Overview of Damage Prevention and Control Methods
- Good sanitation – reduce sources of food, water, and shelter
- Remove debris and control weeds around structures
- Seal all openings larger than ½ inch wide
- Store food in rodent-proof containers
- Properly store and dispose of refuse and garbage
- Ultrasonic devices do not control rats
- Ro-Pel®; repellents generally are not effective for control of rats
- Anticoagulant rodenticides
- Non-anticoagulant rodenticides
- Aluminum phosphide (Phostoxin® and others)
- Rifles- .177- and .22-caliber
- Shotguns. 12- or 20-gauge
- Snap traps
- Cage and box traps
Other Control Methods
- Some cats and dogs catch individual rats, but they cannot be depended upon for control of Norway rats.
Damage Prevention and Control Methods
Rats should be controlled as soon as they are noticed. Accurate data on damage by rats and associated costs of rat control is difficult to obtain. Estimates of losses of food, structural damage, and the amount of labor and materials expended to control rats usually are only educated guesses.
One rat eats approximately 20 to 40 pounds of feed per year and probably contaminates 10 times that amount with urine and droppings. In a year, a single rat will produce about 25,000 droppings. One estimate from Cornell University puts the cost of damage by rats to grain crops in the US at $19 billion annually.
Sanitation plays an important role in controlling populations of rats. Poor sanitation (Figure 7) contributes to moderate to high populations of rats in urban and suburban areas. Sanitation will not eliminate rats in every situation but it may prevent them from flourishing. Sanitation includes proper storage and handling of food, feed, and garbage. Warehouses, granaries, mills for grain, silos, ports, and similar structures often provide excellent habitat for rats.
Stack foods that are in sacks or boxes in rows on pallets in a way that allows thorough inspection for evidence of rats. Keep stored materials away from walls. A 12-inch, white band painted on the floor adjacent to the wall will aid in detecting droppings of rodents and other signs. Sweep floors frequently to permit detection of fresh signs. Pet food is often a source of food for rats in and around homes. For pets that are kept outdoors, provide only enough food and water to satisfy pets in a single meal.
Refuse should be collected regularly, before storage containers are filled to excess. Open dumps often are infested by Norway rats. At a properly operated sanitary landfill, garbage and rubbish are compacted and covered with earth daily. Modern incinerators completely burn refuse, and the resulting residue does not provide food for rats.
Sewers are inhabited by Norway rats in some towns and cities. Rats enter sewers at outlets and through manholes, catch basins, broken pipes, and drains. Traps for water do not impede movements of Norway rats. Rats in sewers usually are the biggest problems in places where sanitary sewers are interconnected with storm sewers, thus providing multiple entry points for rats. The domestic sewage from an average community provides enough food to sustain a large number of rats, a problem that has increased as a result of the recent prevalence of garbage disposals in new homes. Rats are excellent swimmers, capable of staying afloat up to 72 hours.
Regular removal of debris (including stacked lumber, firewood, and other materials) from around structures will reduce shelter available to rats. In some instances, a strip of heavy gravel placed adjacent to building foundations or other structures will reduce burrowing by rats. Gravel should be at least 1 inch in diameter and lay in a band at least 2 feet wide and 6 inches deep. Keep grass mowed to prevent grass from seeding, which provides a potential source of food, and to allow easier detection of rodents.
Physical barriers can prevent rats from gaining entry to structures. “Rat-proofing” is an important, and often neglected, aspect of the control of rats. Exclusion is relatively permanent and prevents damage. Plug holes with paper to determine which holes are active prior to securing them. To exclude rats, seal all holes and openings larger than ½ inch across. Rodent-proofing should be done with heavy materials that resist gnawing, including concrete mortar, 24-gauge galvanized sheet metal, and 18-gauge hardware cloth.
Store all pet food and bulk foods in rodent-proof containers or sealed rooms. A proper container should be heavy-duty, resistant to rust, rats, and damage, and equipped with a tight-fitting lid. Galvanized steel containers in good condition provide greater resistance to entry by rats than vinyl or plastic.
Garbage and rubbish from homes, restaurants, and farms should be stored properly and disposed of regularly. Racks or stands prevent corrosion or rusting of containers, reduce shelter for rats under containers, and minimize the chance of containers being overturned (Figure 8). Containers for bulk storage of refuse, such as those used at apartment buildings, businesses, and housing projects, should be rodent-proof.
Large, metal containers for refuse (dumpsters) sometimes have holes for drainage to facilitate cleaning. Holes for drainage should be fitted with a ¼-inch wire mesh or a removable plug. Otherwise, the container becomes a station for feeding rodents.
Rats are wary and are frightened easily by unfamiliar sounds that come from new locations. Unusually loud, novel sounds will frighten rats and may cause temporary avoidance that lasts from a few minutes to a few weeks. However, rats quickly can become accustomed to new sounds.
Ultrasonic devices are available that are claimed to control rodents. Little evidence is available, however, to suggest that rats respond to nonspecific, high-frequency sound differently than to sound within the range that humans can hear. Ultrasound is directional, does not travel well around corners, and creates sound-shadows or voids. Ultrasound rapidly loses intensity as it leaves the source. Tests of commercial ultrasonic devices have indicated that rodents may be repelled from the immediate area for a few days or longer. Other tests have shown the degree of repellency depends upon the particular frequencies used, the intensity, and pre-existing conditions. Ultrasound will not evict rats that are established in an area. Commercial ultrasonic devices are not recommended as a solution to infestations of rodents. Ultrasonic sound has very limited usefulness in control of rodents.
Rats find some types of tastes and odors objectionable, but chemical repellents are seldom practical for controlling infestations of rats. Other solutions, including rodent-proof construction and reduction of populations, are usually more permanent and cost-effective. Ro-Pel® is registered for use in repelling Norway rats from gnawing on trees, poles, fences, shrubs, garbage, and other objects. Little information currently is available on its effectiveness against rats.
The Environmental Protection Agency (EPA) has taken steps to limit access to rodenticides based on active ingredients and types of application. General Use Pesticides (GUP) are toxicants that homeowners can purchase over-the-counter without a license for use on their own property. Restricted Use Pesticides (RUP) are toxicants that require a license or certificate to purchase and apply any of those toxicants on someone else’s property. Always check the pesticide label and most recent regulations to ensure you are in compliance. Rodenticides are classified as anti-coagulant and non-anticoagulant.
Anticoagulant (slow-acting, chronic toxicants) rodenticides have been the preferred materials for controlling rats since their development following World War II. They are acceptable to rats, do not cause bait-shyness, are easy to apply, and if used properly, present a low risk to livestock, pets, and humans.
Rats that have been poisoned with anticoagulants die from internal bleeding, as the blood loses the ability to clot and capillaries are damaged. An animal killed by anticoagulants may show extreme lack in color of the skin, muscles, and viscera. Hemorrhages may be found in any part of the body. Prior to death, the animal becomes increasingly weak due to loss of blood.
Anticoagulants are classified as first-generation and second-generation. First-generation anticoagulants (warfarin, chlorophacinone and diphacinone) require multiple feedings over a period of several days before death is achieved. Relatively low, chronic doses are fatal, whereas the same amount of toxicant ingested at a single feeding may produce no significant effect on rodents. The effect may vary for different anticoagulants. Treatment does not have to be on consecutive days, though when anticoagulants are eaten daily, death may occur by the third or fourth day. For optimal, lethal effect, several feedings should occur within a 10-day period with no longer than 48 hours between feedings.
Warfarin was the first marketed anticoagulant and has become the best known and most widely used. It is effective against Norway rats.
Chlorophacinone and diphacinone are more toxic, and are formulated at lower concentrations. Chlorophacinone and diphacinone may kill some rats in a single feeding, but multiple feedings are needed to give adequate control of an entire population.
Second-generation anticoagulants (brodifacoum, bromadiolone, difethialone, and difenacoum) were developed in the 1970’s. They may be more effective than first-generation anticoagulants because of their potential to be lethal in a single feeding.
All anticoagulant baits are used in similar fashion because they have similar modes of action. Anticoagulant rodenticides are slow-acting, so subsequent illness is not associated with the bait even if a sub-lethal dose is consumed. Bait-shyness usually does not occur so prebaiting is not needed. Directions on the label commonly instruct the user to “maintain a continuous supply of bait for 10 days or until feeding ceases,” thus ensuring the entire population of rats has ample opportunity to ingest a lethal dose of the bait.
Anticoagulants have the same effect on nearly all warm-blooded animals, but the sensitivity to the toxicants varies among species. In general, the hazard of secondary poisoning from anticoagulants is relatively low. If misused, anticoagulant rodenticides can be lethal to non-target animals such as dogs, pigs, and cats. Residues of anticoagulants, particularly second-generation anticoagulants, present in the bodies of dead or dying rodents can be toxic to scavengers and predators. Concern regarding chronic and sub-lethal effects from secondary poisoning, as well as bioaccumulation of second generation toxicants, is growing. Fortunately, vitamin K is an antidote for all anticoagulant intoxication.
Resistance to anticoagulants is a concern. Within any population of Norway rats, some individuals are less sensitive to anticoagulants than others. Where anticoagulants have been used over long periods at a particular location, there is a potential for a population to become somewhat resistant to the lethal effects of the toxicant. Populations that are resistant have been identified at several locations in the US.
Before concluding that a population of rats may be resistant to an anticoagulant, consider that failure to achieve control, even when baits are highly accepted, may be due to:
- too short of a period of exposure to bait,
- insufficient amount of bait,
- insufficient replenishment of bait,
- too few bait stations and too far apart (stations should be within 20 to 30 feet of each other in areas where rats are active),
- small area of control that permits rats to move in from adjacent areas, and
- genetic resistance to the anticoagulant; although unlikely, it should be suspected if the same amount of bait is taken on a daily basis for several weeks.
If you conclude that resistance is likely, change to another active ingredient or apply a non-anticoagulant toxicant.
Failure to achieve control with anticoagulant baits that are poorly accepted may be due to:
- poor choice of bait or bait that is improperly formulated,
- improperly placed bait stations,
- abundance of alternative foods, and
- bait that is moldy, rancid, infested with insects, or contaminated with other material that reduces acceptance.
Periodically remove old bait, dispose of it according to the instructions on the label, and replace it with fresh bait.
Older rodenticides (e.g., ANTU, arsenic trioxide, phosphorus, and Compound 1080), formerly referred to as acute toxicants, no longer are registered for control of rats. The widespread availability of ready-to-use anticoagulants and their relative effectiveness have resulted in less need for the older materials over the last 40 years. Non-anticoagulant rodenticides, particularly zinc phosphide, are useful in achieving quick reductions in populations of rats. When large numbers of rats are present, the cost of baiting with these materials may be lower than for anticoagulants. Three non-anticoagulant rodenticides are registered by the EPA for control of Norway rats: bromethalin, cholecalciferol, and zinc phosphide. All are useful for controlling anticoagulant-resistant populations of rats. Always follow the directions on the label for all pesticide products.
Bromethalin and cholecalciferol are formulated to serve as chronic rodenticides. They are applied so that rats have the opportunity to feed on the baits 1 or more times over 1 to several days. Acceptance of bait generally is good. Bromethalin is formulated in ready-to-use bait. Bromethalin is slow-acting, so bait-shyness usually is not a problem, and prebaiting (offering similar, but nontoxic, bait prior to applying the toxicant-treated bait) is not necessary for good control in most situations. Cholecalciferol is formulated similarly. Death occurs 3 or 4 days after ingestion of a lethal dose. Cholecalciferol also is slow-acting, which eliminates bait-shyness. Rodents typically cease feeding once a lethal dose has been ingested.
Zinc phosphide is a dark-gray, water-insoluble powder that has been used extensively for controlling rodents. It has a garlic-like odor that appears to be attractive to rodents that are not bait-shy. Zinc phosphide is a fast-acting toxicant, causing death in as little as 17 minutes. This rapid mortality can lead to bait-shyness in surviving rats if zinc phosphide is used for more than a few days. Prebaiting is recommended to increase acceptance.
The following tips are recommended to obtain good bait acceptance, and therefore good control of Norway rats, when using zinc phosphide:
- prebait for a minimum of 3 to 5 days to accustom rats to the nontoxic bait;
- do not change the type of bait during prebaiting or baiting;
- apply prebait at all locations where rats are active,
- if the bait is eaten completely overnight, double the amount at that location the next day, and;
- repeat this procedure until the amount of bait eaten every night no longer increases.
Wait until consumption of prebait has peaked before applying toxic bait. Remove uneaten prebait and replace it with toxic bait at the same locations. Usually, the amount of toxic bait needed will be about half the amount used on the last day that prebait was applied. It may be helpful to wait 1 day between the last application of prebait and application of toxic bait to increase the intensity of hunger in the rats and therefore the motivation to eat.
Safe Use of Toxicants
Avoid handling rodenticides with bare hands; use latex, vinyl, or other chemical-resistant gloves. Clean tools or containers used for dispersing bait, or safely dispose of them along with the packaging materials. Always consider the threat to non-targets when applying baits. Confine or restrain pets and livestock that could feed on the bait. Use tamper-resistant bait stations to house baits.
Follow the application of toxic bait by picking up and disposing of available dead rats and bait that goes uneaten by incineration or deep burial. Normally, the greatest consumption occurs on the first night. Control remaining rats by using anticoagulant baits, traps, or fumigants in burrows.
Bait Selection and Formulation
Use only high-quality grains and fresh ready-to-use baits. Rats prefer fresh, high-quality foods and will reject spoiled or inferior food items when given a choice. Anticoagulant baits commonly are formulated into wax and extruded blocks. Blocks can be secured in bait stations and are resistant to spoilage in moist areas, such as sewers. Rats accept paraffin block baits less readily than loose or pelleted baits, but acceptance of extruded bait blocks is high.
People trained in control of rodents often prefer to mix their own baits. Ground cereal grains are often mixed with 5% powdered sugar and 3% to 10% vegetable oil. A concentrate of the toxicant is added to the mixture in the proper amount. Certain anticoagulants and zinc phosphide can be purchased in concentrate forms for use in bait formulation. Under some conditions, baits made with fruits, vegetables, meat, or fish may be highly accepted. However, use of such bait materials may increase the risk of poisoning non-target animals. Where rats have access to abundant amounts of grain, meat or canned, fish-flavored cat food may be a good substitute. Obtain a sufficient quantity to complete the project without changing brands or flavors. Corn, oats, wheat, and barley are grains preferred by Norway rats. Preference varies among populations of rats and individuals. Baits similar to foods rats are accustomed to eating often are a good choice, particularly if their normal foods are limited or can be made less available.
To determine bait preference in a population of rats, conduct a bait-choice test by placing 4 ounces of each of several non-toxic baits about 1 foot apart in locations where rats are present. Check baits for the next few days to determine which foods are preferred. Rats may not accept new bait until the third or fourth day. Grain-based baits in loose meal or pelleted form are available in bulk or packaged in small plastic, cellophane, or paper “place packs” (Figure 9).
The packets keep bait fresh and make it easy to place baits into burrows, walls, or other locations. Rats will gnaw into bags to feed on acceptable bait. Pelleted bait can more easily be carried by rats to other locations. Hoarding of food by rats may result in bait being moved to where it is undetected or difficult to recover.
Sodium salts of anticoagulants also are available as concentrates to be mixed with water, as liquid baits. Rats must consume water every day, and they can be drawn to water stations when other sources of water are scarce. Liquid bait is particularly useful in storage structures and warehouses. Rodents can detect anticoagulants in water bait more easily than in food bait, so up to 5% sugar may be added to liquid bait to increase acceptance of the solution. Water is attractive to most animals, so use water bait in ways that prevent non-target animals from accessing them, such as bait boxes.
Bait stations (Figures 10a and 10 b) or boxes can contain solid, liquid, or paste baits and traps. They increase the effectiveness and safety of rodenticides.
Best Practices for Bait stations:
- protect bait from moisture and dust;
- provide a protected place for rodents to feed;
- keep non-target animals and children away from hazardous bait;
- allow placement of bait in locations where it would otherwise be difficult because of weather or hazards to non-targets;
- help prevent the accidental spilling of bait; and
- allow easy inspection of bait to see if rodents are feeding on it.
Types of Bait Stations
Bait stations can be purchased from commercial suppliers or made at home. Manufactured bait boxes are designed to withstand different levels of abuse. The EPA has established 4 tiers of bait stations based on ability to protect bait against weather, pets, and children less than 6 years old (Table 1).
Table 1. Explanation of tiers for bait stations.
|Tier 1||Tamper-resistant for children and dogs; weather resistant; tested according to EPA protocols; indoor and outdoor use|
|Tier 2||Tamper-resistant for children and dogs; tested according to EPA protocols; indoor use only|
|Tier 3||Tamper-resistant for children; tested according to EPA protocols; indoor use only|
|Tier 4||Self-certification; packaging not anticipated to release other than small quantities of bait; resistant to opening by a child less than 6 years old; indoor use only; non-refillable.|
Use a bait station with the appropriate rating for your situation. It is advisable to increase protection of bait, as conditions may change. Secure bait boxes to buildings by nailing or gluing them to walls or floors so nothing can knock them over or shake out the bait.
Bait stations are completely enclosed and can contain liquid and solid bait. A hinged lid with a child-proof latch can be used for convenience in inspecting permanent stations. Bait stations for rats should have at least 2 openings, each about 2½ inches in diameter. The holes should be on opposite sides of the station so rats can see an escape route as they enter the station.
Provide enough fresh bait to allow rodents to eat all they want. When bait boxes initially are deployed, check them daily and add fresh bait as needed. After a short time, numbers of rodents and feeding will decline and you will only need to check the boxes every 2 to 4 weeks. If the bait becomes moldy, musty, soiled, or insect-infested, empty the box, and dispose of spoiled or uneaten bait in accordance with the label. Wisk or wipe stations clean with a dry or moist cloth. Avoid using chemicals, as they may dissuade rats from entering. Leave several droppings inside to signal to the remaining rats that the location is a suitable feeding site. Follow all directions on the label of the product being used.
Placement of bait stations is just as important as bait selection. Norway rats will not visit bait stations, regardless of their contents, if they are not conveniently located in areas where rats are active. Where possible, place the bait between shelter and food used by rats. Place bait boxes near burrows, against walls, or along travel routes. Rats frequently are suspicious of new or unfamiliar objects, so it may take several days for them to enter and feed in bait stations.
In confinement buildings for swine, it may be possible to attach bait boxes to ledges on walls or the top of walls that divide pens. Bait boxes may be placed in attics or along the floors and alleys where rodents are active. Tracks and droppings of rodents can be found on dusty surfaces and often give clues to where they are active.
Avoid placement of tamper-resistant bait stations where children or non-target animals may interact with them. Tamper-resistant stations are not tamper-proof. Where buildings are not rodent-proof, place permanent bait stations inside, along the outside of the foundation, and around the perimeter. Bait stations will help keep numbers of rodents at a low level when regularly maintained with fresh anti-coagulant bait. Rodents moving in from nearby areas will be controlled before they can reproduce and cause serious damage.
Tracking powders have been used successfully for many years to control rats. When rodents walk through a patch of toxic powder, they pick it up on their feet and fur, and later ingest it while grooming. Tracking powders are useful for controlling rats where food is plentiful and good acceptance of bait is difficult to achieve. Bait-shyness rarely occurs when using tracking powders.
The amount of material a rat may ingest while grooming is small, so the concentration of active ingredient in tracking powders is considerably higher than in food baits. Therefore, tracking powders are more hazardous. Tracking powders are available only to licensed pesticide applicators.
Place tracking powders only where they cannot contaminate food or animal feed, or where non-target animals cannot come into contact with them. Do not place tracking powders where rats can track the material onto food that is intended for humans or domestic animals. Tracking powders generally are not recommended for use in and around homes because of potential hazards to children and pets. Where possible, remove tracking powder after the program for control of rodents is completed.
Fumigants are most often used to control rats in burrows outdoors. Aluminum phosphide is registered as a burrow fumigant, although it is a highly restricted pesticide. Fumigation Management Plans must be established prior to use of aluminum phosphide. Do not use fumigants where the occupants of a building might be exposed to the fumes. Only licensed pest control operators may use fumigants. To fumigate burrows of rats, close the opening of the burrow with soil or sod immediately after introduction of the fumigant. Burrows of rats often have multiple entrances, and all openings must be sealed for fumigants to be effective. Fumigants are less effective in very porous or dry soils.
Rats can be shot with a pellet gun or .22-caliber firearm loaded with birdshot. Shotguns may be used, also. However, shooting is rarely effective as a method for control.
The use of traps can be effective for controlling Norway rats, but it requires more skill and labor than most other methods. Trapping is recommended where toxicants are inadvisable. It is the preferred method to try first in homes, garages, and other structures where few rats are present. The use of traps has several advantages:
- it does not rely on hazardous rodenticides,
- it permits users to view their success, and
- it allows for disposal of the rat carcasses, thereby eliminating odor problems from decomposing carcasses that may remain with toxicants.
Leave traps unset until the bait has been taken at least once to reduce the chance of rats escaping the trap and becoming trap-shy. Keep traps clean and in good condition. Plastic and metal traps can be cleaned with hot water and a stiff brush. Always wear proper personal protective equipment (PPE). Odors from humans or dead rats are not known to reduce success of trapping. Place traps across obvious runways or where runs are confined.
Several kinds of body-gripping traps are available for capturing and killing rats (Figures 11a and 11b). Research on the narrow-trigger versus the expanded-trigger snap-trap (Figure 11a) showed that expanded-trigger traps had fewer bait losses and almost twice the capture rate of the narrow-trigger version.
Several guidelines will improve your success:
- use plenty of traps;
- locate traps where rats are traveling, such as along walls;
- position traps with the trigger facing the wall so that rats will pass directly over the trigger;
- leave traps unset for several days to help rats become accustomed to them;
- refresh and replace bait as needed;
- set traps using a combination of baited and blind sets;
- use appropriate bait for traps (e.g., hot dog, bacon, pepperoni, or nutmeat tied securely to the trigger; peanut butter, marshmallows, and commercially available non-allergenic baits);
- avoid placing traps directly in front of holes to reduce refusals; and
- place traps inside bait stations, under slanted boards, or in difficult-to-access areas to reduce likelihood of non-target capture.
Wire-mesh cage traps such as the National®, Tomahawk®, and Havahart® can effectively be used to live-capture rats. Wire funnel-entrance traps also have been used to live-capture rats.
Glueboards catch and hold rats that attempt to cross them, in the same way fly-paper catches flies. They are less effective for capturing rats than mice, as rats frequently extricate themselves from the glue. Place glueboards along walls or wherever rats travel. Do not use glueboards where non-targets may be caught. Glueboards lose effectiveness in areas that are dusty, unless they are covered. Temperature extremes may affect the tackiness of some glue. Some glueboards are formulated specifically for use in cold conditions.
Dead rats, particularly when toxicants are used, should be disposed of to prevent scavengers from feeding on the carcasses.
Norway rats are invasive to the US, so relocation is not recommended.
Norway rats are invasive in the US, so translocation is not recommended.
Euthanize rats by asphyxiation with carbon dioxide or use a baton to apply a sharp blow to the base of the skull.
Check your state regulations regarding disposal of carcasses.
Other Control Methods
It is common to find rats living in close association with cats and dogs, relying on pet food for nourishment. Rats often live beneath dog houses and feed when dogs are absent or asleep. Some dogs and cats catch and kill rats. However, pets rarely control large populations of rats. Around most structures, rats can find many places to hide and rear their young out of the reach of predators. Cats cannot eliminate large existing populations of rats, but in some situations they may be able to prevent re-infestation after rats have been controlled. Cats increase the risk of exposure to toxoplasmosis and the impact of free-ranging cats on native wildlife is a concern.