Commensal Rodents

Roof rat, Rattus rattus

The three species of commensal rodents (those that live primarily around human habitation) are Norway Rats, roof (black) rats, and House Mice. These omnivorous rodents consume millions of bushels of grain each year in the field, on the farm, in the elevator, mill, store, and home, and in transit. They also waste many more millions of bushels by contamination. These rodents typically drop 25 to 150 pellets and void 1/3 to 2/3 ounce (10 to 20 ml) of urine every 24 hours and constantly shed fine hairs.

Rats cause extensive damage to sugar-cane in Hawaii and Florida, and roof rats are serious pests in Hawaiian macadamia nut plantations. These rodents will feed on poultry chicks and occasionally even attack adult poultry, wild birds, newborn pigs, lambs, and calves. Health departments annually report hundreds of human babies bit-ten by rats. Many viral and bacterial diseases are transmitted to humans by rodent feces and urine that contaminate food and water (Weber 1982).

Gnawing by rodents causes consider-able property damage. Fires are some-times started when rats and mice gnaw the insulation of electric wiring. They will also use materials such as oily rags and matches for building nests, which can result in fires by spontaneous combustion. Extensive damage to foundations and concrete slabs is sometimes done when Norway rats burrow under buildings. Burrows into dikes and outdoor embankments cause erosion.

Signs of commensal rodents are gnawing, droppings, tracks, burrows, and darkened or smeared areas along walls where they travel. Reviews of problems caused by these species and methods of control are provided by Meehan (1984), Jackson (1987), Baker et al. (1993), Marsh (1994), and Timm (1994).

Summary of Damage Prevention and Control Methods

Many control methods are essentially the same for roof rats as for Norway rats.

Exclusion and Rodent-proofing

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).

Frightening

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.

Repellents

None are effective.

Toxicants

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).

Fumigants

Structure or commodity fumigation. Burrow fumigants are of limited use.

Trapping

Snap traps.

Box-type kill traps. Live traps.

Glue boards.

Shooting

Limited usefulness where legal and not hazardous.

Predators

Cats may occasionally catch roof rats, as will barn owls. Predators are of little, if any, value in controlling roof rats.