B-105

NORWAY RATS

Damage Prevention andControl Methods

Exclusion

Seal all openings larger than 1/2 inch(1.3 cm) wide.

Habitat Modification

Good sanitation practices reducesources of food, water, and shelter.

Store foodstuffs in rodent-proofstructures or containers.

Store and dispose of refuse andgarbage properly.

Control weeds and remove debrisfrom around structures.

Frightening

Ultrasonic devices have not beenproven to control rats.

Robert M. TimmSuperintendent and Extension

Wildlife SpecialistHopland Research and

Extension CenterUniversity of CaliforniaHopland, California 95449

Fig. 1. Norway rat, Rattus norvegicus

Repellents

Ro-pel®.

Toxicants

Anticoagulant rodenticides (slow-acting chronic-type toxicants)Brodifacoum (Talon®).Bromadiolone (Maki®, Contrac®).Chlorophacinone (RoZol®).Diphacinone (Ramik®, Ditrac®).Pindone (Pival®, Pivalyn®)Warfarin (Final® and others).

Toxicants other than anticoagulants(may be acute or chronic toxicants)Bromethalin (Assault®, Vengeance®).Cholecalciferol (Quintox®).Red Squill.Zinc phosphide (Ridall Zinc®, ZP®

rodent bait).

Fumigants

In some situations, outdoor burrowfumigation may be effective.Aluminum phosphide (Phostoxin®and others).Chloropicrin.Gas cartridges.Methyl bromide.

Trapping

Snap traps.

Live traps.

Glue boards.

Other Methods

Clubbing.

Shooting.

Dogs and cats are of limited value insome situations.

PREVENTION AND CONTROL OF WILDLIFE DAMAGE — 1994

Cooperative Extension DivisionInstitute of Agriculture and Natural ResourcesUniversity of Nebraska - Lincoln

United States Department of AgricultureAnimal and Plant Health Inspection ServiceAnimal Damage Control

Great Plains Agricultural CouncilWildlife Committee

B-10

Field Identification of Domestic Rodents

Tail Body Ear Eye Nose

Roof Rat Rattus rattus

Norway Rat Rattus norvegicus

Young Rat

House MouseMus musculus

HeadFeet

Longer thanhead + body

Shorter thanhead + body

LightSlender

HeavyThick

Large

SmallSmall

Large

PointedLarge Large

SmallSmall

Fig. 2. Characteristics of commensal rodents

Blunt

Identification

The Norway rat (Rattus norvegicus, Fig.1) is a stocky burrowing rodent, unin-tentionally introduced into NorthAmerica by settlers who arrived onships from Europe. Also called thebrown rat, house rat, barn rat, sewerrat, gray rat, or wharf rat, it is aslightly larger animal than the roof rat(Fig. 2). Adult Norway rats weigh anaverage of 1 pound (454 g). Their fur iscoarse and usually brownish or red-dish gray above and whitish gray onthe belly. Blackish individuals occur insome locations.

Range

First introduced into the United Statesaround 1775, the Norway rat has nowspread throughout the contiguous 48states. It is generally found at lowerelevations but may occur whereverhumans live.

6

Habitat

Norway rats live in close associationwith people. In urban or suburbanareas they live in and around resi-dences, in cellars, warehouses, stores,slaughterhouses, docks, and in sewers.On farms they may inhabit barns, gra-naries, livestock buildings, silos, andkennels.

They may burrow to make nests underbuildings and other structures,beneath concrete slabs, along streambanks, around ponds, in garbagedumps, and at other locations wheresuitable food, water, and shelter arepresent. Although they can climb, Nor-way rats tend to inhabit the lowerfloors of multistory buildings.

Food Habits

Norway rats will eat nearly any typeof food. When given a choice, theyselect a nutritionally balanced diet,choosing fresh, wholesome items overstale or contaminated foods. They pre-fer cereal grains, meats and fish, nuts,and some types of fruit. Rats require

1/2 to 1 ounce (15 to 30 ml) of waterdaily when feeding on dry foods butneed less when moist foods are avail-able. Food items in household garbageoffer a fairly balanced diet and alsosatisfy their moisture needs.

General Biology,Reproduction, andBehavior

Norway rats are primarily nocturnal.They usually become active aboutdusk, when they begin to seek foodand water. Some individuals may beactive during daylight hours when ratpopulations are high.

Rats have poor eyesight, relying moreon their hearing and their excellentsenses of smell, taste, and touch. Theyare considered color-blind. Therefore,for safety reasons, baits can be dyeddistinctive colors without causingavoidance by rats, as long as the dyedoes not have an objectionable taste orodor.

Rats use their keen sense of smell tolocate food items and to recognize

Escape hole lightlycovered with dirt

Escape hole

Nest

Entrance

12"

Fig. 3. Norway rat burrow system beneath a pile of boards.

other rats. Their sense of taste is excel-lent, and they can detect some con-taminants in their food at levels as lowas 0.5 parts per million.

Norway rats usually construct nests inbelow-ground burrows or at groundlevel (Fig. 3). Nests may be lined withshredded paper, cloth, or other fibrousmaterial. Litters of 6 to 12 young areborn 21 to 23 days after conception.Newborn rats are hairless and theireyes are closed, but they grow rapidly.They can eat solid food at 2 1/2 to 3weeks. They become completely inde-pendent at about 3 to 4 weeks andreach reproductive maturity at 3months of age.

Females may come into heat every 4 or5 days, and they may mate within aday or two after a litter is born. Breed-ing often peaks in spring and fall, withreproductive activity declining duringthe heat of summer and often stoppingcompletely in winter, depending onhabitat. These seasonal trends are mostpronounced in more severe climates.The average female rat has 4 to 6 littersper year and may successfully wean 20or more offspring annually.

Norway rats have physical capabilitiesthat enable them to gain entry to struc-tures by gnawing, climbing, jumping,swimming, and other tactics. For moredetailed information on their physicalabilities and the resulting need todesign rodent-proof structures, see thechapter Rodent-Proof Constructionand Exclusion Methods.

Studies indicate that during its dailyactivities, a rat normally travels anarea averaging 100 to 150 feet (30 to 45m) in diameter. Rats seldom travel far-ther than 300 feet (100 m) from theirburrows to obtain food or water.

Rats constantly explore and learnabout their environment, memorizingthe locations of pathways, obstacles,food and water, shelter, and other ele-ments in their domain. They quicklydetect and tend to avoid new objectsplaced into a familiar environment.Thus, objects such as traps and baitstations often are avoided for severaldays or more following their initialplacement. Place baits and bait stations

near, but not on, rat runways. Rats willquickly find them and after a shortperiod of avoidance, will cautiouslyinvestigate them. Baited but unsettraps will aid in overcoming rats’ fearof them; expanded-trigger traps setdirectly on travel routes may immedi-ately catch rats.

Rats will at first avoid novel fooditems placed in their environment.They may eat very small amounts, andsubsequent feeding will depend on theflavor of the food and its physiologicaleffect. If the food contains poison orsome other substance that soon pro-duces an ill effect but not death, thefood will often be associated with theillness. This “bait shyness” was a ma-jor problems when single-dose acutetoxicants were the main rodenticidesin use. Today, only two rodenticidesregistered for Norway rat control, redsquill and zinc phosphide, possesscharacteristics that make bait shyness apotential problem.

Bait shyness can persist for weeks ormonths and may be transferred tonontoxic foods of similar types. Pre-baiting, that is, training rats to feedrepeatedly on nontoxic bait for aperiod of days prior to applying thetoxicant in the bait, will largely preventsublethal doses and thus bait shyness.It will also lead to successful control,with very few rats left to become bait-shy. Prebaiting will almost alwaysincrease control success when zincphosphide or red squill baits are used.

Because anticoagulant rodenticides areslow-acting, the rats’ subsequent ill-ness is not associated with the baiteven if a sublethal dose is consumed;thus, bait shyness does not usually

occur. These baits serve, in effect, astheir own prebait.

Damage and DamageIdentification

Norway rats consume and contami-nate foodstuffs and animal feed. Theymay damage crops in fields prior toand during harvest, and during pro-cessing and storage. Rats also damagecontainers and packaging materials inwhich foods and feed are stored.

Rats cause structural damage to build-ings by burrowing and gnawing. Theyundermine building foundations andslabs, cause settling in roads and rail-road track beds, and damage thebanks of irrigation canals and levees.Rats also may gnaw on electrical wiresor water pipes, either in structures orbelow ground. They damage struc-tures further by gnawing openingsthrough doors, window sills, walls,ceilings, and floors. Considerable dam-age to insulated structures can occuras a result of rat burrowing and nest-ing in walls and attics.

Among the diseases rats may transmitto humans or livestock are murinetyphus, leptospirosis, trichinosis,salmonellosis (food poisoning), andratbite fever. Plague is a disease thatcan be carried by a variety of rodents,but it is more commonly associatedwith roof rats (Rattus rattus) than withNorway rats.

Rat Sign

The presence of rats can be determinedby a number of signs described below:

B-107

B-1

Fig. 4. Tracks left in dust by (a) Norway rat and (b) house mouse.

Fig. 5. Rub marks along beams, rafters, or other travel routes are evidence of rat activity.

Droppings may be found along run-ways, in feeding areas, and near shel-ter. They may be as large as 3/4 inch(2 cm) long and 1/4 inch (0.6 cm) indiameter. Fresh droppings are soft intexture.

Tracks, including footprints or tailmarks, may be seen on dusty surfacesor in mud (Fig. 4). A tracking patchmade of flour can be placed in path-ways overnight to determine if rodentsare present.

Urine, both wet and dry, will fluo-resce under ultraviolet light. Urinestains may occur along travelways orin feeding areas.

Runs or burrows may be found nextto walls, along fences, next to build-ings, or under bushes and debris. Ratsmemorize pathways and use the sameroutes habitually.

Smudge marks (rub marks) mayoccur on beams, rafters, pipes, andwalls as a result of oil and dirt rubbingoff rats’ fur along frequently traveledroutes (Fig. 5).

Gnawing may be visible on doors,ledges, in corners, in wall material, onstored materials, or other surfaceswherever rats are present. Fresh accu-mulations of wood shavings, insula-tion, and other gnawed materialindicate active infestations. Size ofentry holes (often 1 1/2 inches [4 cm]in diameter or less for mice, 2 inches[5 cm] or larger for rats) or toothmarks can be used to distinguish ratfrom mouse gnawing. Rats keep theirpaired incisor teeth, which grow con-tinuously at the rate of about 5 inches(13 cm) per year, worn down by gnaw-ing on hard surfaces and by workingthem against each other.

Sounds such as gnawing, climbing inwalls, clawing, various squeaks, andfighting noises are common where ratsare present, particularly at times of theday when they are most active.

Estimating Rat Numbers

Rat sign and visual sightings are oflimited value in accurately estimatingrat numbers, but they are the simplestand often the only practical method

08

available. Search premises thoroughlywhen looking for rats. In structures,searches should include attics, base-ments, around foundations, crawlspaces, and behind and under storedmaterials. The following estimates canthen be made:

No sign: no rats or few present. Ifonly a few rats are present they mayhave invaded only recently.

Old droppings and gnawing com-mon, one or more rats seen by flash-light at night, or no rats observed indaytime: medium numbers present.

Fresh droppings, tracks, and gnaw-ing present, three or more rats seenat night, or rats seen in daytime:large numbers present.

Since rats are normally nocturnal andsomewhat wary of humans, usuallymany more rats are present than willbe seen in the daytime. Under certainconditions, rats may become quitebold in the presence of humans, andthen a high percentage of the popula-tion may be visible.

A conservative estimate of rat num-bers can be made from measuringtheir food consumption. You can dothis by feeding the rats for a while onfinely ground grain (whole grains orpelleted foods may be carried offuneaten). When offered over a periodof time, the ground grain will usuallybe accepted and eaten by rats. Con-sumption may gradually increase to amaximum level over the period of a

Fig. 6. Where repeated use of toxicants did not give lasting control, the use of sanitation (reduction ofrats’ food and shelter) assisted in reducing rat numbers.

ToxicantToxicant

Sanitation

Norway Rat Population Subjected toToxicant and Finally to Sanitation

150

125

100

75

50

25

0

ToxicantToxicant Toxicant

1943 1944 1945 1946 1947 1948 1949 1950

week or so as the rats’ natural fear ofnovel foods is overcome. Divide thetotal amount of food eaten per day by1/2 ounce (15 g); this will give a mini-mum estimate of the rats present.Some rats eat more than 1/2 ounce (15g) daily, but rats will probably also beusing other foods in their environ-ment. If too much alternative food isavailable, this technique will not givean adequate estimate.

Legal Status

Norway rats are not protected by law.They may be controlled with any pesti-cide registered by federal or state au-thorities for this purpose, or they maybe controlled by use of mechanicalmethods such as traps.

Damage Prevention andControl Methods

Exclusion

Physical barriers can prevent rats fromgaining entry to structures wherefood and shelter are available. “Rat-proofing” is an important and oftenneglected aspect of rat control. It is arelatively permanent form of rodentcontrol that prevents damage fromoccurring.

To exclude rats, seal all holes andopenings larger than 1/2 inch (1.3 cm)across. Rodent-proofing should bedone with heavy materials that willresist rodent gnawing. These includeconcrete mortar, galvanized sheetmetal, and heavy-gauge hardwarecloth.

For more detailed information onrodent-proofing techniques, see thechapter Rodent-Proof Constructionand Exclusion Methods.

Habitat Modification

In addition to the above-mentionedtechniques of excluding rodents fromsources of food and shelter, sanitationcan play an important role in control-ling rat populations (Fig. 6). Poor sani-tation is one of the basic reasons forthe continued existence of moderate tohigh rat populations in urban and sub-urban areas. In agricultural environ-ments, proper sanitation cannotalways eliminate rat populations, but itcan often prevent rats from flourishingin large numbers.

Sanitation involves good housekeep-ing, including proper storage and han-dling of food materials, feed, andedible garbage. Warehouses, granariesand grain mills, silos, port facilities,and similar structures may provideexcellent habitat for rats. Store bulk

foods in rodent-proof containers orrooms. Stack sacked or boxed foods inorderly rows on pallets in a way thatallows thorough inspection for evi-dence of rats. In such storage areas,keep stored materials away fromwalls. A 12-inch (30-cm) white bandpainted on the floor adjacent to thewall will aid in detecting rodent drop-pings and other rat sign (Fig. 7). Sweepfloors frequently to permit ready de-tection of fresh sign.

Pet foods often are a source of food forrats in and around homes. Keep allsuch materials stored in metal rodent-proof containers. Feed pets only whatthey will eat at a single time.

Garbage and rubbish from homes, res-taurants, farms, and other suchsources should be properly stored andsubsequently removed for disposal. Aproper refuse storage container isheavy-duty, rust-resistant, rat- anddamage-resistant, and equipped with atight-fitting lid. Galvanized steel trashcontainers in good condition are betterthan those made of vinyl or plastic.Racks or stands prevent corrosion orrusting of containers, reduce rat shelterunder containers, and minimize thechance of containers being overturned(Fig. 8).

Bulk storage containers for refuse,such as those used at apartments, busi-nesses, and housing projects, shouldbe similarly rodent-proof. Large metalrefuse containers (dumpsters) some-times have drain holes to facilitatecleaning. These drain holes should befitted with a wire mesh screen or aremovable plug; otherwise, the con-tainer becomes a huge feeding stationfor rodents (Fig. 9).

Refuse should be collected regularlyand before refuse storage containersbecome filled to excess. Sanitary land-fills and incinerators seldom have con-ditions that will allow rat populationsto exist. On the other hand, openrefuse dumps are often infested byNorway rats. At a properly operatedsanitary landfill, garbage and rubbishare compacted and covered with earthdaily. Modern incinerators completelyburn refuse, and the resulting residuedoes not provide food for rats.

B-109

B-110

12" white band

Fig. 7. A 12-inch (30-cm) white painted band makes inspection for rodent sign easier and reminds personnel to practice good sanitation.

Fig. 8. Sturdy refuse containers on rackseliminate a source of food and shelter for rats.

Fig. 9. Large metal refuse bins often havedrainage holes (arrow). These should beplugged or screened to prevent rodent entry.

↑

Sewers are inhabited by Norway ratsin some towns and cities. Rats mayenter at outlets and through manholes,catch basins, broken pipes, or drains.Since Norway rats are excellent swim-mers, water traps do not impede theirmovement; in fact, they can travelupstream against a current. The prob-lem of rats in sewers is usually greatestin places where sanitary sewers are in-terconnected with storm sewers, thusproviding multiple entry points forrats. The domestic sewage of an aver-age community provides enough foodto sustain a large number of rats; thisproblem has increased as a result ofthe recent prevalence of garbage dis-posal units in most newer homes.

Regular removal of debris and controlof weeds from around structures willreduce the amount of shelter availableto rats. In some instances, a strip ofheavy gravel placed adjacent to build-ing foundations or other structureswill reduce rat burrowing at theselocations. Gravel should be at least 1inch (2.5 cm) in diameter and laid in aband at least 2 feet (0.6 m) wide and1/2 foot (15 cm) deep. In any event,keep the perimeter of buildings andother structures clean of weeds anddebris (including stacked lumber, fire-wood, and other stored materials) to

discourage rat activity and to alloweasier detection of rat sign.

Frightening

Rats are wary animals and can befrightened easily by unfamiliar soundsor sounds coming from new locations.Most rodents, however, can quicklybecome accustomed to new soundsheard repeatedly. For years, devicesthat produce ultrasonic sound that isclaimed to control rodents have comeand gone on the market. There is littleevidence to suggest that rodents’ re-sponses to nonspecific, high-frequencysound is any different from their re-sponse to sound within the range hu-man of hearing.

What is known about rodents andsound?

— Unusually loud, novel sounds,including ultrasonic sounds, whichrats can hear, will frighten them andmay cause temporary avoidance last-ing from a few minutes to weeks ormonths.

What is known about ultrasonicsound?

— It is very directional and does nottravel around corners well; thus,sound shadows or voids are created.

— Ultrasound does not travel very far.It loses its intensity rapidly as it leavesthe source.

— Ultrasound has not been shown todrive established rodents out of build-ings or areas, nor has it been proven tocause above-normal mortality in theirpopulations. While it is possible tocause convulsions or permanentphysiological damage to rodents withultrasound, the intensity of suchsounds must be so great that damageto humans or domestic animals wouldalso be likely. Commercial ultrasonicpest control devices do not producesound of such intensity.

Tests of commercial ultrasonic deviceshave indicated that rats may berepelled from the immediate area ofthe ultrasound for a few days, but thenwill return and resume normal activi-ties. Other tests have shown the degreeof repellency to depend upon the par-ticular ultrasonic frequencies used,their intensity, and the preexistingcondition of the rodent infestation.Ultrasonic sound has very limited use-fulness in rodent control. The advertis-ing claims for many commercialdevices are unsubstantiated by scien-tific research. Since commercial ultra-sonic devices are often expensive andof questionable effectiveness, they can-not be recommended as a solution torodent problems.

Table 1. Anticoagulants used for Norwa

Common name andtypical trade names Chemical name

Hydroxycoumarins

Warfarin (Final® and others) 3-(α-acetonylben

Brodifacoum (Talon®)* 3-[3(4'-bromo[1,11-naphthalenyl]-

Bromadiolone (Maki®, 3-[3-(4'-bromo[1Contrac®)* phenylpropyl]-4

Difethialone* [(bromo-4'-0[bipnapthyl-1] 3-hyd

Indandiones

Chlorophacinone (RoZol®) 2-[(p-chlorophen

Diphacinone (Ramik®,Ditrac®) 2-diphenylacety

Pindone (Pival®, Pivalyn®) 2-pivalyl-1,3-ind

*Second-generation anticoagulants especially usefu

Repellents

Rats find some types of tastes andodors objectionable, but chemicalrepellents are seldom a practical solu-tion to rat infestations. Substances suchas moth balls (naphthalene) or house-hold ammonia, in sufficient concentra-tion, may have at least temporaryeffects in keeping rats out of certainenclosed areas. The above materials,however, are not registered by theEPA as rat repellents.

Ro-pel® is registered for use in repel-ling Norway rats and other rodentsfrom gnawing on trees, poles, fences,shrubs, garbage, and other objects.Little information is currently availableon its effectiveness against rats.

Other solutions to rat problems,including rodent-proof constructionand methods of population reduction,are usually more permanent and cost-effective.

Toxicants

Rodenticides were formerly classifiedinto two groups, single-dose (acute)toxicants and multiple-dose (chronic)rodenticides. However, the complexityin mode of action of newer rodenti-cides makes these classifications out-dated. A classification into twogroups, the first including all anti-coagulants, and the second all other

y rat control in the United States.

Usual tFoodBait

zyl)-4-hydroxycoumarin X

’biphenyl]-4-yl)-1,2,3,4-tetrahydro-4-hydroxy-2H-1-benzopyran-2-one X

,1’biphenyl]-4-yl)-3-hydroxy-1--hydroxy-2H-1-benzopyran-2-one X

henyl-1-1'] yl-4) 3-tetrahydro-1,2,3,4- Xroxy-4, 2H-1-benzo-thiopyran-2-one

yl)phenylacetyl]-1,3-indandione X

l-1,3-indandione X

andione X

l for the control of warfarin-resistant rats and mice.

compounds (“non-anticoagulants”), iscurrently more useful.

Anticoagulants (slow-acting,chronic toxicants). The anticoagu-lant rodenticides have been the mostpreferred materials for controlling ratssince their initial development follow-ing World War II. They are quite ac-ceptable to rats, do not cause baitshyness, are easy to apply, and if usedproperly, are relatively safe to usearound livestock, pets, and humans.

Rats poisoned with anticoagulants diefrom internal bleeding, the result ofloss of the blood’s clotting ability anddamage to the capillaries. Animalskilled by anticoagulants may show ex-treme lack of color of the skin,muscles, and viscera. Hemorrhagemay be found in any part of the body.Prior to death, the animal exhibits in-creasing weakness due to blood loss.

Several anticoagulant compounds areregistered for controlling Norway rats(Table 1). With the exception of two(bromadiolone and brodifacoum),multiple feedings over a period of sev-eral days are usually required to causedeath. Relatively low, chronic dosesare fatal, whereas the same amount oftoxicant ingested at a single feedingmay produce no significant effect tothe rodent. However, this may varyfor different anticoagulants. Feeding

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ypes of formulations PercentTracking active ingredient

Liquid Powder used in food bait

X 0.025

0.005

0.005

0.0025

X 0.005

X 0.005

X 0.025

B-1

does not always have to be on con-secutive days. When anticoagulantsare eaten daily, however, death mayoccur as early as the third or fourthday. For optimal lethal effects, severalfeedings should occur within a 10-dayperiod with no longer than 48 hoursbetween feedings.

All anticoagulants provide good toexcellent Norway rat control whenprepared in acceptable baits. A newsecond-generation anticoagulant,difethialone, is presently beingdeveloped and EPA registration isanticipated in the near future. Thecharacteristics of the various anti-coagulant rodenticides are describedfurther in the Pesticides section.

Because of their similarity in mode ofaction, all anticoagulant baits are usedin a similar fashion. Label directionscommonly instruct the user to “main-tain a continuous supply of bait for 15days or until feeding ceases,” thusensuring that the entire rat populationhas ample opportunity to ingest alethal dose of the bait. Anticoagulantshave the same effect on nearly allwarm-blooded animals, but the sensi-tivity to these toxicants varies amongspecies. If misused, anticoagulantrodenticides can be lethal to nontargetanimals such as dogs, pigs, and cats.Additionally, residues of anticoagu-lants which are present in the bodies ofdead or dying rodents can cause toxiceffects to scavengers and predators. Ingeneral, however, the secondary poi-soning hazard from anticoagulants isrelatively low.

Brodifacoum and bromadiolone baits,because of their potential to be lethal ina single feeding, can be more effectivethan the other anticoagulants in certainsituations.

Chlorophacinone (RoZol®) anddiphacinone (Ramik®, Ditrac®) aresimilar in potency and are more toxicthan the anticoagulant compoundsdeveloped earlier. Thus, they are for-mulated at lower concentrations.Chlorophacinone and diphacinonemay kill some rats in a single feeding,but multiple feedings are needed togive adequate control of an entire ratpopulation.

12

Pindone (Pival®, Pivalyn®) is also lesspotent than chlorophacinone ordiphacinone, and it is regarded asslightly less effective than warfarinagainst Norway rats. It has some prop-erties that resist insects and growth ofmold in prepared baits.

Warfarin (Final® and other tradenames) was the first marketed antico-agulant and therefore became the bestknown and most widely used. It iseffective against Norway rats,although some products may containsmall quantities of contaminants thatapparently can reduce bait acceptance.This problem was resolved by thedevelopment of micro-encapsulatedwarfarin.

Anticoagulant Resistance. Withinany population of Norway rats, someindividuals are less sensitive to antico-agulants than others. Where antico-agulants have been used over longperiods of time at a particular location,there is an increased potential for theexistence of a population that is some-what resistant to the lethal effects ofthe baits. Such resistant populations ofrats have been identified at a numberof locations throughout the UnitedStates. Although not common, resis-tance may be underestimated becausedocumentation of resistance is usuallynot pursued by persons involved inoperational rat control programs.

Resistance, if and when it occurs, is oflittle consequence in the control ofNorway rats, especially with thenewer rodenticides presently available.When anticoagulant resistance to thefirst-generation anticoagulants isknown or suspected, use of these com-pounds should be avoided in favor ofthe second-generation anticoagulantsor one of the non-anticoagulantproducts.

Anticoagulant Bait Failure. Resis-tance is only one (and perhaps theleast likely) reason for failure in thecontrol of rats with anticoagulant baits.Control with baits that are highly ac-cepted may fail for one or more of thefollowing reasons:

— Too short a period of bait exposure.

— Insufficient bait and insufficientreplenishment of bait (none remainsfrom one baiting to the next).

— Too few bait stations and/or too farapart. In some situations, stationsmay have to be within 20 to 30 feet(7 to 10 m) of one another.

— Too small a control area, permittingrats to move in from untreated adja-cent areas.

— Genetic resistance to the anticoagu-lant. Although this is unlikely, itshould be suspected if about thesame amount of bait is taken dailyfor a number of weeks.

Control with anticoagulant baits thatare poorly accepted may fail for one ormore of the following reasons:

— Poor bait choice, or bait is formu-lated improperly. Other foods aremore attractive to the rats.

— Improperly placed bait stations.Other foods are more convenient tothe rats.

— Abundance of other food choices.

— Tainted bait: the bait has becomemoldy, rancid, insect-infested, orcontaminated with other materialthat reduces acceptance. Discardold bait periodically, and replace itwith fresh bait.

Occasionally, rats accept bait well andan initial population reduction is suc-cessful. Then bait acceptance appearsto stop although some rats remain. Insuch instances it is likely that theremaining rats never accepted the baiteither because of its formulation orplacement. The best strategy is then toswitch to a different bait formulation,place baits at different locations, and/or use other control methods such astraps.

Other Rodenticides. The olderrodenticides, formerly referred to asacute toxicants, such as ANTU, arsenictrioxide, phosphorus, and Compound1080, are no longer registered for ratcontrol. The widespread availability ofready-to-use anticoagulants and theirrelative effectiveness have resulted inthe reduced use of these older materi-als over the last 20 years.

Table 2. Other (non-anticoagulant) rodenticides used to control Norway rats in the United States.

Acute oral PercentLD50 for Time activerats to ingredient Relative

Common Name Chemical Name mg/kg death Odor Taste in food bait Hazard Mode of Action

Bromethalin N-methyl-2,4-dinitro-N- 2-5 2-4 days None Slight 0.01 Moderate Central nervous(Assault®, (2,4,6-tribromophenyl)- system depressionVengeance®) 6-(trifluoromethyl) and paralysis

benzenamine

Cholecalciferol 9,10-Seocholesta-5,7,10 10-50 3-4 days Slight None 0.075 Low to Mobilizes calcium(Quintox®, (19)-trein-3 betaol moderate resulting in deathvitamin D3) from hypercalcemia

Red squill scilliroside glycoside* 200-490 < 24 hrs. Medium Strong 10 Low Heart

Zinc phosphide zinc phosphide 27-40 1/2-20 hours Strong Strong 1.0-2.0 Moderate Phosphine gasenters circulatorysystem; heartparalysis, gastro-intestinal and liverdamage

* principal active ingredient

At present, four non-anticoagulant ro-denticides (Table 2) are registered byEPA against Norway rats: bromethal-in, cholecalciferol (vitamin D3), redsquill, and zinc phosphide. All arepotentially useful for controlling anti-coagulant-resistant populations of rats.

Of these active ingredients, bromethal-in and cholecalciferol are formulatedto serve as chronic rodenticides, ap-plied so that rats will have the oppor-tunity to feed on the baits one or moretimes over the period of one to severaldays. Bait acceptance is generally goodwhen formulations appropriate forrats are selected. Zinc phosphide andred squill differ in that prebaiting(offering rats similar but nontoxic baitprior to applying the toxicant-treatedbait) is recommended to increase baitacceptance. These two rodenticides arenot designed to be left available to ratsfor more than a few days, as continuedexposure is likely to result in bait shy-ness within the population. Be sure tofollow label recommendations on anyspecific product to achieve best suc-cess.

Non-anticoagulant rodenticides, par-ticularly zinc phosphide, remain usefultools to achieve quick reductions in ratpopulations. When rat numbers arelarge, the cost of baiting with thesematerials may be lower than for theanticoagulants.

Bromethalin (Assault®, Vengeance®) isformulated in a ready-to-use bait as achronic rodenticide, applied so thatrats will have the opportunity to feedon the bait one or more times over aperiod of one to several days. Becauseit is a slow-acting in comparison tozinc phosphide or red squill, bait shy-ness is not usually a problem, nor isprebaiting necessary to get good con-trol in most situations.

Cholecalciferol (vitamin D3, Quintox®)is similarly formulated in a ready-to-use bait, serving as a chronic rodenti-cide. Death occurs 3 or 4 days afteringestion of a lethal dose. Because thetoxicant is slow-acting, bait shyness isnot reported to occur. It is claimed thatrodents cease feeding once a lethaldose has been ingested.

Red squill is a relatively selective andsafe toxicant for use only against Nor-way rats. It acts as an emetic, whichprovides some degree of protection tocertain nontarget species that mightaccidentally consume the bait. Rats,which cannot vomit, are unable to ridthemselves of the toxicant once it isconsumed. In the past, one problemwas the variation in the quality of thematerial, which is derived from aplant. Red squill must be stored in asealed container, as moisture willcause loss of potency.

Zinc phosphide is a dark gray powder,insoluble in water, that has been usedextensively in the control of rodents. Itis available in ready-to-use dry baitsand also in concentrates for use by per-sons trained in rodent control whomay wish to prepare their own baits.Its strong garlic-like odor appears to beattractive to rodents that are not bait-shy. Oils and fats make excellent bind-ers for zinc phosphide and increaseabsorption of the toxicant wheningested. An effective bait is madefrom mixing zinc phosphide with meatsuch as canned fish-flavored cat food.Rats will readily accept this bait, espe-cially if adequate prebaiting has beendone beforehand.

The following general steps are recom-mended to obtain good bait accep-tance, and therefore good rat control,when using zinc phosphide baits:

1. Prebait rats for a minimum of 3 to 5days to get the rats accustomed toeating the nontoxic bait material. Donot change types of bait during theprebaiting or baiting operation.Apply prebait at many locations,wherever there is rat activity.Where bait is completely eatenovernight, double the amount ofprebait at that location the next day.Repeat this procedure until theamount of bait eaten every night nolonger increases.

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Fig. 10. Various types of place packs containingready-to-use rodenticides are commerciallyavailable.

2. Use only high-quality grains andfresh ready-to-use baits. Where ratshave access to abundant amounts ofgrain, meat such as canned fish-flavored cat food may be a goodsubstitute. Obtain a sufficient quan-tity to complete the project withoutchanging brands or flavors.

3. Wait until prebait consumption haspeaked before applying toxic baits.Remove any uneaten prebait andplace the toxic bait at the same loca-tions that the prebait was applied.Usually, the amount of toxic baitneeded will be about half theamount used on the last day thatprebait was applied. It may be help-ful to wait one day between the lastapplication of prebait and applica-tion of toxic bait. That way, rats willbe hungrier. Mix the toxicant intothe bait ingredients according tolabel directions, if preparing yourown baits from a concentrate.

4. Avoid handling the toxic bait orrodenticide concentrate with barehands; use rubber or latex gloves.Clean thoroughly any tools or con-tainers used in bait mixing, or safelydispose of them as well as baitpackaging materials.

5. Confine or restrain any pets, live-stock, or other animals that mayotherwise gain access to and feedon the bait. It may also be necessaryto place prebait and toxic bait intobait boxes for safety.

6. Following toxic bait application,pick up and dispose of availabledead rats and all uneaten bait byincineration or deep burial. Nor-mally, bait should be exposed foronly 1 or 2 nights; the greatest con-sumption occurs on the first night.

7. Control remaining rats by using an-ticoagulant baits or by using trapsor burrow fumigants.

Bait Selection and Formulation

Contrary to popular belief, rats preferfresh, high-quality foods and willreject spoiled or inferior foods itemwhen given a choice. Therefore, rodentbaits should be made from high-quality food materials.

14

Usually corn, oats, wheat, or barley arethe grains most preferred by Norwayrats. Preference will vary between ratpopulations and among individualrats. Baits similar to foods rats areaccustomed to eating are often a goodchoice, particularly if their normalfoods are limited or can be made lessavailable to them. Some people trainedin rodent control prefer to mix theirown baits. Ground cereal grains areoften mixed with 5% powdered sugarand 3% to 10% vegetable oil. A toxi-cant concentrate is added to this mix-ture in the proper amount. Certainanticoagulants, as well as zinc phos-phide, can be purchased in concentrateforms for use in formulating baits. Un-der some conditions, baits made withfruits, vegetables, meat, or fish may behighly accepted. Use of such bait mate-rials, however, may increase the risk ofpoisoning cats, dogs, domestic ani-mals, and other nontarget species.

To determine bait preference in rats,conduct a bait-choice test by placingabout 4 ounces (115 g) of each of sev-eral nontoxic baits about one foot (30cm) apart in several locations whererats are present. Check baits for thenext few days to find out which foodsrats preferred. Keep in mind that ratsare suspicious of new objects andnovel foods; therefore, they may notaccept a new bait until the third orfourth day.

The ready-to-use baits most availableto the public are anticoagulant rodenti-cides. Several types are available.Grain-based baits in a loose meal orpelleted form are available in bulk orpackaged in small, 4- to 16-ounce (112-to 454-g) plastic, cellophane, or paper“place packs” (Fig. 10). These packetskeep bait fresh and make it easy toplace baits into burrows, walls, orother locations. Rats will gnaw intothese bags to feed on acceptable baits.Pelleted baits can more easily be car-ried by rats to other locations. Suchhoarding of food by rats is not uncom-mon. It may result in amounts of baitbeing moved to places where it is un-detected or difficult to recover andmay, if accessible, be hazardous tonontarget species.

Anticoagulant baits have also been for-mulated into wax and extruded blocks(Fig. 11). These are particularly usefulin sewers or where moisture maycause loose grain baits to spoil. Ratsaccept paraffin block baits less readilythan loose or pelleted grain baits, butacceptance of extruded bait blocks ishigh.

Sodium salts of anticoagulants areavailable as concentrates to be mixedwith water, making a liquid bait (Fig.12). Since rats require water daily, theycan be drawn to water stations whereother water sources are scarce. Waterbaits are particularly useful in grainstorage structures, warehouses, andother such locations. Rodents are moreeasily able to detect anticoagulants inwater baits than in food baits; there-fore, up to 5% sugar is sometimesadded to liquid baits to increase rats’acceptance of the bait solution. Sincewater is attractive to most animals, usewater baits in ways that prevent non-target animals from drinking them.

Bait Stations

Bait stations (bait boxes) may increaseboth the effectiveness and safety ofrodenticides. They came into generaluse after the development of the first-generation anticoagulants, whichrequire that a continuous supply ofbait be made available to rodents. Baitstations are useful because they:

— protect bait from moisture anddust;

— provide a protected place forrodents to feed, allowing them tofeel more secure;

Fig. 11. Wax and extruded bait blocks are usefulin damp locations where loose baits becomespoiled quickly.

Fig. 13. Examples of commercially manufacturedrodent bait stations.

Fig. 12. Liquid baits can be placed in fonts orother similar containers.

POISON

— keep other animals (pets, livestock,desirable wildlife) and childrenaway from hazardous bait;

— allow placement of bait in locationswhere it would otherwise be diffi-cult because of weather or potentialhazards to nontarget animals;

— help prevent the accidental spillingof bait;

— allow easy inspection of bait to seeif rodents are feeding on it.

Kinds of Bait Stations. Bait sta-tions can contain solid baits liquidbaits, or both. Bait boxes can be pur-chased from commercial suppliers ormade at home. Manufactured baitboxes made of plastic, cardboard, ormetal (Fig. 13) are sold to pest controlcompanies and to the public in sizesfor rats or mice. Some farm supply andagricultural chemical supply storeshave them in stock or can order them.

Bait boxes can be built from scrapmaterials, and homemade stations canbe deigned to fit individual needs.Make them out of sturdy materials sothey cannot be easily knocked out ofplace or damaged. Where children,pets, or livestock are present, be care-ful to construct the stations so that thebait is accessible only to rodents.Locks, seals, or concealed latches areoften used to make bait boxes moretamperproof. In some situations, sta-tions should be secured in place.Clearly label all bait boxes or stationswith “Poison” or “Rodent Bait — DoNot Touch,” or with a similar warning.Some rodenticides or situations mayrequire use of tamper-resistant baitstations. If so, use only bait boxes orstations which are so designated, andalso be sure to secure them to build-ings by nailing or gluing them to wallsor floors in a way that will not permita person or animal to knock them overor shake the bait out.

Bait Station Design. Bait stationsshould be large enough to allow sev-eral rodents to feed at once. They canbe as simple as a flat board nailed at anangle to the bottom of a wall (Fig. 14),or a length of pipe into which bait canbe placed (Fig. 15). More elaborate sta-tions are completely enclosed and can

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18"

18"

2 1/2"

Fig. 14. A flat board nailed to a wall protectsrodent bait from nontarget animals and allowsrodents to feed in a sheltered location. The boardshould be at least 18 inches (46 cm) long to keeppets and children from reaching the bait.

Fig. 15. Rodent bait station made from alength of pipe. Pipe diameter can be 2 to3 inches (5 to 8 cm) for mice; 3 1/2 to 6inches (9 to 15 cm) for rats.

Fig. 16. A homemade rodentbait station can contain liquidas well as solid baits.

contain liquid as well as solid baits(Fig. 16). A hinged lid with a child-proof latch can be used for conve-nience in inspecting permanentstations.

Bait stations for rats should have atleast two openings approximately2 1/2 inches (6 cm) in diameter. Thetwo holes should be on opposite sidesof the station because rodents can seean alternate escape route as they enterthe station.

Bait Station Maintenance. Baitsmust be fresh and of high quality. Ratswill reject spoiled or stale foods. Pro-vide enough fresh bait to allow ro-dents to eat all they want. When youfirst put bait boxes out, check themdaily and add fresh bait as needed. Af-ter a short time, rodent numbers andfeeding will decline, and you will needto check the boxes only every 2 weeksor once a month. If the bait becomesmoldy, musty, soiled, or insect-in-fested, empty the box and clean it, andthen refill it with fresh bait. Dispose ofspoiled or uneaten bait in accordancewith the label. Follow all label direc-tions for the product you are using.

Placement of Bait Stations.Proper placement of bait stations isjust as important as bait selection. Ratswill not visit bait stations, regardless oftheir contents, if they are not conve-niently located in areas where rodentsare active.

Where possible, place bait between therodents’ source of shelter and theirfood supply. Put bait boxes nearrodent burrows, against walls, oralong travel routes. Since rats are oftensuspicious of new or unfamiliarobjects, it may take several days forthem to enter and feed in bait stations.

On farmsteads, bait station placementdepends on building design and use.In swine confinement buildings, it maybe possible to attach bait boxes to wallledges or the top of pen dividing walls.Bait boxes may be placed in attics oralong the floors or alleys whererodents are active (Fig. 17). Rodenttracks visible on dusty surfaces andtheir droppings often give clues towhere they are active.

Fig. 17. Rodent bait box attached to the top of a pen dividing wall in a swine confinement facility.When used in such locations, bait boxes must be securely fastened and out of pigs’ reach.

Never place bait stations where live-stock, pets, or other animals can knockthem over. Spilled bait may be apotential hazard, particularly tosmaller animals.

Where buildings are not rodent-proof,permanent bait stations can be placedinside buildings, along the outside ofbuilding foundations, or around theperimeter. Bait stations will help keeprodent numbers at a low level whenmaintained regularly with fresh anti-coagulant bait. Rodents moving infrom nearby areas will be controlledbefore they can reproduce and causeserious damage.

Tracking Powders. Toxic dusts orpowders have been successfully usedfor many years to control rats andmice. When rodents walk through apatch of toxic powder, they pick someof it up on their feet and fur and lateringest it while grooming. Trackingpowders are useful in controlling ratswhere food is plentiful and good baitacceptance is difficult to achieve. Ratsare more likely to ingest a lethalamount of a poorly accepted toxicantapplied by this method than if it ismixed into a bait material. There islittle likelihood of toxicant shyness

developing when using trackingpowders.

Because the amount of material a ratmay ingest while grooming is small,the concentration of active ingredientin tracking powders is considerablyhigher than in food baits that utilizethe same toxicant. Therefore, thesematerials can be more hazardous thanfood baits. For the most part, trackingpowders are used by professional pestcontrol operators and others trained inrodent control. Currently, the onlytracking powders registered for useagainst Norway rats contain anti-coagulants.

Place tracking powders in rat burrows,along runways, in walls, behindboards along walls, or on the floor ofbait stations. Placement can be aidedby using various types of sifters, shak-ers, or blowers. Dampness may causethe powder to cake and lessen its effec-tiveness. Care must be taken to placetracking powders only where they can-not contaminate food or animal feed,or where nontarget animals cannotcome into contact with them. Do notplace tracking powders where rats cantrack the material onto food intended

for use by humans or domestic ani-mals. Tracking powders are not gener-ally recommended for use in andaround homes because of potentialhazards to children and pets. Wherepossible, remove tracking powderafter the rodent control program iscompleted.

Fumigants

Fumigants (toxic gases) are most com-monly used to control rats in theirburrows at outdoor locations. Com-pounds including aluminum phos-phide, chloropicrin, and gas cartridges,are registered for this purpose. Theincendiary gas cartridge burns, pro-ducing carbon monoxide and othergases that suffocate rodents in theirburrows. Methyl bromide is presentlyregistered only for fumigation of struc-tures by qualified professionals. Anhy-drous ammonia is not recommendedfor use as a burrow fumigant becauseit is not registered for this purpose. Forfurther information on fumigants, seethe Pesticides section.

Fumigants should only be used bypeople familiar with the necessary pre-cautions because they are highly toxicto humans and other animals. Do notuse fumigants in any situation thatmight expose the occupants of a build-ing to the fumes. Only licensed struc-tural pest control operators should usefumigants in buildings or other struc-tures.

To fumigate rat burrows, close theburrow opening with soil or sodimmediately after introduction of thefumigant. Rat burrows often have mul-tiple entrances, and all openings mustbe sealed in order for fumigants to beeffective. Fumigants are less effectivein soils that are very porous or dry.

Trapping

Trapping can be an effective method ofcontrolling rats, but it requires moreskill and labor than most othermethods. Trapping is recommendedwhere toxicants are inadvisable. It isthe preferred method to try first inhomes, garages, and other small struc-tures where there may be only a fewrats present.

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Trapping has several advantages: (1) itdoes not rely on inherently hazardousrodenticides; (2) it permits the user toview his or her success; and (3) itallows for disposal of the rat carcasses,thereby eliminating odor problemsfrom decomposing carcasses that mayremain when poisoning is done withinbuildings.

The simple, inexpensive, wood-basedsnap trap is available in most hard-ware and farm supply stores. Trapsshould be baited with a small piece ofhot dog, bacon, or nutmeat tiedsecurely to the trigger. Peanut butteror marshmallows also may be used asbait. Baits that become stale lose theireffectiveness.

Set traps close to walls, behind objects,in dark corners, and in places whererat activity is seen. Place the traps sothat when rats follow their naturalcourse of travel (usually close to awall) they will pass directly over thetrigger (Fig. 18). Set traps so that thetrigger is sensitive and will spring eas-ily. Effectiveness can be increased byenlarging the trigger. Attach a squareof cardboard, metal, or screen wirethat fits just inside the wire deadfall(Fig. 19).

8

Single trap set with trigger next to wall.

Wrong—trigger not next to wall.

Fig. 18. Placement of snap traps

Leaving traps unset until the bait hasbeen taken at least once reduces thechance of rats escaping the trap andbecoming trap-shy. Other kinds oftraps are also effective in catching rats.Wire-mesh cage traps such as theNational®, Tomahawk®, andHavahart® can be used effectively tocapture rats alive (Fig. 20). Wire fun-nel-entrance traps have also been usedto capture rats alive.

Keep traps reasonably clean and ingood working condition. They can becleaned with a hot detergent solutionand a stiff brush. Human and dead-ratodors on traps are not known toreduce trapping success.

An alternative to traps are glue boards,which catch and hold rats attemptingto cross them, much the same way fly-paper catches flies. Place glue boardswherever rats travel — along walls orin established runways. Do not useglue boards where children, pets, ordesirable wildlife can contact them.Glue boards lose their effectiveness industy areas unless covered, and tem-perature extremes may affect the tacki-ness of some glues. They areconsidered less effective for capturingrats than for mice. Glue boards can be

The double set increases your success.

Wrong—parallel set with triggers on the inside.

purchased ready-to-use or they can bemade. Euthanize live, trapped rodentsby asphyxiation with carbon dioxide,or use a stick to kill them with sharpblows to the base of the skull. For fur-ther information on glue boards, seethe Supplies and Materials section.

Other Methods

In some situations, rats can be killedmanually with a club or other imple-ment. When rats have access to astructure through only one or a fewentrances, it may be possible to drivethem out en masse. Then they can beclubbed or shot with a pellet gun or .22firearm loaded with birdshot.

Some dogs and cats will catch and killrats. There are few situations, how-ever, in which they will do so suffi-ciently to control rat populations.Around most structures, rats can findmany places to hide and rear theiryoung out of the reach of such preda-tors. Cats probably cannot eliminateexisting rat populations, but in somesituations they may be able to preventreinfestations once rats have been con-trolled. Farm cats, if sufficient in num-ber and supplementally fed, may servethis function.

Double set placed parallel to the wall withtriggers to the ouside.

Wrong—trap too far from wall.

A box or board placedto advantage mayguide rat into trap.

Place traps acrossobvious runways,or where runs areconfined.

Expanded trigger

Fig. 19. Expanded-trigger traps, when properlyplaced, can be very effective.

Fig. 20. Wire cage traps can be used to capturerats alive. Use enough traps to make thecampaign short and decisive.

In urban and suburban areas, rats maybe present because people have pets. Itis not uncommon to find rats living inclose association with cats and dogs,relying on cat and dog food for nour-ishment. Rats frequently live beneathdog houses and soon learn they canfeed when the dog is absent or asleep.

Economics of Damageand Control

Accurate data on rat damage, control,and their cost is difficult to obtain. Es-timates of losses of foodstuffs, struc-tural damage, and the amount of laborand materials expended to control ratsare usually only educated guesses.One study found that a small colony ofNorway rats (10 to 26 animals), whengiven access to a ton of sacked wheat,would contaminate 70% of the grainafter 12 to 28 weeks. The sacks wereheavily damaged as well. Total dam-age equaled 18.2% of the total value ofthe wheat and the sacks.

One rat will eat approximately 20 to 40pounds (9 to 18 kg) of feed per yearand probably contaminates 10 timesthat amount with its urine and drop-pings. In a year’s time, a single rat willproduce some 25,000 droppings. A1973 estimate states rats may cost theUnited States between $500 millionand $1 billion annually in direct eco-nomic losses. In most cases, the cost ofrat control—particularly when it isdone in a timely fashion—is far lessthan the economic loss caused by ratdamage.

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Acknowledgments

I thank Rex E. Marsh for reviewing this chapterand providing many helpful comments. Thematerial it contains includes information takenfrom Brooks 1973, Howard and Marsh 1981, andPratt et al. 1977, among other sources.

Figures 1 and 3 from Schwartz and Schwartz(1981) adapted by Jill Sack Johnson.

Figures 2, 4, 6, 7, 8, 9, 19 and 20 adapted fromPratt et al. (1977) by Jill Sack Johnson.

Figures 5 and 18 adapted from Howard andMarsh (1981) by Jill Sack Johnson.

Figures 10, 11 and 12 by Jill Sack Johnson.

Figures 13, 16 and 17 by Frances I. Gould.

Figures 14 and 15 adapted from Pratt et al.,(1977) by Frances I. Gould.

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EditorsScott E. HygnstromRobert M. TimmGary E. Larson