Also called the brown rat, house rat, barn rat sewer rat, gray rat or wharf rat, it is a slightly larger animal than the roof rat. Adult Norway rats weigh an average of 1 pound. Their fur is coarse and usually brownish or reddish-gray above and whitish gray on the belly. Blackish individuals occur in some locations. Although they can climb, Norway rats tend to inhabit the lower floors of multistory buildings.
Range and Habitat
The Norway rat (Rattus norvegicus) lives in close association with people. In urban or suburban areas they live in and around residences, in cellars, warehouses, stores, slaughterhouses, docks and in sewers. On farms they may inhabit barns, granaries, livestock buildings, silos, and kennels. They may burrow to make nests under buildings and other structures, beneath concrete slabs, along stream banks, around ponds, in garbage dumps and at other locations where suitable food, water and shelter are present.
First introduced into the United States around 1775, the Norway rat has now spread throughout the contiguous 48 states.
Norway rats are primarily nocturnal. They usually become active about dusk, when they begin to seek food and water. Some individuals may be active during daylight hours when rat populations are high.
Rats have poor eyesight, relying more on their hearing and their excellent senses of smell, taste and touch. They are considered color-blind.
Rats use their keen sense of smell to locate food items and to recognize other rats. Their sense of taste is excellent, and they can detect some contaminants in their food at levels as low as 0.5 parts per million.
Norway rats usually construct nests in below-ground burrows or at ground level. Nests may be lined with shredded paper, cloth, or other fibrous material.
Litters of 6 to 12 young are born 21 to 23 days after mating. Newborn rats are hairless and their eyes are closed, but they grow rapidly. They can eat solid food at 2 1/2 to 3 weeks. They become completely independent at about 3 to 4 weeks and reach reproductive maturity at 3 months of age.
Norway rats have physical capabilities that enable them to gain entry to structures by gnawing, climbing, jumping, swimming, and other tactics.
Rats will at first avoid novel food items placed in their environment. They may eat very small amounts, and subsequent feeding will depend on the flavor of the food and its physiological effect.
During it's daily activities, a rat normally travels an area averaging 100 to 150 feet (30 to 45 m) in diameter. Rats seldom travel farther than 300 feet (100 m) from their burrows to obtain food or water.
Rats constantly explore and learn about their environment, memorizing the locations of pathways, obstacles, food and water, shelter, and other elements in their domain. They quickly detect and tend to avoid new objects placed into a familiar environment. Thus, objects such as traps and bait stations often are avoided for several days or more following their initial placement. Place baits and bait stations near, but not on, rat runways. Rats will quickly find them and after a short period of avoidance, will cautiously investigate them.
Norway rats consume and contaminate foodstuffs and animal feed. They may damage crops in fields prior to and during harvest, and during processing and storage. Rats also damage containers and packaging materials in which foods and feed are stored.
Rats cause structural damage to buildings by burrowing and gnawing. They undermine building foundations and slabs, cause settling in roads and railroad track beds, and damage the banks of irrigation canals and levees. Rats also may gnaw on electrical wires or water pipes, either in structures or below ground. They damage structures further by gnawing openings through doors, window sills, walls, ceilings, and floors. Considerable damage to insulated structures can occur as a result of rat burrowing and nesting in walls and attics.
Among the diseases rats may transmit to humans or livestock are murine typhus, leptospirosis, trichinosis, salmonellosis (food poisoning), and ratbite fever. Plague is a disease that can be carried by a variety of rodents, but it is more commonly associated with roof rats (Rattus rattus) than with Norway rats.
The presence of rats can be determined by a number of signs described below:
Droppings may be found along runways, in feeding areas, and near shelter. They may be as large as 3/4 inch (2 cm) long and 1/4 inch (0.6 cm) in diameter. Fresh droppings are soft in texture.
Tracks, including footprints or tail marks, may be seen on dusty surfaces or in mud. A tracking patch made of flour can be placed in pathways overnight to determine if rodents are present.
Urine, both wet and dry, will fluoresce under ultraviolet light. Urine stains may occur along travelways or in feeding areas.
Runs or burrows may be found next to walls, along fences, next to buildings, or under bushes and debris. Rats memorize pathways and use the same routes habitually.
Smudge marks (rub marks) may occur on beams, rafters, pipes, and walls as a result of oil and dirt rubbing off rats’ fur along frequently traveled routes.
Gnawing may be visible on doors, ledges, in corners, in wall material, on stored materials, or other surfaces wherever rats are present. Fresh accumulations of wood shavings, insulation, and other gnawed material indicate active infestations. Size of entry holes (often 1 1/2 inches [4 cm] in diameter or less for mice, 2 inches [5 cm] or larger for rats) or tooth marks can be used to distinguish rat from mouse gnawing. Rats keep their paired incisor teeth, which grow continuously at the rate of about 5 inches (13 cm) per year, worn down by gnawing on hard surfaces and by working them against each other.
Sounds such as gnawing, climbing in walls, clawing, various squeaks, and fighting noises are common where rats are present, particularly at times of the day when they are most active.
Norway rats are not protected by law. They may be controlled with any pesticide registered by federal or state authorities for this purpose or they may be controlled by use of mechanical methods such as traps.
Damage Prevention and Control Methods
Seal all openings larger than 1/2 inch (1.3 cm) wide.
Rats are wary animals and can be frightened easily by unfamiliar sounds or sounds coming from new locations. Most rodents, however, can quickly become accustomed to new sounds heard repeatedly. For years, devices that produce ultrasonic sound that is claimed to control rodents have come and gone on the market. There is little evidence to suggest that rodents’ responses to nonspecific, high-frequency sound is any different from their response to sound within the range human of hearing.
Good sanitation practices reduce sources of food, water, and shelter. Store foodstuffs in rodent-proof structures or containers. Store and dispose of refuse and garbage properly. Control weeds and remove debris from around structures.
Rats find some types of tastes and odors objectionable, but chemical repellents are seldom a practical solution to rat infestations. Substances such as moth balls (naphthalene) or household ammonia, in sufficient concentration, may have at least temporary effects in keeping rats out of certain enclosed areas. The above materials, however, are not registered by the EPA as rat repellents. Ro-pel® is registered for use in repelling Norway rats and other rodents from gnawing on trees, poles, fences, shrubs, garbage, and other objects. Little information is currently available on its effectiveness against rats. Other solutions to rat problems, including rodent-proof construction and methods of population reduction, are usually more permanent and cost-effective.
Anticoagulant rodenticides (slowacting chronic-type toxicants) Brodifacoum (Talon®). Bromadiolone (Maki®, Contrac®). Chlorophacinone (RoZol®). Diphacinone (Ramik®, Ditrac®). Pindone(Pival®, Pivalyn®) Warfarin (Final® and others). Toxicants other than anticoagulants (may be acute or chronic toxicants) Bromethalin (Assault®, Vengeance®). Cholecalciferol (Quintox®). Red Squill. Zinc phosphide (Ridall Zinc®, ZP® rodent bait).
In some situations, outdoor burrow fumigation may be effective. Aluminum phosphide (Phostoxin® and others). Chloropicrin. Gas cartridges. Methyl bromide.
Snap traps. Live traps. Glue boards.
The above information was adapted from PREVENTION AND CONTROL OF WILDLIFE DAMAGE with permission of the editors, Scott E. Hygnstrom, Robert M. Timm, and Gary E. Larson (Cooperative Extension Division, Institute of Agriculture and Natural Resources University of Nebraska-Lincoln, United States Department of Agriculture Animal and Plant Health Inspection Service, Animal Damage Control, Great Plains Agricultural Council Wildlife Committee).