ROOF AND NORWAY RAT
Rattus rattus and  Rattus norvegicus

INTRODUCTION

Management of rats and other rodents always begins with a systematic survey and evaluation of the potentially infested site. Estimates of pest numbers and a map of the infested site should be generated. The potentially infested site should be cleaned, removing all possible food sources for the rodents. The site should be altered to eliminate all favorable nesting habitat and to eliminate any passageways that might exist form outside points into adjacent structures. When a structure is infested, intense trapping, using large numbers of traps should be attempted along with the structural and habitat modifications. All park staff and visitors should be fully informed of the mission of the control effort and the reasons for the infestation. These cultural, mechanical, and physical methods of addressing a pest problem should always be carried out before the use of a chemical is considered.

BIOLOGY

Roof rats and Norway rats have similar Life Histories and habits. Where differences are important for management purposes, however, the differences will be highlighted. Rats have poor vision. Rats are wary of anything new that appears in their territory, avoiding the new object for a few days until the rats become familiar with it.

Life cycle: A mature female rat can give birth to about 20 young in a year (4 to 6 at a time). The average life span of a rat in the field is less than one year, with females living longer than males. The young are born in nests. They are hairless and their eyes and ears are closed. Within two weeks their eyes and ears open, they grow fur, and they begin exploring the nest area. In the three week they begin to switch to solid food, and imitate their mother to learn pathways to food, escape routes, and danger zones. If the mother rat has become wary of rodenticides or traps, many of her young will learn to avoid them. This learning experience can make management difficult in sites where long-term rodent suppression programs have been unsuccessful in the past.

Young are totally weaned when 4 or five weeks old. They then weigh about an ounce and a half. At three months the young are independent of their mother and able to mate and continue the cycle, either in the same location or after migrating to a new, unoccupied are.

Outdoor populations tend to peak in summer to early fall. Indoor populations may remain at the same levels throughout the year, limited only by periodic shortages of food, water, or nesting sites.

Social Behavior: Rats are social animals and live in colonies with well defined territories that they mark with urine and glandular secretions. Rats are aggressive, and social conflicts are most common at feeding sites, prime nesting sites, and territorial boundaries. Females fiercely defend their nest and young from other rats.

Environmental conditions that favor development: The Norway rat and the Roof rat have different nesting and feeding preferences that should be considered when management programs are formulated.

Natural Enemies: Dogs, cats, snakes, birds of prey, and other rats.

MANAGEMENT

Medical importance: Rats can spread disease. Sometimes they transmit disease directly by contaminating food with their urine or feces or by biting people. Sometimes they transmit disease indirectly, as when fleas bite a disease-infected rat, then a person. Some of the important diseases associated with commensal rats are: Plague (not currently a problem in the NE USA), Rat-Bite Fever (a seldom serious bacterial infection that occurs occasionally after being bitten by a rat.), Salmonella Food Poisoning (a problem where rats have access to human food sources), and Leptospirosis (another disease that is spread to humans through contaminated food, water, and through cuts in the skin).

Potential Damage: Rat burrows can cause structural damage by undermining the foundations of buildings, roads, and walkways; can cause damage by gnawing, damaging plastic and lead pipes, door frames, upholstery, and electric wires; and cause damage through the destruction and contamination of food crops and stored foods.

Threshold: The Roof and Norway rats are not native to the United States and are thus considered to be exotic. For that reason, the sighting of just one Norway or Roof rat is adequate reason to initiate management efforts addressed at eliminating the rats. Management efforts should continue until all visible signs of the rats are absent. Monitoring for the detection of a new rat infestation should continue year-round.

Monitoring: Monitoring is a systematic survey conducted at regular intervals.

1. Keep good monitoring records of what you see, smell, hear, and when you saw it, and under what conditions you saw it. These records are important in determining how to best manage a pest.
2. Visual Sightings - an inspection using a powerful light, just after dark, is the best way to see rats. If you see rats during the daytime, population levels are very high!
3. Sounds - when it is quiet in a building, you can sometimes hear squeaks, fighting noises, or clawing and scrambling in the walls. A stethoscope can help pinpoint activity.
4. Droppings - A single rat may produce 50 droppings daily. One way to determine if a rat population is active is to sweep up old droppings and reinspect a week later for new droppings. Fresh rat droppings are black or nearly black, wet and may glisten, and soft. Older droppings lose the black coloration.
5. Urine - Both wet and dry urine stains will glow blue-white under an ultra-violet light (blacklight). Blacklight inspections work best at night and in dark rooms.
6. Grease marks - Oil and dirt rub off of a rat’s coat when it rubs against things. These grease marks build up in often used runways and soon become noticeable. These marks are commonly found along wall/floor junctions, on pipes and ceiling joists, and on sill plates.
7. Runways - Outdoors, runways tend to appear as beaten paths on the ground where rats constantly travel the same route. These can be found next to fences, under bushes, and along buildings.
8. Tracks, Gnawing damage, burrows - physical signs or damage are important indicators of the presence of rats, the size of the population, and the location of the infestation. A tracking patch (a light dusting of an inert material such as clay, talc, or limestone) can be place in sites where you suspect rat activity. The rats walk through the dust and you will be able to track the pests movement. The size and number of burrows is a useful indicator of the significance of the infestation.

Tools needed for Monitoring: A flashlight; ultraviolet light; talc; map of the site showing important observations; monitoring record form or sheet.

Management Strategies:

Chemical Control

Rodenticides should not be used indoors in NPS facilities, particularly in historic buildings, except under extreme circumstances. Rodents that have ingested a toxic dose of a rodenticide may crawl into wall voids and other inaccessible areas to die. The decaying carcass can produce foul odors and attract insects such as dermestid beetles or blowflies, which feed on the dead animal. Once they have consumed the carcass, the insects will seek other food sources, and may become pests themselves, feeding on fabrics, furs, stored foods, and historic artifacts.