NORWAY RATS

The Norway Rat is one of the most well-known and common rats, and also one of the largest. Thought to have originated in northern China, this rodent has now spread to all continents and is the dominant rat in Europe and much of North America.

Identification
The Norway rat (Rattus norvegicus) Norway rats live in close association with people. In urban or suburban areas they live in and around residences, in cellars, warehouses, stores, docks, and in sewers. On farms they may inhabit barns, livestock buildings, silos, and beneath concrete slabs, along stream dish gray above and whitish gray on the belly. They occur wherever suitable food, water, and shelter are present. Although they can climb, Norway rats tend to inhabit the lower floors of multistory buildings. First introduced into the United States around 1775, the Norway rat has now spread throughout the contiguous 48 states.

General Biology, Reproduction, and Behavior
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. Therefore, for safety reasons, baits can be dyed distinctive colors without causing avoidance by rats, as long as the dye does not have an objectionable taste or odor.

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 conception. 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.

Females may come into heat every 4 or 5 days, and they may mate within a day or two after a litter is born. Breeding often peaks in spring and fall, with reproductive activity declining during the heat of summer and often stopping completely in winter, depending on habitat. These seasonal trends are most pronounced in more severe climates. The average female rat has 4 to 6 litters per year and may successfully wean 20 or more offspring annually.

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. If the food contains poison or some other substance that soon produces an ill effect but not death, the food will often be associated with the illness. This “bait shyness” was a major problems when single-dose acute toxicants were the main rodenticides in use. Today, only two rodenticides registered for Norway rat control, red squill and zinc phosphide, possess characteristics that make bait shyness a potential problem.

Bait shyness can persist for weeks or months and may be transferred to nontoxic foods of similar types. Pre-baiting, that is, training rats to feed repeatedly on nontoxic bait for a period of days prior to applying the toxicant in the bait, will largely prevent sublethal doses and thus bait shyness. It will also lead to successful control, with very few rats left to become bait-shy. Prebaiting will almost always increase control success when zinc phosphide or red squill baits are used.

Because anticoagulant rodenticides are slow-acting, the rats’ subsequent illness is not associated with the bait even if a sublethal dose is consumed; thus, bait shyness does not usually occur. These baits serve, in effect, as their own prebait.

Studies indicate that during its 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. Baited but unset traps will aid in overcoming rats’ fear of them; expanded-trigger traps set directly on travel routes may immediately catch rats.

Damage and Damage Identification
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.

Rat Signs
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.

Estimating Rat Numbers
Rat sign and visual sightings are of limited value in accurately estimating rat numbers, but they are the simplest and often the only practical method
available. Search premises thoroughly when looking for rats. In structures, searches should include attics, basements, around foundations, crawl spaces, and behind and under stored materials. The following estimates can then be made: No sign: no rats or few present. If only a few rats are present they may have invaded only recently. Old droppings and gnawing common, one or more rats seen by flashlight at night, or no rats observed in daytime: medium numbers present.

Fresh droppings, tracks, and gnawing present, three or more rats seen at night, or rats seen in daytime: large numbers present. Since rats are normally nocturnal and somewhat wary of humans, usually many more rats are present than will be seen in the daytime. Under certain conditions, rats may become quite bold in the presence of humans, and then a high percentage of the population may be visible.

A conservative estimate of rat numbers can be made from measuring their food consumption. You can do this by feeding the rats for a while on finely ground grain (whole grains or pellet foods may be carried off uneaten). When offered over a period of time, the ground grain will usually be accepted and eaten by rats. Consumption may gradually increase to a maximum level over the period of a week or so as the rats’ natural fear of To exclude rats, seal all holes and novel foods is overcome. Divide the openings larger than 1/2 inch (1.3 cm) total amount of food eaten per day by across. Rodent-proofing should be 1/2 ounce (15 g); this will give a minimum estimate of the rats present. Some rats eat more than 1/2 ounce (15 concrete mortar, galvanized sheet g) daily, but rats will probably also be metal, and heavy-gauge hardware using other foods in their environment. If too much alternative food is available, this technique will not give an adequate estimate.

Economics of Damage and Control
Accurate data on rat damage, control, and their cost is difficult to obtain. Estimates of losses of foodstuffs, structural damage, and the amount of labor and materials expended to control rats are usually only educated guesses. One study found that a small colony of Norway rats (10 to 26 animals), when given access to a ton of sacked wheat, would contaminate 70% of the grain after 12 to 28 weeks. The sacks were heavily damaged as well. Total damage equaled 18.2% of the total value of the wheat and the sacks.

One rat will eat approximately 20 to 40 pounds (9 to 18 kg) of feed per year and probably contaminates 10 times that amount with its urine and droppings. In a year’s time, a single rat will produce some 25,000 droppings. A 1973 estimate states rats may cost the United States between $500 million and $1 billion annually in direct economic losses. In most cases, the cost of rat control—particularly when it is done in a timely fashion—is far less than the economic loss caused by rat damage.

Legal Status
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

Exclusion
Seal all openings larger than 1/2 inch (1.3 cm) wide.

Frightening
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.

What is known about rodents and sound?
Unusually loud, novel sounds, including ultrasonic sounds, which rats can hear, will frighten them and may cause temporary avoidance lasting from a few minutes to weeks or months.

What is known about ultrasonic sound?
It is very directional and does not travel around corners well; thus, sound shadows or voids are created. Ultrasound does not travel very far. It loses its intensity rapidly as it leaves the source.

Ultrasound has not been shown to drive established rodents out of buildings or areas, nor has it been proven to cause above-normal mortality in their populations. While it is possible to cause convulsions or permanent physiological damage to rodents with ultrasound, the intensity of such sounds must be so great that damage to humans or domestic animals would also be likely. Commercial ultrasonic pest control devices do not produce sound of such intensity.

Tests of commercial ultrasonic devices have indicated that rats may be repelled from the immediate area of the ultrasound for a few days, but then will return and resume normal activities. Other tests have shown the degree of repellency to depend upon the particular ultrasonic frequencies used, their intensity, and the preexisting condition of the rodent infestation. Ultrasonic sound has very limited usefulness in rodent control. The advertising claims for many commercial devices are unsubstantiated by scientific research. Since commercial ultrasonic devices are often expensive and of questionable effectiveness, they cannot be recommended as a solution to rodent problems.

Habitat Modification
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.

Repellents
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.

Toxicants
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).

Fumigants
In some situations, outdoor burrow fumigation may be effective. Aluminum phosphide (Phostoxin® and others). Chloropicrin. Gas cartridges. Methyl bromide.

Trapping
Snap traps. Live traps. Glue boards.

Other Methods
Clubbing. Shooting. Dogs and cats are of limited value in some situations.



ACKNOWLEDGEMENTS
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).


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