Damage Prevention and Control Methods
When nuisance problems occur in and around buildings, exclusion is the most effective method of eliminating damage. Woodrats may be excluded from buildings by the same methods used to exclude Norway and roof rats (see Rodent-proof Construction and Exclusion Methods). Since several species of woodrats are agile climbers, all entrances to buildings, including those at the attic level, must be closed. Cracks and openings in building foundations, and any openings for water pipes, electric wires, sewer pipes, drain spouts, and vents must be sealed. Also check for openings in attic vents, broken roof shingles, or other gaps next to the eaves. No hole larger than 1/2 inch (1.3 cm) should be left unsealed. Make sure doors, windows, and screens fit tightly. If gnawing is a problem, edges can be covered with sheet metal. Coarse steel wool, wire screen, and lightweight sheet metal are excellent materials for plugging gaps and holes. Plastic sheeting, wood, or other less sturdy materials will likely be gnawed away. When rodent-proofing, be sure the woodrat is not trapped inside the building. One way to accomplish this is to install a temporary gravity door made of sheet metal or rigid mesh wire, hinged at the top, over entrance holes. The woodrats can push it open to exit but cannot reenter.
No repellents aer registered by the EPA for woodrats. In general, chemical repellents are not considered a practical solution to woodrat problems.
Toxicants available for woodrat control include anticoagulants and zinc phosphide, registered under Special Local Needs 24(c) provisions. Registered products vary among states. When using toxic baits, follow label instructions carefully.
Anticoagulants are effective for woodrat control and are especially suited for use around structures because of their low hazard to pets and children. Most baits formulated for commensal rats and house mice give effective woodrat control. Anti-coagulants work by interfering with the blood-clotting mechanism. Death usually occurs 4 to 5 days after feeding on bait begins. With most anticoagulants, such as chlorophacinone or diphacinone, feeding must occur daily for 4 to 5 days. Finely ground or meal-type anticoagulant baits are recommended. Since woodrats have a tendency to pack away items, pellet bait should be avoided since it is often cached at the nest site. Cached bait is probably not effective in minimizing reinvasions of the area, so it is essentially wasted and may present hazards to nontarget species.
Anticoagulants are usually put out in bait boxes, but woodrats tend to fill boxes with sticks and other debris. Therefore, use open bait containers. Bait exposed in this manner must be placed so nontarget species, pets, and children do not have ready access to it. Access to the bait by pets can be minimized by inverting a wooden crate over the bait tray. Baiting sites should be located near existing woodrat runways, feeding sites, or nests.
Anticoagulant paraffin bait blocks have also proven valuable for woodrat control. Because of the paraffin, the bait has more resistance to molding caused by moisture and, therefore, lasts longer. These bait blocks are particularly useful in mountain cabins or other structures where woodrats gain access when the building is unoccupied. The bait block should be nailed or tied down to prevent the woodrat from packing it away. When the label permits, bait blocks may also be wired to tree limbs or other elevated locations. For additional information on anticoagulant baits see Norway Rats, Roof Rats, and Vertebrate Pesticides.
In agricultural situations, zinc phosphide is a Restricted Use Pesticide and must be applied by a certified applicator. Steam-rolled oats or oat groats treated with 2.0% zinc phosphide are generally very effective on woodrats. Usually, tablespoon (4 g) amounts are scattered in runways near the nest site. Zinc phosphide bait should be applied in late afternoon just prior to woodrats’ night-time feeding. Feeding on a sub-lethal amount of zinc phosphide bait can result in bait shyness. Therefore, do not use zinc phosphide more than once per 6-month period.
In some cases, the use of second generation anticoagulants (for example, brodifacoum, bromadiolone) or other toxicants (cholecalciferol) may be permitted for woodrat control. Based on Section 2 of FIFRA (Federal Insecticide, Fungicide, and Rodenticide Act), EPA ruled that it is legal, unless otherwise specifically prohibited, to use a pesticide against a target species not listed on the label if the label directions for a listed pest are followed. The site to be treated must be mentioned on the label and there must be reason to believe the application will be effective. For example, the use of cholecalciferol to control woodrats in or around buildings could be permissible because the label lists Norway or roof rats and specifies in or around buildings. Not all states accept the EPA ruling. Check with the appropriate pesticide enforcement agency prior to pursuing this course of action.
The majority of woodrat problems in structures can be dealt with by using one or several traps. Woodrats show little fear of new objects in their environment and are easily trapped. The standard rat snap trap is quite effective for woodrats. Trap bait should be wedged into or tied to the treadle. Good baits include nut meats, bacon rind, peanut butter and oatmeal, prunes, raisins and other dried fruit, and biscuits.
Live catch traps, using the same baits as above, can be used for woodrats. Release of trapped animals is not recommended and may be against local fish and game regulations. Also, many studies have shown that animals released into new areas often die from exposure, predation, or competition with resident animals.
Burrow-entrance traps such as the No. 110 Conibear® trap may also be useful for woodrat control. The trap is placed in nest openings or other restricted travel ways and is triggered when the woodrat passes through the trap opening. When traps are set in this manner, baiting is not necessary, but care must be taken to avoid nontarget animals.
Glue boards are also effective for trapping woodrats. These work on the same principle as flypaper; when a rat attempts to cross a glue board, it gets stuck. Glue boards tend to lose their effectiveness in dusty areas, and temperature extremes may affect the tackiness of the adhesive. In many cases, woodrats trapped on glue boards will not die immediately. If they don’t, they can be euthanized by placing the board in a plastic bag and adding carbon dioxide gas.
Remember, all traps and glue boards should be placed so that children, pets, and other nontarget animals do not have access to them.
Destroying woodrat nests has been suggested as a method of control. When a nest is destroyed, the animals may run for cover, thus exposing them to predation by humans or dogs. This method of control is time-consuming and probably of limited value. Once the woodrats in an area are controlled, however, destroying their nests may reduce invasion by other woodrats.
Summary of Damage Prevention and Control Methods
Woodrats can be permanently excluded from buildings.
Not generally useful. Trim lower branches of citrus trees.
None are registered or considered effective at this time.
Anticoagulants (registered in some states).
Zinc phosphide (registered in some states).
Rat snap trap.
Other Control Methods
Destruction of dens.