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Controlling Commensal Rodents

Commensal Rodents

Rats make up the largest single group of mammals on earth--one-third of the earth's total mammal population.

Several kinds of rats and mice are found around the United States. However, only the Norway rat, the roof rat, and the house mouse are considered important pests around farms and homes. They are referred to as "commensal rodents" because of their intimate relationships with humans.

The Norway Rat

The Norway rat is slightly larger than the roof rat and is primarily a burrowing animal but can climb when necessary. It prefers to live in a burrow 8 to 18 inches below ground. It is sometimes called the sewer rat, house rat, wharf rat, or barn rat.

The Roof Rat

The roof rat is a good climber and seldom burrows in the ground. It lives above ground in attics, between walls, in cabinets and shelves, and in barn lofts. The roof rat, compared to the Norway rat, has a more slender, streamlined body, more pointed nose, ears and eyes, and its tail is longer than its body.

Other than the differences listed, the Norway and roof rats are similar. Both are good swimmers; because their front incisor teeth grow an average of 5 inches a year, they gnaw almost constantly to keep them worn down. They can fall 50 feet without serious harm. They usually feed twice at night just after dark and just before dawn. They usually stay within a 100-foot radius if food and water are available but have been known to move almost a mile a day in search of plentiful water and shelter. Adult rats eat about 1 ounce of food and 1.5 ounces of water per day. Without food, weakness begins after about 3 days, but without water, weakness begins in 1 to 2 days. When weakness begins from lack of food or water, they begin to move elsewhere. Overpopulation also causes some rats to seek new locations.

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Reproduction of Rats

Rats breed at 3 to 4 months of age and probably continue until about 18 months old. Gestation is 21 to 25 days. The young are weaned at 3 weeks old, often just before the arrival of another litter. A female can breed only one day after giving birth. If fertilization does not occur, she will come into heat about every 5 days. A female averages six litters per year, with nine young per litter. However, under ideal conditions, litters may contain 20 young, and 14 litters have been recorded during one year. The babies from one pair of rats would be more than 3.5 million in 3 years under ideal conditions and ignoring the death rate. In natural conditions, however, many die, but in a year, as many as 60 to 70 offspring from one female may mature. Breeding is greatest in spring and fall, drops some in summer, and drops substantially in winter.

The House Mouse

The common house mouse depends less on humans than rats do. It commonly inhabits grassy fields and cultivated grain crops and adapts well to living away from humans. For example, some have been captured on open tundra in Alaska, miles away from human settlements. On the other hand, they adapt well to living with humans, as indicated by a report of their living 1,800 feet below ground in a coal mine, probably feeding on lunch scraps of the miners.

The house mouse has a small range. Home range tends to be from a few feet up to 25 feet. This is important to know when determining the frequency and distance to place poison bait or traps. Mice, unlike rats, show almost no fear of new objects placed in their ranges.

Rats and mice have poor vision. Rats see clearly only up to about 2 feet and mice even less--6 inches. However, they can detect movement beyond their clear-vision range. Their activity patterns are based on their keen senses of smell, taste, hearing, and touch.

Reproduction of the House Mouse

Reproduction of the house mouse is similar to rats. The average gestation is 20 days, and litter size is about six, with 6 to 10 litters per breeding life of the female.

Diseases Rats and Mice Spread

Rats are known to spread 35 diseases to humans and animals. Some human diseases rats spread are Salmonellosis, rabies, tularemia, leptospirosis, amoebic dysentery, typhus, jaundice, trichinosis, rickettsialpox, lymphocytic choriomeningitis, ray fungus, and ringworm. Also, they transport and host ectoparasites, especially mites. Mites living temporarily on rats and mice in their nests and burrow can, following treatment of the house and birds for mites, quickly re-invest the premises with mites. When you sell birds on mite-infested farms, migrating rodents can transport mites to adjacent farms. Rats can transport 18 different kinds of mites, lice, fleas, and ticks.

Controlling Rat and Mouse Populations

The best way to control rats and mice is to close all access routes into buildings, but this is difficult to do in poultry houses. The second best control method is to remove all shelter, food, and water, which again, poultry farmers are not able to do. That leaves using one or a combination of poisoning, trapping, or using cats.

Using Cats

Some people consider cats a nuisance. However, when properly managed, cats do control mice but cannot be relied upon to reduce rat populations severely, although they will restrict a population buildup. Young (less than 5 years old) female cats make the best hunters. Males only occasionally hunt well.

Cats should be well fed and watered to maintain good vigor. They do not have to be starved to cause them to hunt mice. A bowl kept supplied with a dry commercial cat food and fresh water nearby is a good feeding program.

Provide a litter box with commercial litter, sand, or sandy soil and an open-topped sleeping box with some sort of soft bedding.


Trapping is a practical way to remove rats and mice on relatively small farms, but in commercial operations you need too many traps and it takes too much attention to remove dead rats and rebait the traps to be practical.

If you use traps, many foods make good baits--peanut butter, meat of nuts, doughnuts, cake, fresh crisp-fried bacon, cheese, raisins, strawberry jam, milk chocolate, apples, gumdrops, prunes, and pineapple.

Enlarging the trap trigger with cardboard makes it more effective. Place the traps across or near paths rats or mice normally use. Both rats and mice, because of their poor eyesight and for protection, like to run close to walls. Because mice travel only short distances, set traps every 10 feet. With rats, place traps every 25 to 50 feet.

Rats and mice are accustomed to human odors. Therefore, you don't need to boil or handle traps with gloves. Remove dead animals from the traps regularly.

Using a Rodenticide

Rodenticides are usually mixed with some bait material or materials. Selecting the right bait is important, especially where a plentiful supply of good feed is available, as in poultry houses. Also, the Norway and roof rats and the mouse each have bait preferences. Therefore, it is important to know which of these rodents you plan to poison so you can choose the right bait material. Remember: If you use rodent control for rats only, mice will multiply rapidly once the rat population is under control. (The mice do not have to compete with the rats.)

Warning: Using poison baits should be done with care. Children, pets and livestock must not have access to the bait traps or the dead rodents.

Bait Preferences and Care

House Mouse

House mice prefer canary seed (bird seed), prunes, pineapple, jelly beans, peanut butter, chopped apples, corn, wheat, oatmeal, Milo, doughnuts, cookies, and sweet chocolate candy. They also like the juices of prunes and pineapple.

House mice are nibblers and like to try new foods. Using baits different from the usual food source often works well on mice, with two or three choices of baits in small amounts, instead of using more of one bait.

Roof Rat

The roof rat is a finicky eater, wary of everything new in its environment, including food, and does not readily accept meat or fish. The roof rat likes cereal grains, chopped apples, sweetpotatoes, melons, prunes, pineapple, cookies, doughnuts, sweet chocolate candy, peanut butter, and tomatoes.

Norway Rat

Norway rats readily accept fresh meat and fish. They usually prefer a bait higher in protein and fat than their normal diets. Also, they like peanut butter, sweet chocolate candy, lettuce, tomatoes, apples, carrots, bananas, corn, Milo, wheat, and doughnuts. Norway rats are gluttons and accept a greater variety of baits than do roof rats. Also, they are not as wary about new objects or food in their territory as is the roof rat. This makes them a little easier to bait and trap.

Many ingredients are added to bait mixtures to enhance the bait's acceptance by rats and mice, but about the only truly effective, readily available enhancers are 5 percent sugar, bacon drippings, and peanut or corn oil added to the bait mixture. Five percent sugar and 5 percent oil may be added.

As you can see, you can use a variety of baits with rodenticides. The important thing to remember with any bait material is that baits and bait containers must be fresh and clean for best acceptance. Thinking that rats and mice prefer spoiled, unclean food is false. The truth is, they are actually little different from other mammals in that they prefer fresh, clean food.

Rats and mice move in search of food before eating stale, sour, moldy, or feces and urine-contaminated food. It is estimated rats and mice eliminate 80 percent of their daily feces and urine waste as they are feeding. Therefore, you should present baits to them in a manner that tends to decrease rat or mouse contamination. Because of this, bait stations that dispense anticoagulant-type poison bait as it is being eaten are normally better than an open-top bait container that lets rats and mice contaminate the bait with their body wastes. Shallow trays, dishes, and boards may work well for quick-kill bait, since it should be removed and replaced every day or two anyway.

Establishing Bait Stations

When dispensing poison bait or establishing bait stations, consider these points:

  1. Because rats and mice have poor eyesight, they tend to run beside walls or other stationary objects and use their keen sense of touch in their whiskers and the guard hairs on their bodies to help guide them. These sensitive hairs help them travel in the dark, in their burrows, and in search of food and water. They do not often leave their established pathways unless the environment or food and water supplies change.
  2. Neither rats nor mice travel any farther than necessary to reach food and water.
  3. Place baits where rats and mice live and travel--not scattered at random or just where placement is convenient.
  4. Rats are social animals and, within the same species, will use the same food, water source, and runways--and might even nest close together. They range, if necessary, as far as 150 feet to get food and water but prefer to travel much shorter distances if food and water are available. Therefore, you should put rat baits every 25 feet.

The house mouse, however, is a "loner." In each territory there are one or more females, food, and shelter. The male mouse does not willingly share his territory with another adult male mouse. Therefore, you can control mice only by many bait placements--at least one in each territory. A territory is usually not more than a 10-foot 10-foot area. Some mice, for example, spend their entire lives in a pallet of feed. Mice require very little water and get much of what they do need from foodstuff, which aids their being able to occupy such a relatively small space. Place baits for mice 10 feet apart.

Amount of Poison Bait To Dispense

When you dispense poisoned baits, place enough bait to feed all rodents present. Otherwise, some animals will receive just enough of the bait to make them sick. Then they become bait shy.

The amount of poison bait needed depends on the rodent species, the size of the infestation, and the toxicant used. Since each location contains so many variables, prebaiting and test baiting are the best ways to determine what bait to use, how much bait to use, and where to place it.

Continued re-use of the same bait and rodenticide in the same location generally results in decreased acceptance, bait shyness, and poor control. Baits and rodenticides should, therefore, be rotated periodically.

Treat for Rats, Mice, or Both

It is not uncommon to find rats and mice living in the same area. However, it is less common to find Norway and roof rats occupying the same area. The Norway rat is larger and more aggressive and can drive away roof rats.

In cases where rat populations have been drastically reduced by poisoning or some other method, mice will often move in and rapidly increase in numbers.

Characteristics of Commensal Rodents

CharacteristicsNorway RatRoof RatHouse Mouse
Weight10 to17 oz8 to12 oz to oz
Total Length
(nose to tip of tail)
12 to18 in13 to17 in6 to 7 in
Head and BodyBlunt muzzle; heavy thick body
7 to 10 in
Pointed muzzle, slender body
6 to 8 in
2 to 3 in
TailShorter than head plus body, carried with much less movement, comparatively, than roof rat; lighter colored on under side
6 to 8 in
Longer than head plus body, generally moving whip-like, uniform coloring top and bottom at all ages and for all subspecies,
7 to 10 in
Equal to or a little longer than body plus head,
3 to 4 in
EarsSmall, close set, appear half buried in furLarge, prominent, stand well out from furProminent, large for size of animal
FurCoarse, generally red-brown to gray-brown colorBlack to slate gray; tawny above, gray white below, or, tawny above, white to lemon bellySilky, dusky gray

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Used By Permission
Written By:

Dr. Ralph Kamel