The New Jersey Roof Rat is a common long-tailed rodent of the genus Rattus (Old World rodents) and the subfamily murinae (Murine rodents). The species originated in tropical Asia and spread through the Near East in Roman times before reaching Europe in the 8th Century and spreading with Europeans across the world.
The Roof rat (Rattus Rattus) is one of two introduced rats found in the contiguous 48 states. The Norway rat (R. Norvegicus) is the other species and is better known because of its widespread distribution. A third rat species, the Polynesian rat (R. Exulans) is present in the Hawaiian Islands but not on the mainland. Rattus rattus is commonly known as the roof rat, black rat, and ship rat. Roof rats were common on early sailing ships and apparently arrived in North America by that route. This rat has a long history as a carrier of the plague.
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 differentiation. 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.
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.
The food habits of roof rats outdoors in some respects resemble those of tree squirrels since they prefer a wide variety of fruit and nuts. They also feed on a variety of vegetative parts of ornamental and native plant materials. Like Norway rats, they are omnivorous and, if necessary, will feed on almost anything. In food-processing and storage facilities, they will feed on nearly all food items, though their food preferences may differ from those of Norway rats. They do very well on feed provided for domestic animals such as swine, dairy cows, and chickens, as well as on dog and cat food. There is often a correlation between rat problems and the keeping of dogs, especially where dogs are fed outdoors. Roof rats usually require water daily, though their local diet may provide an adequate amount if it is high in water content.
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 after conception. At birth they are hairless, and their eyes are closed. The 5 to 8 young in the litter develop rapidly, growing hair within a week. Between 9 and 14 days, their eyes open, 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.
Young rats generally cannot be trapped until about 1 month old. At about 3 months of age, they are completely independent of the mother and are reproductively mature.
Breeding seasons vary in different areas. In tropical or semitropical regions, the season may be nearly year-round. Usually, the peaks in breeding occur in the spring and fall. Roof rats prefer to nest in locations off of the ground and rarely dig burrows for living quarters if off-the-ground sites exist.
Rats usually begin searching for food shortly after sunset. If the food is in an exposed area and too large to be eaten quickly, but not too large to be moved, they will usually carry it to a hiding place before eating it. Many rats may cache or hoard considerable amounts of solid food, which they eat later. Such caches may be found in a dismantled woodpile, attic, or behind boxes in a garage.
When necessary, roof rats will travel considerable distances (100 to 300 feet [30 to 90 m]) for food. They may live in the landscaping of one residence and feed at another. They can often be seen at night running along overhead utility lines or fences. They may live in trees, such as palm, or in attics, and climb down to a food source. Traditional baiting or trapping on the ground or floor may intercept very few roof rats unless bait and/or traps are placed at the very points that rats traverse from above to a food resource. Roof rats have a strong tendency to avoid new objects in their environment and this neophobia can influence control efforts, for it may take several days before they will approach a bait station or trap. Neophobia is more pronounced in roof rats than in Norway rats. Some roof rat populations are skittish and will modify their travel routes and feeding locations if severely and frequently disturbed. Disturbances such as habitat modifications should be avoided until the population is under control.
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.
The social behavior of free-living roof rats is very difficult to study and, as a result, has received less attention than that of Norway rats. Most information on this subject comes from populations confined in cages or outdoor pens.
Rats tend to segregate themselves socially in both space and time. The more dominant individuals occupy the better habitats and feed whenever they like, whereas the less fortunate individuals may have to occupy marginal habitat and feed when the more dominant rats are not present.
Knowledge is limited to interspecific competition between the different genera and species of rats. At least in some parts of the United States and elsewhere in the world, the methods used to control rats have reduced Norway rat populations but have permitted roof rats to become more prominent, apparently because they are more difficult to control. Elsewhere, reports indicate that roof rats are slowly disappearing from localized areas for no apparent reason.
It has often been said that Norway rats will displace roof rats whenever they come together, but the evidence is not altogether convincing.
Rat densities (numbers of rats in a given area) are determined primarily by the suitability of the habitat—the amount of available nutritional and palatable food and nearby protective cover (shelter or harborage).
The great adaptability of rats to human-created environments and the high fertility rate of rats make for quick recuperation of their populations. A control operation, therefore, must reduce numbers to a very low level; otherwise, rats will not only reproduce rapidly but often quickly exceed their former density for a short period of time.
Unless the suitability of the rat’s habitat is destroyed by modifying the landscaping, improving sanitation, and rat-proofing, control methods must be unrelenting if they are to be effective.
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.
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.
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.
Roof rats undoubtedly cause millions of dollars a year in losses of food and feed and from damaging structures and other gnawable materials. On a nationwide basis, roof rats cause far less economic loss than Norway rats because of their limited distribution.
There are approximately 30,000 professional structural pest control operators in the United States and about 70% of these are primarily involved in general pest control, which includes rodent control. It is difficult to estimate how much is spent in structural pest control specifically for roof rats because estimates generally group rodents together.
Sugarcane, citrus, avocados, and macadamia nuts are the agricultural crops that suffer the greatest losses. In Hawaii, annual macadamia loss has recently been estimated at between $2 million and $4 million.
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.
Seal all openings that provide entry to structures. Rat guards (for overhead utility lines).
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 (slow-acting 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.
Limited usefulness where legal and not hazardous.
Cats may occasionally catch roof rats, as will barn owls. Predators are of little, if any, value in controlling roof rats.
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