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Norway Rats


Quick Facts


  • The Norway rat is not really from Norway. Its name doesn’t point out to its country of origin, but rather to the means of transportation – a Norwegian ship – it used to travel from Asia to Europe.
  • The brown rat breeds more often than nearly any other mammal. One female can give birth to up to 200 young per year.
  • Rats don’t eat everything they forage – they store it in piles, as most rodents do, in one of the many chambers of the burrows they dig.
  • They have an excellent physical condition that allows them to climb on creeping plants, swim, or even walk along telephone lines to find food.
  • Rats can squeeze through very small holes, but they don’t have collapsible skeletons. Just like other rodents, their skeleton is comprised of bones and cartilage.
  • Although they are considered some of the most disgusting and dangerous pests in the world, rats are also beneficial for the environment, as they disperse seeds when they feed and excrete them from their bodies in new places, thus helping plants spread.




Rattus norvegicus (the Norway rat or the brown rat) is a rather large mammal of the mouse family, with a head-body length of up to 28 cm (40 cm head-to-tail) and weight between 150 g and 500 g. It has, as its name suggests, brown fur that can vary in color from gray and white to reddish brown or even black. The fur on its back is always darker than on the underside; their ears (which are shorter than those of related species) and tail (typically scaly) are bald. Males are typically larger and heavier than females. Norway rats are often confused with the closely related roof rat, but can be distinguished by their larger size and shorter tails. Also, the temporal ridges of the brown rat are straight, while those of the black rat are curved.


Diet, Behavior & Habits




The brown rat is an extraordinarily adaptable species and can live in a wide range of environments, usually preferring those inhabited by humans. Originally native to northern China, this species has successfully colonized nearly all regions of the world, except only for deserts and arctic regions. In their feral state, they live in forests, brushy areas with tall vegetation, open fields, woodlands, or along water courses in coastal areas. However, they are most common in human-modified environments, whether rural or urban, preferring basements, sewers, garbage dumps, port buildings, and any other structure that provides them the food and shelter they need. The brown rat is a ground-dwelling species but is also a strong swimmer, being able to swim half a mile in open sea and tread water for up to three days.


Brown rats are professional foragers. With their incredible senses and the ability to pick up skills by watching their peers, rats are able to survive in any type of environment that provides a steady source of food. In urban environments, they feed on discarded human food, while those living near bodies of water have been seen catching fish and diving for mollusks. Rats that live in woodlands or near farms feed primarily on chicks, birds and their eggs, insects, mice, small rabbits, and lizards; some populations of rats even know how to hunt small ducks. As animals with a very opportunistic diet, rats feed on almost anything edible, from nuts and plants to small animals and substances such as soap and beeswax.


A very sociable species, the brown rat lives in large, patriarchal groups established in a previously inhabited area. Their nests are built from leaves, garbage, and wood chips, and are usually intricate, consisting of chambers with different purposes: food storage, nesting, and emergency exits. Dominant males are those that occupy prime areas in the territory and mate with several females. If necessary conditions of food and shelter are ensured, females can become pregnant immediately after birth, raising up to 13 liters per year (each consisting of 6-9 young, on average). Babies leave the nest at about one month old and reach sexual maturity after another two months, further contributing to the incredible reproduction rate of their colony.


One of the greatest skills the brown rat possesses is an incredibly sensitive hearing, being able to hear and produce ultrasonic sounds, above the range of human hearing. Pet rats have been recorded producing squeals, soft chirps, hisses, whistles, and even a particular ultrasonic sound called by scientists “rat laughter.” Although they also possess good vision, they rely primarily on their whiskers, as well as on their touch, taste, and hearing, to determine what’s in front of them. Some species of rats (particularly the giant pouched rat) are raised as mine-detection animals and trained to find other remnants of war, as well.


Signs of Infestation




Brown rats are considered by some the greatest pest mammals in the world. Statistically speaking, in the past 1000 years, they have taken more human lives than all wars and revolutions combined. They carry a significant number of pathogens that can cause bubonic plague, tularemia, Weil’s disease, Q fever, typhus, infectious jaundice, toxoplasmosis, and many other infectious diseases. In addition, brown rats also cause a great deal of damage to residential and commercial properties, as they constantly gnaw and burrow inside and outside of buildings, destroying electrical circuits and increasing the risk of fire hazards and structural collapse. These are some ways in which rats may cause property damage:


  • Contaminate food (both of humans and pets)
  • Damage packaging and containers where food is stored
  • Gnaw on electrical cables
  • Tear up insulation in walls and ceilings
  • Damage crops and landscape plantings
  • Weaken the structure of buildings with their burrowing activities
  • Gnaw on wooden doors, ledges, wall materials


Here is how to identify a brown rat infestation on your property:


  • Droppings: Dark brown and long, similar to brown rice grains; found individually or in groupings.
  • Noises: Brown rats are adept crawlers and can easily gain access under decks and floorboards, so grinding, chattering noises at night may be indicative of their presence on your property.
  • Marks: Just like ants and other pests, rats use established routes between forage sites and their nest, and their bodies (often covered in grease and dirt) may leave smudges and marks along floor boards and objects they come into contact with.
  • Urine: Another indication of rats’ presence is puddles or trails of urine typically found on walls and baseboards, as rats are dribbling excrements with nearly every step they make.
  • Nesting: Rats prefer nesting in fuzzy materials that will keep them warm, so thoroughly inspect stored fabrics, furniture stuffing, cotton, twigs, and other nesting materials for signs of excrements and trails.


Management & Control and Prevention


Once they establish their nest inside your house, rats are extremely difficult to remove; a successful rat extermination strategy combines sanitation measures, rodent proofing, and population control. Here are the most effective methods pest control professionals will use to eliminate rat populations from your property:




Exclusion. The most effective – but often ignored – way of making sure rats will never bother you again is to set up physical barriers and “build them out.” Brown rats can run along cables, pipes, fences, electrical wires vines, and trees to access entry points to a building. They can also crawl along pipes and utility lines or gnaw through various materials (anything from steel to concrete) to gain entry to your house. To prevent rodent entry, all holes and openings greater than 1.3 cm must be properly sealed or covered using durable materials – steel wool or screen wire to temporarily cover holes or patching plaster for more long-lasting results. Monitor vents, windows, and exterior doors to make sure there are no unnecessary ledges and openings that may serve as access routes.


Sanitation. Sanitation is a fundamental aspect of effective rat management and control and must be continuous in order to prevent infestation. Poor sanitation is oftentimes the reason why rats develop and thrive in urban and suburban areas. In rural regions, sanitation alone may not be able to completely eliminate rat colonies, but it can significantly aid in population control.




Sanitation measures include regular housekeeping inside and outside of the building, proper storage of foodstuffs, feed, and garbage (to allow inspection for evidence of infestation), and immediate elimination of waste and refuse. Lumber, pipes, gardening equipment, and other goods must be stored off the ground. Garbage and trash must be collected and kept in containers with tight-fitting covers.


Repellents. This is perhaps the least effective method to be used for the control of large rat populations, but may provide a temporary effect for preventing rats’ access in certain closed areas. Moth balls, ammonia, peppermint, and other substances with strong odors may convince rodents to stay away from certain areas, but will not eliminate their colonies for good. For long-lasting effects, homeowners must consider a mix of rodent-proof construction, sanitation, and population control methods.


Rodenticides. Ever since its initial development right after World War II, anticoagulant rodenticides have been the preferred chemical method of killing rats and mice. They are especially effective for larger rat populations (where traps and repellents would have little to no effect). These toxic substances cause death as a result of internal hemorrhaging, which starts as soon as the rat ingests the poison and its blood starts losing its clotting capabilities. This is a method with a delayed effect, meaning that rats must be fed the poison for a number of days for it to have effect. There are, however, certain substances, such as brodifacoum or bromadiolone, which can cause death after a single serving.




Traps. Trapping is the preferred method for rat control in homes, garages, and other residential structures where only a limited number of rats are present. Since it doesn’t rely on toxic substances that could be ingested by children and pets and allows for the easy disposal of rat carcasses, trapping is sometimes a better alternative to rodenticides. Wood-based snap traps or wire cage traps can be purchased from most hardware supply stores at a convenient price. To make the extermination campaign effective and permanent, a sufficient number of traps must be placed around the infested area, preferably intersecting rats’ established paths. Electrocution traps, glue boards, and live traps are also extensively used by homeowners across the country to control small rodent populations.


Although there are plenty of DIY methods of controlling small rat populations, the most effective extermination methods are those applied by pest control professionals, who know to recognize the signs of infestation and can administer effective and humane rodent treatments to eliminate current infestations.