Norway Rat

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, eating and contaminates stored food. It is also of human health importance as a vector of carrier of diseases. It is thought to be of central Asian origin, but now is found worldwide including the United States.

Sign of Infestations

  1. Gnaw marks. New gnawing or holes tend to be rough, whereas old gnawings are smooth from wear and old holes are often greasy.
  2. Droppings. Fresh droppings are soft and moist, whereas old droppings are dried and hard; adult Norway's are about 3/4 inch with blunt ends vs. adult roof's about 1/2 inch with pointed ends.
  3. Tracks/footprints. Front foot has four toes and print is in front of the usually longer hind print with five toes. Fresh tracks are clear and sharp, whereas old tracks are at least partially obscured by dust.
  4. Rub marks are dark, greasy markings on vertical surfaces. Fresh marks are soft, greasy, and easily smeared, whereas old marks are without the grease, dry and flaky.
  5. Burrows. Found in earthen banks, under concrete slabs, and under walls. If active, they are free of dust and cobwebs. Main openings usually have hard packed soil. Rub marks may be visible.
  6. Runways. Consistently follow the same paths, usually along walls, stacked merchandise, etc. Active runways have a greasy appearance free of dust and cobwebs, with fresh tracks and/or droppings.
  7. Damaged goods. Norway rats prefer meat, fish, and cereal, whereas roof rats prefer fruits, vegetables, and cereals.

Habits

Rats primarily are 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 on furrows in the soil along railroad embankments with at least one entrance hole and at least one bolt-hole or emergency exit which often is hidden under grass, debris, etc. These are social animals and often many burrows will be located within a given area. An opening of greater than 1/2 inch is required for entry into 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 then 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 feet from their harborage for food and/or water. They will gnaw through almost anything to obtain 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 usually include dark rub marks on the vertical surfaces where their oily fur makes contact. Their runway will be free of debris, and outdoors, the grass will be worn away to the bare soil.