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The Norway rat (Rattus norvegicus,) is a stocky burrowing rodent, unintentionally introduced into North America by settlers who arrived on ships from Europe. 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 (454 g). Their fur is coarse and usually brownish or reddish gray above and whitish gray on the belly. Blackish individuals occur in
some locations.
Norway rats live 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. Although they can climb, Norway rats tend to inhabit the lower floors of multistory buildings.
Norway rats will eat nearly any type of food. When given a choice, they select a nutritionally balanced diet,
choosing fresh, wholesome items over stale or contaminated foods. They prefer cereal grains, meats and fish, nuts,
and some types of fruit. Rats require 1/2 to 1 ounce (15 to 30 ml) of water daily when feeding on dry foods but
need less when moist foods are available. Food items in household garbage offer a fairly balanced diet and also
satisfy their moisture needs.
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
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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. For more detailed information on their physical abilities and the resulting need to design rodent-proof structures, see the chapter Rodent-Proof Construction and Exclusion Methods.
Studies indicate that during its daily activities, a rat normally travels an area averaging 100 to 150 feet (30 to 45m) 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. 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 hysiological
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.
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 rat bite 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: near, but not on, rat runways. Rats ill
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.
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. Prebaiting,
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.
Pre-baiting 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 Escape hole lightly covered with dirt.
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 travel ways 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.
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
Novel foods is overcome. Divide the total amount of food eaten per day by 1/2 ounce (15 g); this will give a minimum estimate of the rats present.
Some rats eat more than 1/2 ounce (15 g) daily, but rats will probably also be using other foods in their environment.
If too much alternative food is available, this technique will not give an adequate estimate.
Physical barriers can prevent rats from gaining entry to structures where food and shelter are available. “Ratproofing”
is an important and often neglected aspect of rat control. It is a relatively permanent form of rodent control that revents damage from occurring. To exclude rats, seal all holes and openings larger than 1/2 inch (1.3 cm) across. Rodent-proofing should be done with heavy materials that will resist rodent gnawing. These include concrete mortar, galvanized sheet metal, and heavy-gauge hardware cloth.
In addition to the above-mentioned techniques of excluding rodents from sources of food and shelter, sanitation
can play an important role in controlling rat populations. Poor sanitation is one of the basic reasons for the continued existence of moderate to high rat populations in urban and suburban areas. In agricultural environments, proper sanitation cannot always eliminate rat populations, but it can often prevent rats from flourishing in large numbers.
Sanitation involves good housekeeping, including proper storage and handling of food materials, feed, and edible garbage.
Warehouses, granaries and grain mills, silos, port facilities, and similar structures may provide excellent habitat for rats. Store bulk foods in rodent-proof containers or rooms. Stack sacked or boxed foods in orderly rows on pallets in a way that allows thorough inspection for evidence of rats. In such storage areas, keep stored materials away from
walls. A 12-inch (30-cm) white band painted on the floor adjacent to the wall will aid in detecting rodent droppings
and other rat sign. Sweep floors frequently to permit ready detection of fresh sign.
Pet foods often are a source of food for rats in and around homes. Keep all such materials stored in metal dent proof
containers. Feed pets only what they will eat at a single time. Garbage and rubbish from homes, restaurants, farms, and other such sources should be properly stored and subsequently removed for disposal. A proper refuse storage container is heavy-duty, rust-resistant, rat- and damage-resistant, and equipped with a tight-fitting lid. Galvanized steel trash containers in good condition are better than those made of vinyl or plastic. Racks or stands prevent corrosion or rusting of containers, reduce rat shelter under containers, and minimize the chance of containers being overturned.
Bulk storage containers for refuse, such as those used at apartments, businesses, and housing projects, should be similarly rodent-proof. Large metal refuse containers (dumpsters) sometimes have drain holes to facilitate
cleaning. These drain holes should be fitted with a wire mesh screen or a removable plug; otherwise, the container
becomes a huge feeding station for rodents. Refuse should be collected regularly and before refuse storage containers become filled to excess. Sanitary landfills and incinerators seldom have conditions that will allow rat populations to exist. On the other hand, open refuse dumps are often infested by Norway rats.
At a properly operated sanitary landfill, garbage and rubbish are compacted and covered with earth daily. Modern incinerators completely burn refuse, and the resulting residue does not provide food for rats. Sewers are inhabited by Norway rats in some towns and cities. Rats may enter at outlets and through manholes, catch basins, broken pipes, or drains.
Since Norway rats are excellent swimmers, water traps do not impede their movement; in fact, they can travel
upstream against a current. The problem of rats in sewers is usually greatest in places where sanitary sewers are interconnected with storm sewers, thus providing multiple entry points for rats. The domestic sewage of an average
community provides enough food to sustain a large number of rats; this problem has increased as a result of the recent prevalence of garbage disposal units in most newer homes.
Regular removal of debris and control of weeds from around structures will reduce the amount of shelter available
to rats. In some instances, a strip of heavy gravel placed adjacent to building foundations or other structures
will reduce rat burrowing at these locations. Gravel should be at least 1inch (2.5 cm) in diameter and laid in a
band at least 2 feet (0.6 m) wide and 1/2 foot (15 cm) deep. In any event, keep the perimeter of buildings and
other structures clean of weeds and debris (including stacked lumber, firewood, and other stored materials) to
discourage rat activity and to allow easier detection of rat sign.
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.
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.
Rodenticides were formerly classified into two groups, single-dose (acute) toxicants and multiple-dose (chronic)
rodenticides. However, the complexity in mode of action of newer rodenticides makes these classifications outdated.
A classification into two groups, the first including all anticoagulants, and the second all other compounds (“non-anticoagulants”), is currently more useful. The following general steps are recommended to obtain good bait acceptance, and therefore good rat control, when using zinc phosphide baits:
1. Prebait rats for a minimum of 3 to 5
days to get the rats accustomed to eating the nontoxic bait material. Do not change types of bait during the prebaiting or baiting operation. Apply prebait at many locations, wherever there is rat activity. Where bait is completely eaten overnight, double the amount of prebait at that location the next day. Repeat this procedure until the amount of bait eaten every night no
longer increases.
2. Use only high-quality grains and fresh ready-to-use baits. Where rats have access to abundant amounts of grain, meat such as canned fish flavored cat food may be a good substitute. Obtain a sufficient quantity to complete the project without changing brands or flavors.
3. Wait until prebait consumption has peaked before applying toxic baits. Remove any uneaten pre-bait and place the toxic bait at the same locations that the pre-bait was applied. Usually, the amount of toxic bait needed will be about half the amount used on the last day that pre-bait was applied. It may be helpful to wait one day between the last application of prebait and application of toxic bait. That way, rats will be hungrier. Mix the toxicant into the bait ingredients according to label directions, if preparing your own baits from a concentrate.
4. Avoid handling the toxic bait or rodenticide concentrate with bare hands; use rubber or latex gloves. Clean thoroughly any tools or containers used in bait mixing, or safely dispose of them as well as bait packaging materials.
5. Confine or restrain any pets, livestock, or other animals that may otherwise gain access to and feed on the bait. It may also be necessary to place prebait and toxic bait into bait boxes for safety.
6. Following toxic bait application, pick up and dispose of available dead rats and all uneaten bait by incineration or deep burial. Normally, bait should be exposed for only 1 or 2 nights; the greatest consumption occurs on the first night.
7. Control remaining rats by using anticoagulant baits or by using traps or burrow fumigants.
