People And Rodent Pests

Features - PCT News

Three common wild rodents live with people — the Norway rat, the roof rat and the house mouse. The more we know about these unwanted visitors, the better we can manage them for our customers.

August 15, 2002

Pest management professionals should recognize a basic human feeling — people hate and fear rats. Yet customers or residents may deny a rodent problem because seeing a rat or mouse running around is often a sign of poor housekeeping or run-down structures.

Rodents or their parasites carry many diseases, including plague, salmonellosis, leptospirosis and typhus. The Centers for Disease Control and Prevention reports about five to 10 cases of plague every year. Some human cases come from people contacting wild rodents in natural areas in the course of recreational pursuits. A real danger remains that wild rodents, such as squirrels and field mice, could spread the disease to urban rats and from there to the human population. New housing continues to encroach upon wild rodent habitats in many areas and periodic drought and brush fires in the western U.S. may also serve to bring wild rodent carriers of disease more in contact with people and the rats that live in and around buildings.

While modern construction methods may result in sound structures that may reduce invasion by the Norway rat, it is much more difficult to keep out house mice, which may move in during construction and maintain a population thereafter. Mice are particularly common above suspended ceilings (look for droppings in lighting fixtures). The climbing ability of the roof rat may still allow access to new buildings. This rat is a more effective carrier of some diseases, like plague, than other pest rat species. Roof rats will typically move about along overhead areas, in beams and rafters, presenting special difficulties in managing this species.

RATS AND MICE. House mice are so small they can readily invade structures. They resemble rats, but are smaller. Like rats, they have a long, naked tail. House mice can most easily be confused with juvenile Norway rats or deer mice. Young rats have a much more stocky body and bigger feet, with shorter tails. Deer mice have larger ears and eyes than house mice and have a furred tail. It is not uncommon to find deer mice in urban or suburban areas, where they may often live in parkland, wooded areas, brush piles or firewood. Deer mice, with their role in Hantavirus, should be discouraged from inhabited areas by keeping brush and weeds removed from such areas. Putting stored items and firewood on racks off the ground will help reduce use of these areas by small rodents.

The Norway rat is a larger, more stout rodent than the roof rat. The tail of the Norway rat is shorter, while the roof rat’s tail is longer than its body. Norway rats, roof rats and house mice are omnivorous, eating both plant and animal material. The roof rat may favor fruits, nuts and vegetables to a greater extent than its other commensal counterparts. Yards with fruit trees are especially prone to attracting roof rats, particularly if ripe fruit is not harvested or dropped fruit on the ground is not removed.

The feeding of birds is a popular pastime for many people and the use of feeders designed to prevent squirrels and larger birds from scattering seeds upon the ground will greatly help reduce the attraction of spilled seeds to various wild rodents. Dogs or cats that are fed outside should only be given what they will eat during daylight hours, so as not to support a rodent population with this high-energy food during the night, which is when most rodents are more active.

Pest rats and mice in cities and suburbs are a little different from their country cousins except that, like people, they have adapted to the “asphalt jungle.” The distribution of Norway rats and house mice is throughout the United States. Roof rats are primarily found in warmer, coastal areas, principally in the southeastern and western states.

We often hear the statement that there is “one rat per person” in our metropolitan areas. There is no basis for such estimates. The most extensive city surveys on rat numbers were carried out in Baltimore during the 1940s and 1950s and researchers determined there were between 100 and 200 rats per block in some problem areas.

RODENT CONTROL. Rodent control is a difficult task, often requiring a variety of tools including Integrated Pest Management methods to achieve effective control. Because of rapid rodent reproduction, controlling less than 90 percent of the population can lead to rapid replacement. Ineffective control may be worse than no control at all, because it tends to eliminate the less wary and more juvenile rodents, leaving more food and shelter for the remaining adults. But if you reduce the food, water and living places of pest rats and mice, you will reduce their numbers.


1. Gain entrance through 1/2-inch holes (1/4-inch for mouse)

2. Climb horizontal and vertical wires

3. Climb inside of vertical pipes 1½ to 4 inches in diameter

4. Climb outside of vertical pipes and conduits up to 3 inches in diameter

5. Climb outside of vertical pipes of any size if the pipe is within 3 inches of the wall or other support

6. Crawl horizontally on any type of pipe or conduit

7. Jump vertically as much as 36 inches from a flat surface (12 inches for mice)

8. Jump horizontally 48 inches on a flat surface

9.  Jump horizontally at least 8 feet from an elevation of 15 feet

10.  Burrow down vertically to a depth of 4 feet in soil

11.  Drop 50 feet without serious injury (8 feet for mice)

12.  Climb bricks or other rough exterior walls offering footholds

13.  Climb vines, shrubs or trees or travel along telephone or power lines to gain access to upper stories of buildings

14.  Reach as much as 13 inches along smooth vertical walls

15.  Swim as far as a ½ mile in open water and tread water for up to three days; travel in sewer lines even against substantial current and dive through water plumbing traps

16.  Gnaw through a variety of materials including lead sheathing, cinderblock, aluminum sheeting, glass and improperly cured concrete

Rodenticides are commonly used by pest management professionals in rodent control. Mechanical traps and glueboards also have value in many situations, particularly in sensitive accounts, where a protected placement of toxic bait is not allowed or cannot be made without expected hazards. Traps and glueboards require frequent servicing, both from the standpoint of humaneness concerns regarding any animals trapped, as well as regard to customer risks and concerns. Even dead rats or mice in a trap are still a hazard; parasites can transfer to pets or people and the carcass may contain pathogens that could spread disease. Rodents on glueboards or in traps should be promptly removed by the pest management professional. Wear disposable gloves and place carcasses in a plastic bag for disposal.

Ultrasonic and electromagnetic rodent repellent devices are available on the consumer market but good data supporting their effectiveness is lacking. Controlled studies with commensal rodents have been generally negative. Some chemical repellents list rodents among animals affected. These materials may give temporary protection, such as reducing chewing on trash bags or plastic garbage cans.

However, these materials are also noxious to people and can sometimes be transferred to the hands and mouth in use unless good hygiene is practiced. Moving pest rats and mice from one area to another does not lessen their impact.

Individuals and groups concerned with animal welfare often promote research and use of antifertility drugs to control harmful animals. Again, a sterile rat can still bite, contaminate and chew materials and spread disease. The mating habits of commensal rodents (males are polygamous) make the practical use of any antifertility drug difficult. Most rodent management experts prefer that efforts to feed pest rodents on an introduced diet (such as rodenticides) result in dead rodents that can no longer pose a threat.

Sanitation and rodent proofing are important to reduce harborage, food and entry points in structures to reduce rodent problems. If efforts are taken to patch holes and seal rodents out, it is important that those conducting the work use substantial materials (foam in a can is not a good patch to deter rodents as they will readily chew through it). Appropriate materials are:
• Patching concrete (2 inches thick)
• 24-gauge sheet metal flashing
• 19- to 24-gauge hardware cloth

Seal around doors and windows and screen over ventilators and duct openings with hardware cloth or screening.

In addition, the monitoring of rodent populations can be done with non-poisoned bait, tracking patches (such as powdered chalk) and sightings during active hours.

RODENTICIDES. When high pest rodent populations require removal or structures and commodities are at risk from rodent attack, rodenticides can play a useful management role. Toxic bait placements must be carefully made according to product label directions, which generally direct placements to be made either in inaccessible areas (out of sight and out of reach) or in tamper-resistant bait stations. The importance of sanitation, harborage reduction and rodent exclusion on pest rodent populations in and around structures should not be underestimated. Rodenticide use without concurrent environmental improvement will have little long-lasting effect.

Single-feed products today are available in both anticoagulant and non-anticoagulant categories. The lower toxic doses that these products provide help ensure that rodents that feed only once at a protected bait are likely to eat a lethal dose. The advanced (second-generation) anticoagulants of brodi-facoum, bromadiolone and difethialone will kill rats and mice in a single feeding, while low concentrations and the vitamin K1 antidote can be retained as with older anticoagulant products. These products are also generally effective on rats or mice resistant to the older anticoagulants.

Other types of products also have single-feed claims, including bromethalin and cholecalciferol baits. A variety of formulations of both anticoagulant and non-anticoagulant products are available. Use of bulk pellets by professional applicators is declining. Loose pellets are harder to contain and non-target exposure or site contamination may occur. Placepacks offer advantages in protecting bait until rodents chew packaging. Bait blocks are becoming a more popular formulation. Increased research by manufacturers has raised palatability of these formulations significantly. They can now compete with other available foodstuffs and are well received by rats and mice in most locations. Most importantly, bait blocks can be fastened down in placement to reduce the hazard to children and non-target animals.

The adoption of tamper-resistant stations, the inclusion of human taste deterrents (such as denatonium benzoate) in some rodenticide formulations and the careful placement and monitoring of protected baits by professional applicators all have helped reduce risk to children, pets, domestic animals and wildlife. When accidents occur they can generally be traced to careless or improper placement of bait or a misunderstanding or lack of proper communication with the customer (as to location and hazard of rodenticides or presence and access of pets). Sometimes circumstances change after the pest management professional leaves a site, so ask the right questions, anticipate the unexpected and conduct your management practices accordingly.

The author is technical specialist, Syngenta Professional Products, Wilmington, Del. He can be reached via e-mail at


Here’s a list of the accuracy of rodents’ senses. How do humans compare?

Smell — Odor is a very important element in the world of the rat and mouse. Rodents mark objects with urine and glandular secretions and recognize their own colony and strangers by smell. Odor leads males to females ready to mate and the smell of strange males can inhibit pregnancy in mice. Most odors added to bait as enhancers are ineffective, if not actually repellent.

Vision — People say rats and mice have poor eyesight. In fact, they have poor vision beyond three or four feet, but are sensitive to motion up to 50 feet. Rats and mice appear to be color blind, but light-colored or reflective objects may stand out in their environment and cause initial avoidance.

Taste — Commensal rodents have an enormously well developed sense of taste, including the ability to detect some chemicals at part per billion concentrations. Taste sensitivity also can lead to bait shyness for those rodents that associate any unpleasant or uncharacteristic taste with symptoms of discomfort or illness.

Rats like the same food as people and certainly fresh meats, nuts, some fruits and other food make the most attractive baits, although usually the least practical. Rodents appear to choose their diet on a basis of nutritional need. Garbage is an ideal, well-balanced food. Rats and mice will usually readily accept grains like corn, oats and wheat. The low cost and durability of such cereal grains make them the material of choice for most rodent baits, especially those that are exposed for long periods.

Hearing — Rodents can use hearing to locate objects to within a few inches. Humans hear to 20 kilohertz levels but the range for the rat extends to a much higher frequency of 50 kilohertz or more. Rodents make high frequency noises in various situations, such as in mating, but researchers do not understand the function of these sounds.

Touch — Rodent whiskers are constantly moving as they explore their environment. Much of a rat’s movement in a familiar area relies heavily on the sense of touch to direct it through time-tested movements learned by exploration and knowledge of the rodent’s home range. Other senses (like vision) are apparently "turned down" when traveling such home range territory, evidenced by a rat that repeatedly will run into an object that is placed in a habitual runway. This knowledge is valuable in placing traps, baits or census materials so that a rodent will encounter them. Rats prefer a stationary object on at least one side of them as they travel and so they commonly move along walls.