Damage and Damage Identification

Norway rat, Rattus norvegicus

Norway rats consume and contaminate foodstuffs and animal feed. They may damage crops in fields prior to and during harvest, and during processing and storage. Rats also damage containers and packaging materials in which foods and feed are stored.

Rats cause structural damage to buildings by burrowing and gnawing. They undermine building foundations and slabs, cause settling in roads and rail-road track beds, and damage the banks of irrigation canals and levees. Rats also may gnaw on electrical wires or water pipes, either in structures or below ground. They damage structures further by gnawing openings through doors, window sills, walls, ceilings, and floors. Considerable dam-age to insulated structures can occur as a result of rat burrowing and nesting in walls and attics.

Among the diseases rats may transmit to humans or livestock are murine typhus, leptospirosis, trichinosis, salmonellosis (food poisoning), and ratbite fever. Plague is a disease that can be carried by a variety of rodents, but it is more commonly associated with roof rats (Rattus rattus) than with Norway rats.

Rat Sign

Figure 4. Tracks left in dust by (a) Norway rat and (b) house mouse.

The presence of rats can be determined by a number of signs described below:

Droppings may be found along run-ways, in feeding areas, and near shelter. They may be as large as 3/4 inch (2 cm) long and 1/4 inch (0.6 cm) in diameter. Fresh droppings are soft in texture.

Tracks, including footprints or tail marks, may be seen on dusty surfaces or in mud (Fig. 4). A tracking patch made of flour can be placed in pathways overnight to determine if rodents are present.

Urine, both wet and dry, will fluoresce under ultraviolet light. Urine stains may occur along travel ways or in feeding areas.

Runs or burrows may be found next to walls, along fences, next to buildings, or under bushes and debris. Rats memorize pathways and use the same routes habitually.

Smudge marks (rub marks) may occur on beams, rafters, pipes, and walls as a result of oil and dirt rubbing off rats’ fur along frequently traveled routes (Fig. 5).

Figure 5. Rub marks along beams, rafters, or other travel routes are evidence of rat activity.

Gnawing may be visible on doors, ledges, in corners, in wall material, on stored materials, or other surfaces wherever rats are present. Fresh accumulations of wood shavings, insulation, and other gnawed material indicate active infestations. Size of entry holes (often 1 1/2 inches [4 cm] in diameter or less for mice, 2 inches [5 cm] or larger for rats) or tooth marks can be used to distinguish rat from mouse gnawing. Rats keep their paired incisor teeth, which grow continuously at the rate of about 5 inches (13 cm) per year, worn down by gnawing on hard surfaces and by working them against each other.

Sounds such as gnawing, climbing in walls, clawing, various squeaks, and fighting noises are common where rats are present, particularly at times of the day when they are most active.

Estimating Rat Numbers

Rat sign and visual sightings are of limited value in accurately estimating rat numbers, but they are the simplest and often the only practical method available. Search premises thoroughly when looking for rats. In structures, searches should include attics, basements, around foundations, crawl spaces, and behind and under stored materials. The following estimates can then be made:

No sign: no rats or few present. If only a few rats are present they may have invaded only recently. Old droppings and gnawing common, one or more rats seen by flash-light at night, or no rats observed in daytime: medium numbers present.

Fresh droppings, tracks, and gnawing present, three or more rats seen at night, or rats seen in daytime: large numbers present.

Since rats are normally nocturnal and somewhat wary of humans, usually many more rats are present than will be seen in the daytime. Under certain conditions, rats may become quite bold in the presence of humans, and then a high percentage of the population may be visible.

A conservative estimate of rat numbers can be made from measuring their food consumption. You can do this by feeding the rats for a while on finely ground grain (whole grains or pelleted foods may be carried off uneaten). When offered over a period of time, the ground grain will usually be accepted and eaten by rats. Consumption may gradually increase to a maximum level over the period of a week or so as the rats’ natural fear of novel foods is overcome. Divide the total amount of food eaten per day by 1/2 ounce (15 g); this will give a mini-mum estimate of the rats present. Some rats eat more than 1/2 ounce (15 g) daily, but rats will probably also be using other foods in their environment. If too much alternative food is available, this technique will not give an adequate estimate.

Legal Status

Norway rats are not protected by law. They may be controlled with any pesticide registered by federal or state authorities for this purpose, or they may be controlled by use of mechanical methods such as traps.

Economics of Damage and Control

Accurate data on rat damage, control, and their cost is difficult to obtain. Estimates of losses of foodstuffs, structural damage, and the amount of labor and materials expended to control rats are usually only educated guesses. One study found that a small colony of Norway rats (10 to 26 animals), when given access to a ton of sacked wheat, would contaminate 70% of the grain after 12 to 28 weeks. The sacks were heavily damaged as well. Total damage equaled 18.2% of the total value of the wheat and the sacks. One rat will eat approximately 20 to 40 pounds (9 to 18 kg) of feed per year and probably contaminates 10 times that amount with its urine and droppings. In a year’s time, a single rat will produce some 25,000 droppings. A 1973 estimate states rats may cost the United States between $500 million and $1 billion annually in direct economic losses. In most cases, the cost of rat control—particularly when it is done in a timely fashion—is far less than the economic loss caused by rat damage.