies

Large numbers of fruit flies may indicate unsanitary conditions including spilled or spoiled fruit and vegetables; poorly maintained garbage containers; accumulation of organic matter around drains, grout or broken tiles; and wet areas under and behind equipment. — Texas A&M AgriLife Extension, IPM Action Plan for Small Flies

Read that last sentence as a search list. In my experience the source in a bar is almost never the fruit on the garnish tray; it is the sticky film under the speed rail, the drain under the glass washer, or the recycling bin where the beer cans sit for a week.

The fix

Improve sanitation, especially with fruit and food material, soda cans (recycle), juice boxes, mop and broom heads and even floor drains. — Utah State University Extension, Fruit Flies

Regularly clean floor drains. — Utah State University Extension, Fruit Flies

Cultural, physical and mechanical management options are the best strategies and include posting notices to encourage the cleanup of spills, proper food storage and trash/recycle handling, elimination of standing water, fixing plumbing leaks, drying mops, emptying mop buckets and inspecting incoming produce and rejecting any infested or overripe produce. — Texas A&M AgriLife Extension, IPM Action Plan for Small Flies

Professional synthesis, not law. The order I use:

1. Produce in, produce out. Inspect it at the door, refuse the overripe case, and move produce through the walk-in fast. The Delivery Inspection Card has the line for it.
2. The wet places. Mop heads hung to dry, buckets emptied, the recycling rinsed and taken out, the bar mats and drip trays washed at close, the drain under the glass washer scrubbed on the weekly drain schedule (the same brush as for drain flies).
3. Behind and under. Pull the reach-in and the beer cooler once a month and clean the floor under them. A fermenting puddle under a cooler can feed fruit flies for weeks.
4. Then traps, if you want them. A vinegar trap is a reasonable monitor for fruit flies specifically (it tells you they are still there), but it is not a control, and it does nothing for drain flies.

A fruit-fly population that persists after all four is a sign that the source has not been found, not that a product is needed. No insecticide is a substitute for finding the fermenting thing, and a store-bought spray in a bar is a critical citation (who may apply pesticide).