Damage and Damage Identification

Nature of Damage

Roof rat, Rattus rattus

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.

Rat Sign

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 over-head 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.

Legal Status

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.

Economics of Damage and Control

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.