Washington Roof Rats
Three subspecies have been named, and these are generally identified by their fur color: (1) the black rat (R. Rattus Rattus Linnaeus) is black with a gray belly; (2) the Alexandrine rat (R. Rattus Alexandrinus Geoffroy) has an agouti (brownish streaked with gray) back and gray belly; and (3) the fruit rat (R. Rattus Frugivorus Rafinesque), has an agouti back and white belly. The reliability of using coloration to identify the subspecies is questionable, and little significance can be attributed to subspecies differentiations. In some areas the subspecies are not distinct because more than one subspecies has probably been introduced and cross-breeding among them is a common occurrence. Roof rats cannot, however, cross with Norway rats or any rodent species.
Range and Habitat
Roof rats range along the lower half of the East Coast and throughout the Gulf States upward into Arkansas. They also exist all along the Pacific Coast and are found on the Hawaiian Islands. The roof rat is more at home in warm climates, and apparently less adaptable, than the Norway rat, which is why it has not spread throughout the country. Its worldwide geographic distribution suggests that it is much more suited to tropical and semitropical climates. In rare instances, isolated populations are found in areas not within their normal distribution range in the United States. Most of the states in the US interior are free of roof rats, but isolated infestations, probably stemming from infested cargo shipments, can occur.
Roof rats are more aerial than Norway rats in their habitat selection and often live in trees or on vine-covered fences. Landscaped residential or industrial areas provide good habitat, as does riparian vegetation of riverbanks and streams. Parks with natural and artificial ponds, or reservoirs may also be infested. Roof rats will often move into sugarcane and citrus groves. They are sometimes found living in rice fields or around poultry or other farm buildings as well as in industrial sites where food and shelter are available. Roof rats frequently enter buildings from the roof or from accesses near overhead utility lines, which they use to travel from area to area. They are often found living on the second floor of a warehouse in which Norway rats occupy the first or basement floor. Once established, they readily breed and thrive within buildings, just as Norway rats do. They have also been found living in sewer systems, but this is not common.
Control methods must reflect an understanding of the roof rat’s habitat requirements, reproductive capabilities, food habits, life history, behavior, senses, movements, and the dynamics of its population structure. Without this knowledge, both time and money are wasted, and the chances of failure are increased.
Unfortunately, the rat’s great adaptability to varying environmental conditions can sometimes make this information elusive.
The young are born in a nest about 21 to 23 days mating. At birth they are hairless, and their eyes are closed. There are 5 to 8 young in each litter and they develop rapidly, growing hair within a week. Their eyes open between 9 and 14 days and they begin to explore for food and move about near their nest. In the third week they begin to take solid food. The number of litters depends on the area and varies with nearness to the limit of their climatic range, availability of nutritious food, density of the local rat population, and the age of the rat. Typically, 3 or more litters are produced annually.
The young may continue to nurse until 4 or 5 weeks old. By this time they have learned what is good to eat by experimenting with potential food items and by imitating their mother.
Rats rely more on their keen senses of smell, taste, touch and hearing than on vision. They are considered to be color-blind, responding only to the degree of lightness and darkness of color.
They use their keen sense of smell to locate and select food items, identify territories and travel routes, and recognize other rats, especially those of the opposite sex. Taste perception of rats is good; once rats locate food, the taste will determine their food preferences.
Touch is an important sense in rats. The long, sensitive whiskers (vibrissae) near their nose and the guard hairs on their body are used as tactile sensors. The whiskers and guard hairs enable the animals to travel adjacent to walls in the dark and in burrows.
Roof rats also have an excellent sense of balance. They use their tails for balance while traveling along overhead utility lines. They move faster than Norway rats and are very agile climbers, which enables them to quickly escape predators. Their keen sense of hearing also aids in their ability to detect and escape danger.
In food-processing and food-storage facilities, roof rats do about the same type of damage as Norway rats, and damage is visually hard to differentiate.
In residences where rats may be living in the attic and feeding outdoors, the damage may be restricted to tearing up insulation for nesting or gnawing electrical wiring. Sometimes rats get into the kitchen area and feed on stored foods. If living under a refrigerator or freezer, they may disable the unit by gnawing the electrical wires.
In landscaped yards they often live in overgrown shrubbery or vines, feeding on ornamentals, vegetables, fruits, and nuts. Snails are a favorite food, but don’t expect roof rats to eliminate a garden snail problem. In some situations, pet food and poorly managed garbage may represent a major food resource.
In some agricultural areas, roof rats cause significant losses of tree crops such as citrus and avocados and, to a lesser extent, walnuts, almonds, and other nuts. They often eat all the pulp from oranges while the fruit is still hanging on the tree, leaving only the empty rind. With lemons they may eat only the rind and leave the hanging fruit intact. They may eat the bark of smaller citrus branches and girdle them. In sugarcane, they move into the field as the cane matures and feed on the cane stalks. While they may not kill the stalk outright, secondary organisms generally invade and reduce the sugar quality. Norway rats are a common mammalian pest of rice, but sometimes roof rats also feed on newly planted seed or the seedling as it emerges. Other vegetable, melon, berry, and fruit crops occasionally suffer relatively minor damage when adjacent to infested habitat such as riparian vegetation.
The nature of damage to outdoor vegetation can often provide clues as to whether it is caused by the roof or Norway rat. Other rat signs may also assist, but be aware that both species may be present. Setting a trap to collect a few specimens may be the only sure way to identify the rat or rats involved. Out-of-doors, roof rats may be present in low to moderate numbers with little sign in the way of tracks or droppings or runs and burrows.
There is less tendency to see droppings, urine, or tracks on the floor in buildings because rats may live overhead between floors, above false ceilings, or in utility spaces, and venture down to feed or obtain food. In food-storage facilities, the most prominent sign may be smudge marks, the result of oil and dirt rubbing off of their fur as they travel along their aerial routes.
The adequate inspection of a large facility for the presence and location of roof rats often requires a nighttime search when the facility is normally shut down. Use a powerful flashlight to spot rats and to determine travel routes for the best locations to set baits and traps. Sounds in the attic are often the first indication of the presence of roof rats in a residence. When everyone is asleep and the house is quiet, the rats can be heard scurrying about.
Like the Norway rat, the roof rat is implicated in the transmission of a number of diseases to humans, including murine typhus, leptospirosis, salmonellosis (food poisoning), rat-bite fever, and plague. It is also capable of transmitting a number of diseases to domestic animals and is suspected in the transference of ectoparasites from one place to another.
Roof rats are not protected by law and can be controlled any time with mechanical or chemical methods. Pesticides must be registered for rat control by federal and/or state authorities and used in accordance with label directions.
Damage Prevention and Control Methods
Seal all openings that provide entry to structures. Rat guards (for overhead utility lines).
Habitat Modification and Sanitation
Practice good housekeeping and facility sanitation. Contain and dispose of garbage and refuse properly. Reduce vegetative cover (for example, trim vines from buildings and fences). Cultural practices in agriculture (weed and brush control, pruning).
Ultrasonic devices have not been proven to provide rat control. Lights and other sounds are of limited value. Visual devices such as model owls, snakes, and cats are of no value.
None are effective.
Anticoagulant rodenticides (slowacting chronic-type poisons) Brodifacoum (Talon®, Havoc®). Bromadiolone (Maki®, Contrac®). Chlorophacinone (RoZol®). Diphacinone (Ramik®, Ditrac®). Pindone (Pival®, Pivalyn®). Warfarin (Co-Rax®). Toxicants other than anticoagulants (may be acute or chronic poisons) Bromethalin (Assault®, Vengeance®). Cholecalciferol (Vitamin D3) (Quintox®, Rampage®). Zinc phosphide (Ridall Zinc®, ZP® Rodent Bait).
Structure or commodity fumigation. Burrow fumigants are of limited use.
Snap traps. Box-type kill traps. Live traps. Glue boards.
The above information was adapted from PREVENTION AND CONTROL OF WILDLIFE DAMAGE with permission of the editors, Scott E. Hygnstrom, Robert M. Timm, and Gary E. Larson (Cooperative Extension Division, Institute of Agriculture and Natural Resources University of Nebraska-Lincoln, United States Department of Agriculture Animal and Plant Health Inspection Service, Animal Damage Control, Great Plains Agricultural Council Wildlife Committee).