Figure 1. The Norway rat (Rattus norvegicus)

Identification

The Norway rat 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 (Fig. 2).

Figure 2. Characteristics of commensal rodents.

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.

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 (Fig. 3).

Figure 3. Norway rat burrow system beneath a pile of boards.

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 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.

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.