Damage Prevention and Control Methods
The damage control methods used for roof rats are essentially the same as for Norway Rats. However, a few differences must be taken into account.
Exclusion or Rodent-proofing
When rodent-proofing against roof rats, pay close attention to the roof and roof line areas to assure all accesses are closed. Plug or seal all openings of greater than 1/2 inch (1.3 cm) diameter with concrete mortar, steel wool, or metal flashing. Rodent-proofing against roof rats usually requires more time to find entry points than for Norway rats because of their greater climbing ability. Eliminate vines growing on buildings and, when feasible, overhanging tree limbs that may be used as travel routes. For more detailed information, see Rodent-proof Construction and Exclusion Methods.
Attach rat guards to overhead utility wires and maintain them regularly. Rat guards are not without problems, however, because they may fray the insulation and cause short circuits.
Habitat Modification and Sanitation
The elimination of food and water through good warehouse sanitation can do much to reduce rodent infestation. Store pet food in sealed containers and do not leave it out at night. Use proper garbage and refuse disposal containers and implement exterior sanitation programs. Emphasis should be placed on the removal of as much harborage as is practical. For further information see Norway Rats.
Dense shrubbery, vine-covered trees and fences, and vine ground cover make ideal harborage for roof rats. Severe pruning and/or removal of certain ornamentals are often required to obtain a degree of lasting rat control. Remove preharvest fruits or nuts that drop in backyards. Strip and destroy all unwanted fruit when the harvest period is over.
In tree crops, some cultural practices can be helpful. When practical, remove extraneous vegetation adjacent to the crop that may provide shelter for rats. Citrus trees, having very low hanging skirts, are more prone to damage because they provide rats with protection. Prune to raise the skirts and remove any nests constructed in the trees. A vegetation-free margin around the grove will slow rat invasions because rats are more susceptible to predation when crossing unfamiliar open areas.
Rats have acute hearing and can readily detect noises. They may be frightened by sound-producing devices for awhile but they become accustomed to constant and frequently repeated sounds quickly. High-frequency sound-producing devices are advertised for frightening rats, but almost no research exists on their effects specifically on roof rats. It is unlikely, however, they will be any more effective for roof rats than for Norway Rats. These devices must be viewed with considerable skepticism, because research has not proven them effective.
Lights (flashing or continuously on) may repel rats at first, but rats will quickly acclimate to them.
Products sold as general animal repellents, based on taste and/or odor, are sometimes advertised to repel animals, including rats, from garbage bags. The efficacy of such products for rats is generally lacking. No chemical repellents are specifically registered for rat control.
Rodenticides were once categorized as acute (single-dose) or chronic (multiple-dose) toxicants. However, the complexity in mode of action of newer materials makes these classifications outdated. A preferred categorization would be “anti-coagulants” and “non-anticoagulants” or “other rodenticides.”
Anticoagulants (slow-acting, chronic toxicants).
Roof rats are susceptible to all of the various anticoagulant rodenticides, but less so than Norway rats. Generally, a few more feedings are necessary to produce death with the first-generation anticoagulants (warfarin, diphacinone, and chlorophacinone) but this is less significant with the second-generation anticoagulants (bromadiolone, brodifacoum, and difethialone). All anticoagulants provide excellent roof rat control when prepared in acceptable baits. For the characteristics of the various anticoagulant rodenticides see Norway Rats.
A few instances of first-generation anticoagulant resistance have been reported in roof rats; although not common, it may be underestimated because so few resistance studies have been conducted on this species. Resistance is of little consequence in the control of roof rats, especially with the newer rodenticides presently available. Where anticoagulant resistance is known or suspected, the use of first-generation anticoagulants should be avoided in favor of the second-generation anticoagulants or one of the non-anticoagulant rodenticides like bromethalin or cholecalciferol.
At present three non-anticoagulant rodenticides: zinc phosphide, cholecalciferol (vitamin D3), and bromethalin are registered and available for roof rat control. All can be used to control anticoagulant-resistant populations of roof rats.
Roof rats can be controlled with the same baits used for Norway rats. Most commercial baits are registered for both species of rats and for house mice, but often they are less acceptable to roof rats than to the other species. For best results, try several baits to find out which one rats consume most. No rat bait ingredient is universally highly acceptable, and regional differences are the rule rather than the exception.
Pelleted or loose cereal anticoagulant baits are used extensively in tamper-resistant bait boxes or stations for a permanent baiting program for Norway rats and house mice. They may not be effective on roof rats, however, because of their usual placement. Bait stations are sometimes difficult to place for roof rat control because of the rodents’ overhead traveling characteristics. Anticoagulant paraffin-type bait blocks provide an alternative to bait stations containing pelleted or loose cereal bait. Bait blocks are easy to place in small areas and difficult-to-reach locations out of the way of children, pets, and nontarget species. Where label instructions permit, small blocks can be placed or fastened on rafters, ledges, or even attached to tree limbs, where they are readily accessible to the arboreal rats.
Some of the first-generation anticoagulants (warfarin and diphacinone) are available as soluble rodenticides from which water baits can be prepared. Liquid baits may be an effective alternative in situations where normal baits are not readily accepted, especially where water is scarce or where rats must travel some distance to reach water.
In controlling roof rats with rodenticides, a sharp distinction must be made between control in and around buildings and control away from buildings such as in landfills and dumps, along drainage ditches and streams, in sewer water evaporation ponds, and in parks. Control of roof rat damage in agriculture represents yet another scenario. Distinctions must be made as to which rodenticide (registered product) to use, the method of application or placement, and the amount of bait to apply. For example, only zinc phosphide can be applied on the ground to control rats in sugarcane or macadamia orchards, and the second-generation anticoagulants, cholecalciferol and bromethalin, can be used only in and around buildings, not around crops or away from buildings even in noncrop situations. Selection of rodenticides and bait products must be done according to label instructions. Labels will specify where and under what conditions the bait can be used. Specifications may vary depending on bait manufacturer even though the active ingredient may be the same. The product label is the law and dictates the product’s location of use and use patterns.
Tracking powders. Tracking powders play an important role in structural rodent control. They are particularly useful for house mouse control in situations where other methods seem less appropriate. Certain first-generation anticoagulants are registered as tracking powders for roof rat control; however, none of the second generation materials are so registered. Their use for roof rats is limited to control within structures because roof rats rarely produce burrows.
Tracking powders are used much less often for roof rats than for Norway rats because roof rats frequent over-head areas within buildings. It is difficult to find suitable places to lay the tracking powder that will not create a potential problem of contaminating food or materials below the placement sites.
Tracking powders can be placed in voids behind walls, near points of entry, and in well-defined trails. Tunnel boxes or bait boxes specially designed to expose a layer of toxic powder will reduce potential contamination problems and may actually increase effectiveness. Some type of clean food can be used to entice the rats to the boxes, or the tracking powders can be used in conjunction with an anticoagulant bait, with both placed in the same station.
Since roof rats rarely dig burrows, burrow fumigants are of limited use; however, if they have constructed burrows, then fumigants that are effective on Norway rats, including aluminum phosphide and gas cartridges, will be effective on roof rats. Where an entire warehouse may be fumigated for insect control all rats and mice that are present will be killed. The fumigation of structures, truck trailers, or rail cars should only be done by a licensed pest control operator who is trained in fumigation techniques. Rodent-infested pallets of goods can be tarped and fumigated on an individual or collective basis.
Trapping is an effective alternative to pesticides and recommended in some situations. It is recommended for use in homes because, unlike with poison baits, there is no risk of a rat dying in an inaccessible place and creating an odor problem.
The common wooden snap traps that are effective for Norway rats are effective for roof rats. Raisins, prunes, peanut butter, nutmeats, and gumdrops make good baits and are often better than meat or cat food baits. The commercially available, expanded plastic treadle traps, such as the Victor® Professional Rat Trap, are particularly effective if properly located in well-traveled paths. They need not be baited. Place traps where they will intercept rats on their way to food, such as on overhead beams, pipes, ledges, or sills frequently used as travel routes (Fig. 3).
Some traps should be placed on the floor, but more should be placed above floor level (for example, on top of stacked commodities). In homes, the attic and garage rafters close to the infestation are the best trapping sites.
Pocket gopher box-type traps (such as the DK-2 Gopher Getter) can be modified to catch rats by reversing the action of the trigger. Presently, only one such modified trap (Critter Control’s® Custom Squirrel & Rat Trap) is commercially available. These kill traps are often baited with whole nuts and are most useful in trapping rats in trees. Their design makes them more rat-specific when used out-of-doors than ordinary snap traps that sometimes take birds. Caution should be taken to avoid trapping nontarget species such as tree squirrels.
Wiremesh, live traps (Tomahawk®, Havahart®) are available for trapping rats. Rats that are captured should be humanely destroyed and not released elsewhere because of their role in disease transmission, damage potential, and detrimental effect on native wildlife.
Glue boards will catch roof rats, but, like traps, they must be located on beams, rafters, and along other travel routes, making them more difficult to place effectively for roof rats than for Norway rats or house mice. In general, glue boards are more effective for house mice than for either of the rat species.
Where legal and not hazardous, shooting of roof rats is effective at dusk as they travel along utility lines. Air rifles, pellet guns, and .22-caliber rifles loaded with bird shot are most often used. Shooting is rarely effective by itself and should be done in conjunction with trapping or baiting programs.
In urban settings, cats and owls prey on roof rats but have little if any effect on well-established populations. In some situations in which the rats have been eliminated, cats that are good hunters may prevent reinfestation.
In agricultural settings, Weasels, Foxes, Coyotes, and other predators prey on roof rats, but their take is inconsequential as a population control factor. Because roof rats are fast and agile, they are not easy prey for mammalian or avian predators.
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).
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®). Difethialone (Generation®). Diphacinone (Ramik®, Ditrac®). 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.
Box-type kill traps.
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.