- Determine the signs of an infestation of Norway rats.
- Explain how the biology of Norway rats determines the techniques for their control.
- Explain the options for control of Norway rats.
Norway rats are burrowing rodents that were introduced to North America from Europe. Also called brown rats, house rats, barn rats, sewer rats, gray rats, or wharf rats, they are slightly larger than roof rats. Norway rats often occur in close association with humans and therefore are called “commensal” (table-sharing) rodents.
Norway rats are not protected by laws. Most communities have sanitation regulations that are designed to reduce populations of urban pests.
The fur of a Norway rat is coarse and usually brown or red-gray above and white-gray on the belly. The tail does not reach the tip of the nose. Adult Norway rats weigh an average of 12 to 16 ounces. Their paired incisor teeth grow continuously at the rate of about 5 inches per year. They keep their teeth worn down by gnawing on hard surfaces.
Norway rats have spread throughout the contiguous 48 states, Alaska, and Hawaii. They generally are found at lower elevations but may occur wherever humans live.
Voice and Sounds
Norway rats emit a variety of high-pitched squeaks. Noises from fighting, gnawing, and climbing also may be heard.
Tracks and Signs
Tracks, including footprints and tail marks, may be seen on dusty surfaces or in mud (Figure 2).
Figure 2. Tracks of a Norway rat. Image by Dee Ebbeka.
A tracking patch made of flour can be placed in pathways overnight to determine if rodents are present.
Droppings may be found along runways, in feeding areas, and near shelters (Figure 3).
Scat may be as large as ¾ inch long and ¼ inch in diameter with blunt ends. Rats deposit about 40 to 50 droppings per day, which usually are clumped in small groups. Other rodents tend to disperse their scat. Fresh droppings have a soft texture.
Figure 3. Scat of Norway rats.
Photo by Kurt VerCauteren.
Runs or burrows may be found next to walls and buildings, along fences, and under bushes and debris. Active holes are 2 to 3 inches in width and clean of debris (Figure 4) and have hairs of rats present around the entrance. Rats habitually use the same routes.
Figure 4. Hole of a den of a Norway rat.
Photo by Stephen M. Vantassel.
Figure 5. Marks from gnawing of Norway rats tend to be 1/16 of an inch wide. Photo by Robert M. Timm.
Marks from gnawing may be visible in corners and on wall materials, doors, ledges, and stored materials wherever rats are present. Fresh accumulations of shavings from wood, insulation and other material indicate active infestations.
Entrance holes are 2 to 3 inches in diameter with grooves 1/16 of an inch wide, nearly twice the size of marks left by house mice (Figure 5).
Smudge marks (rub marks) may occur on beams, rafters, pipes, and walls as a result of oil and dirt rubbing off of fur along routes that are frequently traveled.