Common Name: Norway / Brown / House / Wharf / Sewer Rat
Scientific Name: Rattus norvegicus (Berkenhout)
Class/Order/Family: Mammalia/Rodentia/Muridae
Metamorphosis: None
Introduction
The Norway rat is the largest of the commensal rodents and the most common commensal rat in the temperate regions of the world. It not only damages/destroys materials by gnawing, 
eats and contaminates stored food, but it is also of human health importance as a vector or carrier of diseases. It is thought to be of central Asian origin, but is now of worldwide distribution and found throughout the United States.
Recognition
Adult with combined head and body length 7-9.5″ (18-25 cm), tail length 6-8″ (15-21 cm), usual weight about 7-18 oz (200-500 g) but up to 20.5 oz (620 g). Fur coarse, shaggy, brown with scattered black hairs, with underside gray to yellowish white. With muzzle blunt, eyes small, ears small (do not reach eyes) and densely covered with short hairs.
Heavy bodied. With scaly tail bicolored (darker above), shorter than head and body combined. Adult droppings up to 3/4″ (20mm) long, capsule shaped, and with blunt ends SIGNS OF INFESTATION Gnaw marks. New gnawings or holes tend to be rough whereas, old gnawings are smooth from wear and old holes are often greasy.
Droppings. Fresh droppings are soft and moist whereas, old droppings are dried and hard; adult Norway’s about 3/4″ (18-20 mm) with blunt ends vs. adult roof’s about 1/2″ (12-13 mm) with pointed ends.
Tracks/footprints. Front foot 4-toed and print is in front of usually longer hind print with 5 toes. Fresh tracks are clear and sharp whereas, old tracks are at least partially obscured by dust. Rub marks or dark, greasy markings on vertical surfaces. Fresh marks are soft, greasy, and easily smeared whereas, old marks are with the grease dry and flaky.
Burrows. Found in earthen banks, under concrete slabs, and under walls. If active, free of dust and cobwebs. Main opening usually with hard packed soil, rub marks may be visible.
Runways. Consistently follow same paths, usually along walls, stacked merchandise, etc. Active runways with greasy appearance, free of dust and cobwebs, with fresh tracks and/or droppings. Damaged goods. Norway rats prefer meat, fish, and cereal (dry dog food an favorite) whereas, roof rats prefer fruits, vegetables, and cereals.
Signs of Infestation
Gnaw marks. New gnawings or holes tend to be rough whereas, old gnawings are smooth from wear and old holes are often greasy.
Droppings. Fresh droppings are soft and moist whereas, old droppings are dried and hard; adult Norway’s about 3/4″ (18-20 mm) with blunt ends vs. adult roof’s about 1/2″ (12-13 mm) with pointed ends.
Tracks/footprints. Front foot 4-toed and print is in front of usually longer hind print with 5 toes. Fresh tracks are clear and sharp whereas, old tracks are at least partially obscured by dust. Rub marks or dark, greasy markings on vertical surfaces. Fresh marks are soft, greasy, and easily smeared whereas, old marks are with the grease dry and flaky.
Burrows. Found in earthen banks, under concrete slabs, and under walls. If active, free of dust and cobwebs. Main opening usually with hard packed soil, rub marks may be visible.
Runways. Consistently follow same paths, usually along walls, stacked merchandise, etc. Active runways with greasy appearance, free of dust and cobwebs, with fresh tracks and/or droppings. Damaged goods. Norway rats prefer meat, fish, and cereal (dry dog food an favorite) whereas, roof rats prefer fruits, vegetables, and cereals.
Similar Groups
(1) Roof rat (Rattus rattus) with muzzle pointed, eyes large, ears large, almost naked tail uniformly colored and longer than head plus body, droppings spindle-shaped with pointed ends.
(2) Hispid cotton rat (Sigmodon hispidus) with tail about half head-body length combined as less heavy (weight 2 3/4-7 oz/80-198 g), and fur coarse and grizzled, grayish above with mixed buff and black, and whitish below.
(3) House mouse (Mus musculus) with muzzle pointed ears large, tail about as long as head plus body, small (about 1/2-1 oz/14-28 g), shorter (head, body and tail 5.25-7.5″/6.5-10.2 cm), droppings 1/8-1/4″ (3-6 mm) long, rod-shaped with pointed ends.
(4) Most native rats and mice have hairy tails, hairs short or long, or if the tail is almost naked, it is also annulate (appears to be of ringlike segments).
Biology
Norway rats reach sexual maturity in 2-5 months. Pregnancy lasts as average of 23 days (range 21-25). The young are blind and naked at birth. Hair appears in about 7 days and eyes opening in 12-14 days. They are weaned at about 3-4 weeks. The average number of litters is 3-6 per year (range 3-12), each containing an average of 7-8 young (range 6-22), but averaging about 20 weaned/female/year.
Adults live an average of 6-12 months, but much longer in captivity. They have rather poor vision and are color blind, but their senses of hearing, smell, touch, and taste are keenly developed. Touch is via their vibrissae or long whiskers. They are good runners, climbers, jumpers, and swimmers.
A Norway rat requires 3/4-1 oz (21-28 g) of food and 1/2-1 ox (15-30 ml) of water each day, with the water coming from a nonfood source. This results in about 30-180 droppings and ½ oz/3 teaspoons (16 cc) of urine produced each day.
Historically, the disease most commonly thought of involving rats (roof rat primarily) is plague which is transmitted via fleas leaving an infected rat and attacking man. Fortunately, plague has not been found in rats in the United States for many years.
Other transmittable diseases include murine typhus via fleas (also possibly via droppings and urine), infectious jaundice/ leptosprosis/ Weil’s Disease via urine in water or food, rat-bite fever via bites, trichinosis via undercooked pork, and food poisoning or Salmonellosis via droppings. Another problem is tropical rat mite dermatitis which is caused by these mites when they feed on humans.
Habits
Rats are primarily nocturnal in habit and they are cautious. Although they constantly explore their surroundings, they shy away from new objects and changes. Outdoors, Norway rats prefer to nest in burrows in the soil along railroad embankments, stream/river banks, piles of rubbish, under concrete slabs, etc. The burrow will have at least 1 entrance hole and at least 1 bolt-hole or emergency exit which is often hidden under grass, debris, etc. These are social animals and often many burrows will be located within a given area. An opening of greater that 1/2″ (12 mm) is required for entry into buildings.
Indoors, Norway rats usually nest in basements and the lower portions of buildings. Indoors, Norway rats usually nest in basements and the lower portions of buildings in piles of debris or merchandise as long as it is not disturbed. Although Norway rats prefer the ground or lower levels of buildings and sewers, on occasion they may be found in attics, on roofs, and in other high places. Although they will eat practically anything, Norway rats prefer meat, fish, and cereal. If the food material eaten proves to be disagreeable, they are quick to develop food/bait shyness. Once they find an acceptable/preferred food, rats tend to eat their fill at one sitting/place and will return time after time. They almost always require a nonfood or separate source of water.
Norway rats will travel about 100-150 ft (30.5-45.7 m) from their harborage for food and/or water, even plastic or lead pipes. Once established, Norway rats tend to follow the same route or pathway between their harborage and food and/or water sources. As often as possible, they follow vertical surfaces which their vibrissae or long whiskers can contact. Runways along vertical surfaces will usually include dark rub marks on te vertical surfaces where their oily fur makes contact. Their runway will be fee of debris, and outdoors, the grass will be worn away to the bare soil.
Control
The key to any rat control program is pest identification, sanitation, harborage elimination, and rat-proofing the building. Control is based on the behavioral habits of the Norway rat. Some of the most important things to remember are: Rats defecate somewhat indiscriminately within their territories but mostly where they feed. Rat droppings serve as an indication of their presence and where control efforts should be concentrated.
Nontoxic tracking powders can also be used to determine where they are most numerous. Rats will travel 100-150 ft (30.5-45.7 m) for food and/or water along established runways and usually with their vibrissae in contact with vertical surfaces. Look for rub marks and clean runways. Place traps or bait stations along runways and against vertical surfaces.
Rats are gluttons. Place sufficient bait in each bait station for at least 1 meat (stop feed baits), 2 meals (acute/single-feed baits), or more (chronic/multi-feed baits). Once a preferred bait is found, they utilize this bait until feeding stops. Rats are cautious. Minimal disturbance is desirable. Pretrapping with unset snap traps or prebaiting with nontoxic bait may be necessary. Norway rats prefer meat, fish and cereals. Use such high-protein baits and bait the center of glue boards and snap traps with such foodstuffs.
Rats usually have a water source other than their food if their food has a low moisture content. Liquid baits are particularly effective when their normal water source can be reduced or eliminated.
Outdoors, Norway rats usually live in ground burrows. Outdoors, gassing (Fumigating) rat burrows is extremely effective. Follow label directions and do not gas burrows within 15 ft (4.6 m) of, or burrows which may run under or open into an occupied structure.