The Pests

House Mice and Norway Rats: A Quarter Inch Is a Door

Written for: Restaurant owners and managers; maintenance staff; anyone cited for droppings, gnawing, an open bait station or a door gap

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.

Rodent lines are rarer on Wasatch Front restaurant reports than cockroach and fly lines, and the ones that appear are mostly droppings, gnawing, or a bait station that was open or unlabeled (what inspectors cite). The biology of a mouse is the whole reason the back door matters so much.

The sizes

Mice can enter through openings as small as 1/4 inch. — UC IPM, Pest Notes: House Mouse

House mice can squeeze under gaps ¼ inch tall, and through openings 3/8 inch wide. — UC IPM, Pest Notes: House Mouse

Rats of either species, especially young rats, can squeeze beneath a door with only a 1/2-inch gap. — UC IPM, Pest Notes: Rats

All holes larger than 1/4 inch should be sealed to exclude both rats and mice. — UC IPM, Pest Notes: Rats

A quarter inch is the thickness of a pencil. If daylight shows under the back door, a mouse fits. That gap is also the single most-cited pest line in the county, under “Outer Openings-Protected,” and the Food Code’s own fix is “Solid, self-closing, tight-fitting doors” (the Food Code page).

The three elements

Successful rat control typically includes three elements: sanitation measures; structural exclusion (also called “rodent proofing”); and population control. — UC IPM, Pest Notes: Rats

Exclusion comes first and lasts longest.

Exclusion is the most successful and permanent form of house mouse control. — UC IPM, Pest Notes: House Mouse

The most successful and longest-lasting strategy for rat control around structures is exclusion, or “building them out.” Seal cracks and openings in building foundations and any openings for water pipes, electric wires, sewer pipes, drain spouts, and vents. — UC IPM, Pest Notes: Rats

The Exclusion Walk goes door by door and penetration by penetration with these measurements on it. Door sweeps, weatherstripping, copper mesh or sheet metal in the gaps around pipes, screens on vents, a self-closer on the delivery door.

Sanitation keeps going.

Sanitation is fundamental to rat control and must be ongoing. — UC IPM, Pest Notes: Rats

While good sanitation (achieved through cleaning and decluttering) will seldom completely control house mice, poor sanitation can attract them and will permit them to thrive in greater numbers. — UC IPM, Pest Notes: House Mouse

The dumpster lid, the grease bin, the mop sink, the dry storage floor and the gap behind the line are the sanitation list. A mouse needs very little food, and a bag of rice on the floor of dry storage is a month of it.

Population control is the part your pest company does. Snap traps and covered, locked, labeled bait stations, placed along walls and runways and checked on a schedule. The Food Code is explicit on stations:

7-206.12 Rodent Bait Stations. Rodent bait shall be contained in a covered, tamper-resistant bait station. P — FDA Food Code 2022, § 7-206.12, Chapter 7 p. 4

and on why:

Open bait stations may result in the spillage of the poison being used. Also, it is easier for pests to transport the potentially toxic bait throughout the establishment. Consequently, the bait may end up on food-contact surfaces and ultimately in the food being prepared or served. — FDA Food Code 2022, Annex 3, Public Health Reasons, 7-206.12

A rodenticide station in a kitchen is a critical item if it is open, unsecured or unlabeled. Loose bait blocks under a shelf are a citation and a hazard. If your technician’s stations are not locked and labeled with the product and the company’s name and license number, that is the first question to ask (questions to ask).

The records

Professional synthesis, not law. A rodent program is the one most often asked about in a records review, because the stations are numbered and the counts are easy to keep. A station map and a station log (number, location, date checked, what was found, bait replaced) is the record that shows a program was running and working; it is in the Records Binder. The count also tells you when exclusion has done its job: a station that reads zero for three months is a station on a building that is sealed.

Droppings, gnawing and dead rodents

Droppings and gnaw marks are what an inspector usually writes when there is rodent activity. Clean them up with gloves and a disinfectant, not a dry sweep, and write the location down; the location is where the next trap goes. A dead rodent in a trap is its own line if it is left: “Removing Dead or Trapped” applies to traps as much as to glue boards (dead pests count).

If you are not sure whether what you found is a mouse dropping, a cockroach dropping or something else, text a photo to (801) 477-0223.