Norway Rat
A large, stocky burrowing rodent that nests at ground level in and around buildings, contaminates food, gnaws structures, and can carry diseases that affect people.
Key facts
| Scientific Name | Rattus norvegicus |
|---|---|
| Beneficial Status | none |
| Class | Mammalia |
| Family | Muridae |
| Genus | Rattus |
| Kingdom | Animalia |
| Order | Rodentia |
| Organism Type | rodent |
| Pest Status | True |
| Phylum | Chordata |
| Professional Recommended | yes for established or recurring infestations |
| Protected Status | none |
| Risk Level | high |
| Species | Rattus norvegicus |
| Taxon Authority | (Berkenhout, 1769) |
| Treatment Recommended | True |
Overview
The Norway rat is a large, ground-dwelling rodent that burrows around foundations and moves into the lower levels of buildings, where it raids food, gnaws on nearly anything, and can pass diseases to people — handy for the rat, less so for the homeowner who finds its calling card behind the pantry. Source: https://ipm.ucanr.edu/home-and-landscape/rats/pest-notes/
Identification
A Norway rat is heavier and more thickset than the slender roof rat, with coarse brownish fur, a paler gray belly, and a blunt muzzle. Adults weigh roughly 7 to 18 ounces (≈200–510 g). The small ears do not reach the eyes when folded forward, and the scaly tail is shorter than the head and body — laid back over the animal it reaches only about the middle of the skull. Source: https://ipm.ucanr.edu/home-and-landscape/rats/pest-notes/
Lookalikes
The main look-alike is the roof rat, which is sleeker and a strong climber; its tail is longer than its head and body combined, while the Norway rat's tail is shorter than its body. Where both share a building, Norway rats stay in basements and ground floors and roof rats take the attic and upper levels. A house mouse is far smaller and easy to tell from an adult rat by size. Source: https://ipm.ucanr.edu/home-and-landscape/rats/pest-notes/
Biology
Norway rats breed prolifically, so populations build fast. Young are born about 21 to 23 days after mating and reach breeding age in roughly three months. A litter holds six to twelve pups, and an average female has four to six litters a year, weaning more than twenty offspring annually. Source: https://extension.missouri.edu/publications/g9446
Where Found
This is a burrowing rat. Its burrows appear along foundations, under woodpiles and rubbish, and through gardens and fields. Indoors, Norway rats settle in basements and ground floors rather than climbing high. They live throughout the 48 contiguous United States. Source: https://ipm.ucanr.edu/home-and-landscape/rats/pest-notes/
Seasonality
Norway rats are active throughout the year, so it pays to check for signs in any season. Source: https://ipm.ucanr.edu/home-and-landscape/rats/pest-notes/
They breed most actively in spring and fall. Source: https://extension.missouri.edu/publications/g9446
Signs
You usually spot the evidence before the rat. Look for droppings near food-prep, storage, and pet-food areas, plus fresh gnaw marks, runways and burrows next to buildings and fences, and dark smudge or rub marks where rats drag their oily fur against beams and pipes. Source: https://ipm.ucanr.edu/home-and-landscape/rats/pest-notes/
Tracks in mud or dust and gnawing, climbing, or squeaking noises in walls also reveal rats. Source: https://extension.missouri.edu/publications/g9446
Risks
Norway rats pose health, property, and food-safety risks at once. Both UC IPM and CDC document that rats transmit serious diseases to people. UC IPM names typhus, leptospirosis, rat-bite fever, and foodborne illnesses from pathogens such as Salmonella, E. coli, and Campylobacter. CDC, which covers human disease transmission, also lists salmonellosis, hantavirus, and plague — spread directly through contaminated food or contact with rat urine, droppings, and bites, or indirectly through fleas and mites that feed on rats. Source: https://ipm.ucanr.edu/home-and-landscape/rats/pest-notes/ Source: https://www.cdc.gov/healthy-pets/rodent-control/index.html
For property, burrowing can undermine foundations and slabs, and rats gnaw wood, wiring, plastic, and even soft metals, shredding insulation for nests. For food safety, rats eat and contaminate stored food; one rat can eat about 30 pounds of grain a year and foul ten times that much with urine, feces, and hair. Source: https://ipm.ucanr.edu/home-and-landscape/rats/pest-notes/ Source: https://extension.missouri.edu/publications/g9446
Is It A Pest
Yes. The Norway rat is a true pest with no homeowner upside indoors — it contaminates food, damages structures, and carries disease risk. Any confirmed activity in or against a building warrants prompt action. Source: https://ipm.ucanr.edu/home-and-landscape/rats/pest-notes/
Beneficial Notes
There is no recognized beneficial role for Norway rats in homes, yards, or food settings; this is an introduced commensal pest. The one place the species is valued is the laboratory, where domesticated strains are used in research — a far cry from the wild urban and farm rats this page covers. Source: https://ipm.ucanr.edu/home-and-landscape/rats/pest-notes/
When Not To Treat
"Don't treat" here mostly means "confirm it's a rat first." A mouse, an attic squirrel, or a misread dropping calls for correct identification before deploying traps or bait, because Norway rat control (ground-level) differs from roof-rat control (elevated). Once an infestation is confirmed in or against a building, delay only lets the colony grow. Source: https://ipm.ucanr.edu/home-and-landscape/rats/
Prevention
Exclusion is the most durable defense — "build them out." Seal any opening larger than 1/4 inch, which closes gaps to both rats and mice: foundation cracks and the gaps around pipes, wires, drains, and vents. Fit doors, windows, and screens tightly, capping chew-prone edges with sheet metal where gnawing occurs. Pair sealing with sanitation — store food and pet food in rodent-proof containers, and clear woodpiles, rubbish, and dense ground cover that shelter burrows. Source: https://ipm.ucanr.edu/home-and-landscape/rats/pest-notes/
Treatment
Run a full IPM program, not one tactic. Snap trapping is the safest, most effective tool around homes and garages: set traps close to walls, behind objects, and at fresh sign, each at a right angle to the wall with the trigger nearly touching it. Bait with a pea-sized amount of nuts, dried fruit, bacon, or dry pet food tied to the trigger. Where baiting fits, place tamper-resistant bait stations near burrows, walls, and travel routes. Finish with exclusion (seal everything over 1/4 inch) and sanitation so the site does not refill. Source: https://ipm.ucanr.edu/home-and-landscape/rats/pest-notes/ Source: https://extension.missouri.edu/publications/g9446
Inspection
Inspect at ground level and below: check foundation lines, slab edges, woodpiles, rubbish, compost, and dense vegetation for active burrows, and trace runways marked by rub marks and droppings. Indoors, work the basement and ground floor first, following gnaw marks, smudge trails on beams and pipes, and droppings near food. Source: https://ipm.ucanr.edu/home-and-landscape/rats/pest-notes/ Source: https://ipm.ucanr.edu/home-and-landscape/rats/
Kids
A Norway rat is a big brown rat that digs little tunnels called burrows near houses, barns, and gardens. It has small ears, a blunt nose, and a scaly tail a bit shorter than its body. Rats chew through wood and wires and get into food, which is why grown-ups don't want them in the house. The best way to keep rats out is to seal up holes and not leave food lying around — a tidy kitchen is a rat's least favorite restaurant. Source: https://ipm.ucanr.edu/home-and-landscape/rats/pest-notes/
Sources
How we know: Identification, biology, habitat, signs, damage, prevention, and control come from UC IPM (ipm.ucanr.edu) and University of Missouri Extension (extension.missouri.edu). Human-disease facts use two independent sources — UC IPM and CDC (cited only for disease transmission). Taxonomy follows ITIS, which lists Rattus norvegicus (Berkenhout, 1769) in family Muridae. Source: https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=180363 Review status: unreviewed (draft).
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