The Norway Rat, also known as the House Rat, Sewer Rat or Wharf Rat, is the largest of the commensal rodents and the most common Rat in the United States. They not only damage materials by gnawing, they also eat and contaminate stored food, making them an important health concern as well.
Despite its name, the Norway Rat is thought to be of Central Asian origin, but they are now found worldwide. Adult Norway Rats are typically between 7”-10” long, not counting the tail, which will always be shorter than the length of the body. Norway Rats will weigh between 8oz-16oz, but have been found to grow up to 1.5lbs. Their fur is coarse, shaggy and brown, with their underside gray to yellowish white. The Norway Rat has a blunt muzzle, small eyes and ears, and their tail is dark and scaly. Their droppings are usually about 3/4” long with blunt tips.
Norway Rats can begin breeding as early as 2 months old. Each pregnancy lasts just over three weeks; the young are born blind and naked, and are weaned at about 3-4 weeks. Norway Rats have an average of 8 young per litter, about half of which are female. They can have anywhere from 3 to 12 litters per year, depending on the availability of food and shelter. The average Norway Rat will live a little less than a year in the wild.
Norway Rats have very poor vision and are color blind, but their senses of hearing, smell, touch and taste are keenly developed. They are good runners, climbers, jumpers, swimmers and are also extensive tunnelers.
A single Norway Rat needs about 1oz of food and between 1/2oz to 1oz of water each day, with the water coming from a nonfood source. With access to a good food source, each Rat can produce over 100 droppings and up to 1/2oz of urine every day. Historically, the most common disease associated with Rats is plague, though that was primarily from the Roof Rat (Black Rat) which was transmitted via fleas leaving infected Rats and biting humans. Fortunately, plague has not been found in Rats in the United States for many years, though Rats can carry many other types of infectious bacteria, such as Jaundice and Salmonellosis, spreading these diseases through their droppings and urine.
Norway Rats are primarily nocturnal and are very cautious. They will constantly explore their surroundings, but they shy away from any new objects or changes. Norway Rats prefer to nest in burrows, tunneling under piles of rubbish or concrete slabs, often tunneling under foundations and into floor and wall insulation. Their burrows will have at least one entrance hole and at least one emergency exit, which will usually be hidden. Because Norway Rats are such social animals, so there will often be many connected burrows within a given location.
Norway Rats will eat practically anything, though they prefer meat, fish and grains. When they find an acceptable food material they tend to eat their fill at one sitting and return time after time. Norway Rats will travel up to 150ft from their nest to search for new food sources. Once established, they tend to follow the same route between their nest and food source. As often as possible, they follow vertical surfaces or corners where they can feel a wall on one side or the other. Commonly used pathways will show dark rub marks where their oily fur makes contact. Norway Rats will gnaw through almost anything to get to food, from hard plastic to lead pipes and even old concrete.
The key to Rat control is exclusion, or “Rat-proofing” the home. This is often the most difficult portion because adult Norway Rats need an opening only slightly larger than 1/2” in diameter to gain entry, and they are also avid tunnelers. Careful inspection of the exterior of the house is necessary, checking door thresholds and foundation vents, as well as any opening around pipes or wires, to ensure as many entry points as possible are properly sealed. In addition to the exclusion, proper trapping is needed to eliminate the remaining Rats inside the house. It is important NOT to use poisonous bait inside a home for Rats because of the high possibility of the Rat dying inside an inaccessible wall, causing additional pest problems, not to mention horrid odors.