Estimating Rat Numbers
Signs and visual confirmation of rats have limited value in estimating numbers accurately, but they are the simplest and often the only practical methods available. Search the premises thoroughly when looking for rats. Searches should include attics, basements, and crawl spaces, around foundations, and behind and under storage materials.
No signs probably mean that no or few rats are present. If only a few rats are present they may have invaded only recently. Rats normally are nocturnal and wary of humans, resulting in many more rats present than may be seen. Occasionally, rats may be quite bold in the presence of humans, at which time a high percentage of the population may be visible. The following are general rules for estimating numbers of rats:
- old droppings and gnawing, 1 or more rats seen at night, or no rats observed in daytime indicates that moderate numbers are present; and
- fresh droppings, tracks, and gnawing, 3 or more rats seen at night, or rats seen during the day indicate that large numbers are present.
A conservative estimate of the number of rats in an area can be made from measuring consumption of food. Feed the rats finely ground grain. Whole grains or pelleted foods may be carried off uneaten. When it is offered over a week, rats overcome the fear of novel foods and grain usually will be eaten by rats. Consumption may increase gradually to a maximum level during 2 weeks. Divide the total amount of food eaten per day by ½ ounce, which will give a minimum estimate of the number of rats present. Some rats eat more than ½ ounce per day, but they probably supplement their diet with other foods. If too much alternative food is available, this technique will not provide a reliable estimate.
Damage to Structures
Rats cause damage to buildings by burrowing and gnawing. They undermine foundations, cause settling in roads and railroads, and damage the banks of canals and levees. Rats gnaw on electrical wires and water pipes. They gnaw through doors, windows, walls, ceilings, and floors. Activity from burrowing and nesting in walls and attics may cause considerable damage to insulated structures.
Damage to Livestock and Pets
Norway rats occasionally prey on chickens, ducks, racing pigeons, and their eggs.
Damage to Landscapes
Norway rats may damage field crops prior to, during, and after harvest. Burrowing can damage plant roots and reduce harvest yields, as well as cause erosion on slopes.
Health and Safety Concerns
Norway rats transmit diseases to humans and livestock. Murine typhus, leptospirosis, trichinosis, salmonellosis, and rat bite fever are common. Plague is a disease more commonly associated with roof rats than with Norway rats.
Rats may bite infants and adults that are unable to defend themselves in areas with high numbers of rats. Bites typically occur where there are odors from food, such as on the hands and mouth.
Norway rats consume and contaminate food intended for humans or livestock. Rats damage containers and packaging materials in which food and feed are stored. 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.