CALIFORNIA AGRICULTURAL EXTENSION SERVICE 
 
 CIRCULAR 142 
 
 APRIL, 1948 
 
 CONTROL OF 
 RATS AND 
 
 TRACY I. STORER 
 
 
Cooperative Extension work in Agriculture and Home Economics, College of Agriculture, 
 
 University of California, and United States Department of Agriculture cooperating. 
 
 Distributed in furtherance of the Acts of Congress of May 8, and June 30, 1914. 
 
 B. H. Crocheron, Director, California Agricultural Extension Service. 
 
IHE RATS AND MICE of our homes and 
 cities are real pests. These rodents, their habits, and means for 
 their control are discussed in this circular. Brief attention also 
 is given to native white-footed mice and woodrats that often 
 invade cabins in the hills and mountains. 
 
 Food and property worth millions of dollars are destroyed 
 or damaged each year by house rats and mice. These pests may 
 also bring serious diseases to man— plague, endemic typhus, 
 and others— either directly or through fleas or mites. 
 
 The best control is continuous and community-wide. Quiet 
 day-by-day application of control measures by private citizens 
 and local government officials is far better than a much- 
 publicized "rat week" after which the rodents are left undis- 
 turbed. 
 
 Exclusion and proper sanitation are the surest and most last- 
 ing defenses against rats. This circular tells how to rat proof 
 buildings, and describes other control methods, such as traps, 
 poisoned baits, gases, and dusts. 
 
 Rat poisons are dangerous to people and to livestock. Some 
 gases used in rat control are explosive. Stocks of these materials 
 must be locked up, and certain precautions are necessary in 
 their use. A doctor should be called immediately if accidents 
 occur. 
 
 Aid in control may be obtained from any county agricultural 
 commissioner or farm advisor, and in some cities and counties 
 from the health department. 
 
CONTENTS 
 
 PAGE 
 
 The rat problem 3 
 
 Kinds of rats and mice 3 
 
 General features 3 
 
 Norway rat 5 
 
 Roof and black rats 5 
 
 House mouse 6 
 
 Diseases carried by rats and mice .... 6 
 
 Plague 6 
 
 Murine typhus 8 
 
 Leptospirosis 8 
 
 Food poisoning 8 
 
 Rat-bite fever 8 
 
 Trichinosis 8 
 
 Rickettsialpox 8 
 
 Damage by rats and mice 9 
 
 Evidence of rats and mice 1 
 
 Sight 1 
 
 Sound 1 
 
 Droppings 1 
 
 Runways 1 
 
 Smears 1 
 
 Holes 13 
 
 Nests 13 
 
 Urine stains 13 
 
 Methods of control 13 
 
 Exclusion 13 
 
 Foundations 15 
 
 Exterior walls 15 
 
 Openings 15 
 
 Doorways 16 
 
 Corn cribs 17 
 
 Feed storage 17 
 
 Human food supplies 19 
 
 Cabinets and enclosed platforms . 19 
 
 Garbage, boxes, and trash 19 
 
 PAGE 
 Methods of control {continued) 
 
 Traps 21 
 
 Trapping house mice 21 
 
 Trapping rats 21 
 
 Poisoned baits 23 
 
 Poisons 23 
 
 Baits 24 
 
 Prebaiting 24 
 
 Placing baits 25 
 
 Formulas 25 
 
 Red squill 25 
 
 Zinc phosphide 27 
 
 ANTU 28 
 
 Arsenic 28 
 
 Barium carbonate 29 
 
 Strychnine 29 
 
 Yellow phosphorous 29 
 
 Special poisons 29 
 
 Deodorants for dead rats 29 
 
 Poisonous gases 30 
 
 Calcium cyanide 30 
 
 Other gases 31 
 
 Poisonous dusts 31 
 
 Other means of control 32 
 
 Destruction of burrows 32 
 
 Flooding 32 
 
 Blocking 32 
 
 Dogs, cats, and ferrets 32 
 
 Rat virus 33 
 
 Other aspects of control 33 
 
 Legal requirements 33 
 
 Pest control operators 33 
 
 White-footed mice 33 
 
 Wood rats 35 
 
 Literature cited 37 
 
CONTROL OF RATS AND MICE 
 
 TRACY I. STORER 
 
 Professor of Zoology and Zoologist in the Agricultural Experiment Station, Davis 
 
 THE RAT PROBLEM 
 
 The "house" rats and mice have long 
 been unwelcome associates of man. They 
 followed his occupation and development 
 of new countries and now are present in 
 many parts of the world. In contrast to 
 native rats and mice they are "aliens," 
 but well established. In California, as 
 elsewhere, these animals are all too com- 
 mon in cities and on farms, in stores and 
 warehouses, in slaughterhouses and on 
 garbage dumps, and sometimes along 
 streams or in fields well apart from man's 
 dwellings. 
 
 Rats and mice are objectionable in 
 many ways: 
 
 1) They eat and foul all sorts of 
 foods. 
 
 2) They gnaw into packages, boxes, 
 cabinets, and buildings. 
 
 3) They damage furniture, clothing, 
 and other goods in homes and 
 places of business. 
 
 4) They carry diseases that menace 
 human society. 
 
 5) They give fright and annoyance 
 to many persons, and Injure some 
 by rat bites. 
 
 Mankind unfortunately provides rats 
 and mice with their two essentials of life 
 —food and shelter. In consequence, they 
 live and multiply in almost every suitable 
 place to which they have access. Rats and 
 mice have been fought for centuries, but 
 the "rat problem" is still unsolved. 
 
 This circular describes the kinds of 
 rats and mice, their habits, the kinds of 
 damage they may do, the evidence of their 
 presence, and the equipment, materials, 
 and methods used for control. To get rid 
 of these rodents, it is necessary to know 
 
 their habits, reactions, and food prefer- 
 ences, and then use this knowledge in 
 control operations. Often one must ex- 
 periment to find the most suitable bait, 
 the best places for traps, or other satis- 
 factory method of control. No one fixed 
 formula or method will give good results 
 in all cases. 
 
 Advice in control of rats and other 
 rodents may be had from various govern- 
 ment officials and agencies, including 
 county agricultural commissioners and 
 farm advisors, city, county, and state 
 health departments, state departments of 
 agriculture, and the U. S. Fish and WiM- 
 life Service and U. S. Public Heallh Serv- 
 ice. Specialists in these agenci-^s will give 
 technical advice. Some will do control in 
 places where there is serious damage to 
 property or a health hazard. Moreover, 
 these agencies can use materials and meth- 
 ods not available to the general public. 
 
 KINDS OF RATS AND MICE 
 
 General Features 
 
 Rats and mice are rodents or gnawing 
 animals (mammals of the zoological 
 order Rodentia). Like all rodents, they 
 have, at the front of the mouth, two pairs 
 of chisel-like teeth that grow continuously 
 and are self-sharpening. These incisors 
 are used to cut food, to remove shells or 
 coverings of nuts and seeds, to make 
 nests by shredding cloth, paper, or 
 grasses, and to gnaw wood, plaster, or 
 softer materials so that the animals may 
 enter places giving food or shelter. Rats 
 and mice have no canine teeth (tusks). 
 Their check or grinding teeth have small 
 
 ^ This circular replaces part of Extension Cir- 
 cular 79 which will not be republished. For con- 
 trol of field rodents, see Extension Circular 138. 
 
 [3 
 
jjrojections used to chew food finely be- 
 fore swallowing it. 
 
 All of the toes end in sharp claws that 
 help in climbing and digging. When 
 cornered, a rat uses its incisor teeth and 
 claws, and may inflict severe injuries. The 
 long tail serves as a counter-balance to the 
 body in running, jumping, and climbing. 
 
 abruptly or to hurry to safety. They be- 
 come used to ordinary noises, however, 
 and are often active where people, do- 
 mestic animals, or machines are close by. 
 The long "whiskers," or vibrissae, on 
 the nose, and other long hairs above the 
 eyes, serve the sense of touch. There are 
 sensory nerves about the base of each 
 
 Fig. 1.— The Norway rat. Distinctive features (as compared with the roof rat) are the 
 blunt nose, moderate-sized and slightly haired ears, and the tail which does not exceed 
 the combined length of head and body. The total length is up to 16 inches. 
 
 Most rodents have scent glands which 
 leave odors on their droppings, trails, and 
 nests. In rats and mice these glands are 
 just inside the vent or anal opening, be- 
 low the base of the tail. The odor from 
 house rats is mild to the human nose, but 
 that from the house mouse is strong and 
 unpleasant, the "mousy odor." 
 
 Rats and mice have rather poor vision, 
 but the senses of smell, taste, hearing, and 
 touch are keenly developed. Their fre- 
 quent sniffing movements tell them much 
 about their surroundings through odors 
 received. Their choice in foods is un- 
 doubtedly based upon taste preferences. 
 They are frightened by unusual sounds, 
 which may cause them either to stop 
 
 hair. It is the habit of a house rat or 
 mouse to run close beside a wall, against 
 which these sensory hairs touch to give 
 the animal information about its sur- 
 roundings. In the laboratory, rats with 
 the vibrissae removed have been found 
 less skillful in running and finding their 
 way. 
 
 Three kinds of rats and one kind of 
 mouse, all "aliens" from the Old World, 
 are now abundant and of great economic 
 importance in California. They are the 
 Norway rat, the roof rat, the black rat, 
 and the house mouse. The alien rats may 
 be distinguished from the native woodrat 
 (p. 35) by their coarser hair and scaly 
 tail. 
 
 [4] 
 
Norway Rat 
 
 The largest of the alien rats, and the 
 one which does most damage, is the Nor- 
 way rat {Rattus norvegicus) , also known 
 as brown rat, house rat, wharf rat, and 
 sewer rat (fig. 1). 
 
 Full-grown adults are about 16 inches 
 in total length, with the tail 7% inches 
 long, and weigh 11 or 12 ounces; excep- 
 tional individuals weigh as much as 24 
 ounces. The nose is rather blunt, and the 
 ears are of moderate size and slightly 
 haired. The tail is scaly, nearly naked, 
 and blunt-ended; when laid forward it 
 usually does not reach to the end of the 
 nose, and is never longer than the head 
 and body. The coarse body fur is gen- 
 erally brown, with scattered black hairs, 
 and is darkest along the middle of the 
 back. The under parts are pale gray to 
 yellowish-white.^ 
 
 The Norway rat is distributed generally 
 throughout lowland California, along the 
 seacoast and in the interior, both in cities 
 and in the country, but has not invaded 
 the mountains to any extent. It lives 
 about residences, stores, warehouses, 
 slaughterhouses, barns, pigpens, and 
 chicken yards, on garbage dumps, in 
 sewers and tunnels, and along the banks 
 of streams and ditches. 
 
 This rat stays mainly at the ground 
 level, seldom going above the first floor 
 of a building. It climbs, but not as much 
 as the other alien rats. It burrows and 
 makes nests under buildings or platforms 
 on the ground, beneath piles of lumber or 
 stones, in garbage dumps, and in marshy 
 places along both fresh and salt water. 
 The nest, indoors or out, is of trash and 
 not always neatly or well formed as with 
 some other rodents. 
 
 The tunnels are 2 to 3 inches in diam- 
 eter and of various lengths, often with 
 more than one opening on the surface. In 
 addition, each burrow commonly has sec- 
 ondary exits or "bolt holes" which may 
 be hidden under grass or boards. Norway 
 rats take to water when necessary, and can 
 swim easily. r 
 
 Female Norway rats average 8 to 9 
 embryos (extremes of 2 and 17 have been 
 found) at a pregnancy, and may breed 4 
 or more times during a year. Pregnancy 
 lasts 21 to 22 days, and it is thought that 
 the young can shift for themselves when 
 about 3 weeks old. A female may first 
 breed at the age of 4 to 5 months, when 
 her weight is about 5 ounces (140+ 
 grams) . 
 
 It is not known how long wild rats live, 
 but much of the population probably is 
 replaced by new individuals each year. 
 The possible rate of increase is suggested 
 by one laboratory experiment with cap- 
 tive Norway rats, which began with one 
 pair; more than 1,500 were produced by 
 the end of a year ! 
 
 Because of its large size and bold na- 
 ture, the Norway rat has in many places 
 replaced the black rat. When rats are re- 
 duced in numbers by trapping, house 
 mice often increase, suggesting some kind 
 of competition between these various ro- 
 dents. Yet Norway rats and house mice 
 often inhabit the same buildings. 
 
 Roof and Black Rats 
 
 The roof rat {Rattus rattus alexandri- 
 nus), sometimes called Alexandrine, or 
 gray, rat (fig. 2) , grows to a total length 
 of 15 inches, with the tail measuring 8^4 
 to 10 inches ; its weight seldom exceeds 8 
 ounces. The nose is sharp and slender, 
 the ears are rather large, with little or no 
 hair, and the thin, tapering, scaly tail is 
 almost always longer than the combined 
 length of head and body. The back and 
 sides are gray or gray-brown, and the 
 belly white or nearly so. 
 
 ^Additional more technical characters are: 
 
 (a) norvegicus, length of ear from notch, 16-20 
 mm; ear when laid forward normally not ex- 
 tending more than halfway to eye; tail more or 
 less bicolor; hind foot 38-46 mm long; mammae 
 (teats), 12; first molar (cheek tooth) without 
 distinct small cusp or style on first row of cusps. 
 
 (b) rattus and alexandrinus, length of ear from 
 notch, 24-26 mm; ear when laid forward reach- 
 ing halfway to eye; tail not bicolor; hind foot, 
 36-40 mm long; mammae, 10. 
 
 5] 
 
Roof rats live in much of lowland Cali- 
 fornia, coastwise and inland, in cities and 
 rural areas, and have been found scatter- 
 edly in the mountains as high as 5,000 
 feet. They inhabit homes, warehouses, 
 packing sheds, and feed stores, often liv- 
 ing in attics or upper stories, but may 
 also be found at ground level, especially 
 when Norway rats are absent. In some in- 
 land localities they live along stream 
 banks. Roof rats climb readily and travel 
 on the exterior of rough-surfaced build- 
 ings, on electric wires and cables, and in 
 trees. In California they often nest in 
 trees, especially palms, and in dense 
 hedges or vines growing on fences. These 
 rats can also swim easily. 
 
 The roof rat averages about 6 embryos 
 per brood and may breed several times 
 per year. The rate of development and 
 ages at which young become independent 
 and breed are assumed to be about the 
 same as for the Norway rat. 
 
 Except for its almost solidly black 
 color, the black rat {Rattus rattus rattus) 
 resembles the roof rat in size and struc- 
 ture. In California this rat is found only 
 near salt water, in seaports and adjacent 
 towns along the entire coast, but inland 
 only about as far as Martinez, Contra 
 Costa County. 
 
 Roof and black rats are far more com- 
 mon on ships than the Norway rat, but 
 all these alien rats travel in vessels, es- 
 pecially cargo ships. 
 
 House Mouse 
 
 The common house mouse (Mus mus- 
 culus) is 3 to 4 inches in head-and-body 
 length, and the tail is 3 to 3% inches long 
 (fig. 3). The ears and eyes are small (as 
 compared to those of the native white- 
 footed mice), and the tail is scantily 
 haired. The upper surface of the body is 
 almost uniformly brown, and the under 
 parts pale brown to whitish. 
 
 House mice have followed settlers into 
 almost all parts of California. The mice 
 live in any structure they can enter, and 
 
 also entirely apart from man in the fields 
 of some interior lowland districts. Those 
 living outside during the summer tend to 
 move into buildings when it gets cold. 
 
 Since house mice in nests are easily 
 transported in bales of hay, household 
 goods, crates, and boxes, they are con- 
 stantly being moved about and into new 
 localities. These mice live at all levels 
 in buildings, from basements to attics. 
 They climb readily, and can pass through 
 holes % inch square. They seldom bur- 
 row in the ground, but often make com- 
 pact nests of cloth, sacking, or other 
 materials that are shredded finely and 
 worked into a round hollow ball used 
 especially to shelter the young. 
 
 House mice average 5 to 6 young per 
 litter; the young can run about when 21 
 days old and can breed at 42 days. Indi- 
 vidual captive female mice have produced 
 100 young per year. This species is short- 
 lived; most individuals probably do not 
 live much more than one year, hence the 
 population "turnover" is rapid. At times 
 house mice increase to very large num- 
 bers (Hall, 1927; Piper, 1928; Storer, 
 1931). 
 
 DISEASES CARRIED 
 BY RATS AND MICE 
 
 The alien rodents and their insect 
 parasites carry several important 
 human diseases (Hull, 1941). This 
 is a most important reason for con- 
 trol of rats and mice. 
 
 Plague 
 
 Plague is a bacterial disease of wild 
 field rodents and rats, and is usually 
 transmitted by fleas. With the spread of 
 rats by ship-borne commerce, plague in- 
 fections have occurred in several United 
 States seaports. Two epidemics of flea- 
 transmitted bubonic plague in San Fran- 
 cisco (1900, 1909) resulted in 281 cases 
 with 191 deaths. In Los Angeles during 
 October, 1924, plague from rats took the 
 
 [6] 
 
Fig. 2.— The roof rat. Important characteristics are the slender snout, large naked 
 ears, and long slender tail which usually exceeds the combined length of head and 
 body. The total length is up to 15 inches. The black rat is identical in form except for 
 its black coat of hair. 
 
 Fig. 3.— The house mouse. The eyes are small, the body is brown, and the tail scantily 
 haired. The head-and-body length is 3 to 4 inches, and the tail 3 to 3Vi inches. 
 
 pneumonic form in human beings. This 
 form is spread by coughing (droplet in- 
 fection) from one person to another. Of 
 32 human cases, 30 died. 
 
 Plague is now well distributed among 
 field rodents (as sylvatic plague) in many 
 parts of the western United States, and 
 some human cases have resulted. Rats in 
 cities or towns may be reinfected from the 
 
 surrounding field rodents, and thus be 
 a health hazard. A few plague-infected 
 rats have been found in California cities 
 during recent years. Often when neither 
 rats nor field rodents show visible signs 
 of plague, the fleas from these animals 
 prove to contain plague bacteria, thus 
 maintaining the "reservoir" of this dis- 
 ease among rodents. 
 
 [7] 
 
Murine Typhus 
 
 This endemic typhus is a New World 
 disease of rats and man carried by rat 
 fleas. It is related to the louse-borne ty- 
 phus of Europe, and to Rocky Mountain 
 spotted fever, but is not as deadly. Human 
 cases of murine typhus have increased 
 greatly during recent years in states of 
 the Atlantic seaboard and Gulf Region. 
 
 It was first reported in California in 
 1916, and to the end of 1945 there were 
 362 human cases and 17 deaths in the 
 state. Here it occurs mainly in Los An- 
 geles, San Diego, Orange, San Bernar- 
 dino, and Santa Barbara counties. Cases 
 have been reported in all months, but are 
 more numerous in late summer and 
 autumn. The largest percentage of human 
 cases is among food industry workers 
 in large cities, but some cases have been 
 found in other kinds of business estab- 
 lishments, motion picture theaters, and 
 private homes (Beck and Van Allen, 
 1947). Control of rat fleas by DDT, and 
 control of rats, have been effective in re- 
 ducing the number of human cases. 
 
 Leptospirosis 
 
 This disease, also called Weil's disease, 
 infectious jaundice, and Stuttgart disease 
 of dogs, affects the kidneys in rats, and 
 is spread by rat urine contaminating food 
 or water. Workers in mines, rice fields, 
 sewers, etc., in the Orient suffer a heavy 
 mortality, and there are some human 
 cases in the United States. About 40 per 
 cent of the rats in San Francisco show 
 evidence of this disease. Dogs also are 
 susceptible, and one or more epidemics 
 have occurred among city dogs in Cali- 
 fornia. 
 
 Food Poisoning 
 
 Certain bacteria common in rats and 
 their droppings cause food poisoning in 
 man, and epidemics have resulted from 
 human food contaminated by rats. Food 
 dirtied by rats is always a health hazard. 
 
 Rat-bite Fever 
 
 This is another bacterial disease of 
 rats transmissible to man. Several cases 
 have been reported in the United States 
 in recent years. 
 
 Trichinosis 
 
 This disease of rats, swine, and man is 
 also present in cats, bears, and some other 
 animals. It is caused by a microscopic 
 trichina worm that lives as a larva in the 
 muscles of infected animals. When flesh 
 of one infected animal is eaten by another 
 of the susceptible animals, the larvae 
 taken in with the flesh become mature, 
 breed, and produce many new larvae. If 
 the latter are abundant, the poisonous 
 substances they secrete lead to illness or 
 death of the new host. Rats aid in spread- 
 ing this disease among pigs, and man be- 
 comes infected by eating pork or bear 
 meat which has been imperfectly cooked 
 or cured and therefore contains many 
 living trichina larvae. 
 
 Rickettsialpox 
 
 This disease of house mice is trans- 
 mitted by a tiny blood-sucking mite, and 
 was first recognized in 1946, when about 
 80 human cases occurred in two housing 
 centers in New York City. 
 
 Because of the danger of these 
 rodent-borne diseases, no one save a 
 trained public health official should 
 handle alien rats or mice or keep them 
 in cages. 
 
 In the poorer housing areas of large 
 cities some persons are bitten by wild 
 rats. In Baltimore, Maryland, 87 people 
 were treated for rat bites at one hospital 
 during 1939-43. Most of the victims were 
 infants under one year of age, and nearly 
 all were bitten at night while asleep. 
 
 House rats carry mites and several 
 kinds of fleas, some of which also attack 
 
 8 
 
human beings, and may carry disease. 
 The tropical rat mite (Liponyssus hacoti) 
 is so small that it can just be seen by the 
 unaided eye. It is common on rats in Cal- 
 ifornia, and at times bites people, causing 
 serious discomfort to both adults and in- 
 fants. The spots of attack are reddish, 
 with slight swelling and persistent itching 
 that may be mistaken for flea bites. 
 
 DAMAGE BY RATS 
 AND MICE 
 
 The total amount spent for rat and 
 mouse control is small when com- 
 pared with the amount of damage 
 these animals do. 
 
 No one knows how many rats there are, 
 but it has often been assumed that the 
 rat population equals the human popula- 
 tion. In the United States, then, there 
 would be 140,000,000 rats. The average 
 adult Norway rat probably eats about an 
 ounce of food a day. If the rats lived en- 
 tirely on wheat, they would need a daily 
 food supply of 4,375 tons, or 145,833 
 bushels! The annual levy would be more 
 than 53 million bushels, and, with wheat 
 at $1.00 per bushel, the annual bill for 
 supporting the rat population would be 
 $53,229,000. Of course, a higher wheat 
 price means a higher toll. Rats consume 
 much that is unfit for human use, but they 
 often take high-priced foods, and damage 
 more than they eat. Hence this estimate 
 based on wheat gives a rough idea of the 
 cost of these rodents. 
 
 The alien rats and mice may eat prac- 
 tically anything used as food by human 
 beings and by livestock (except hay) . 
 State and federal food inspectors are giv- 
 ing increased attention to foods contami- 
 nated by rats and mice, and can recognize 
 hair, droppings, or urine stains in pack- 
 ing houses. Contaminated food is con- 
 demned for human use, and must be 
 disposed of at a loss. 
 
 Even more important economically 
 than the food actually consumed is the 
 damage to sacks, boxes, and other con- 
 
 tainers, causing the loss of packaged ma- 
 terials. Many thousands of such food 
 containers are thus rendered unsalable 
 in packing houses, warehouses, and retail 
 stores. In homes, cardboard cartons and 
 other packages of food are torn open, 
 spilled, and dirtied so that many must be 
 discarded. Slabs of bacon and sides of 
 other meats may be eaten and gnawed 
 extensively. House mice sometimes live 
 continuously in cold-storage warehouses 
 where they eat nothing but high-priced 
 meat! 
 
 To get nesting materials, rats and mice 
 often gnaw and pull apart clothing, dry- 
 goods, blankets, mattresses, upholstered 
 furniture, and carpets, so that the damage 
 is too great to repair. Cloth or paper bags 
 of grain or cereals in feed warehouses 
 often are so chewed that much labor is 
 needed to resack the contents and to re- 
 pair the sacks. Piles of empty bags some- 
 times are riddled with holes and made 
 useless by the work of rats and mice. 
 
 On poultry and pigeon farms, in addi- 
 tion to destroying feedstuffs and sacks, 
 rats often eat eggs, and at times kill num- 
 bers of young birds. 
 
 Rats, and less often mice, gnaw into 
 buildings, usually at the doors or win- 
 dows, and when inside they cut holes in 
 cabinets and other interior fittings as 
 they search for food or for places to nest. 
 Both indoors and out, rats may put elec- 
 tric circuits out of operation by gnawing 
 away insulation (either lead or fabric), 
 exposing, and sometimes cutting the 
 wires. Mice may enter wiring conduits 
 and travel within them; they also may 
 gnaw off the insulation of wires and pro- 
 duce short circuits. 
 
 Rats and mice have been accused of 
 causing fires by short-circuiting power 
 wires (also by gnawing matches), but 
 conclusive evidence of such damage is 
 difficult to obtain. At times they build 
 nests inside the frames of electric motors. 
 Short circuits caused by nest materials 
 may burn out the motor, which then must 
 be rewound. 
 
 [9] 
 
fc -^^^s 
 
 
 \ ". 
 
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 Fig. 4.— Evidences of rats: Droppings and damage to grain sacks; tracks in dust; a freshly gnawed 
 post, and footprints and tail marks in flour. Courtesy of U. S. Public Health Service. 
 
EVIDENCE OF RATS 
 AND MICE 
 
 Rats and mice may be seen or 
 heard, or their presence may be 
 discovered through other common 
 signs, such as droppings, runways 
 or foot marks, smears, holes, piles 
 of earth from burrows, nests, dam- 
 age to food or property, gnawings, 
 and disappearance of bait. 
 
 Sight 
 
 Rats and mice try to avoid being seen. 
 One may have quite an infestation with- 
 out seeing a single live rodent. Gunder- 
 son (1943), in Iowa, believes that the 
 number of rats present, particularly on 
 farms, may be roughly guessed on the 
 following basis : 
 
 None seen, to 20 present; 
 Occasionally seen, 20 to 200; 
 Often seen, more than 200. 
 
 When rats are numerous on a garbage 
 dump, some can usually be seen in day- 
 light. A dump after dark, seen by a flash- 
 light, often is a spectacular sight. 
 
 Sound 
 
 In any place where fair numbers are 
 present, rats and mice may usually be 
 heard running and squealing after dark. 
 In a home, even one or two mice or rats 
 scampering and dragging food over the 
 rough plaster "keys" in an attic often 
 make enough noise to disturb the people 
 below— and often lead to prompt efforts 
 toward control. 
 
 Droppings 
 
 These signs commonly are blackish, 
 soft, and somewhat shiny when fresh, but 
 become hard and dull later (fig. 4). Un- 
 disturbed droppings last for a long time; 
 hence only those in places often swept 
 are useful to indicate recent activity, and 
 
 whether few or many rodents are present. 
 A Norway rat (captive) may produce 25 
 to 180 droppings per day, more being 
 voided with fruit or vegetable diets. 
 Droppings are found scattered wherever 
 rats run, but may be concentrated in 
 places where the animals stop to feed. 
 Those of the Norway rat often are in 
 groups, are spindle-shaped, and % to % 
 inch long; those of roof or black rats are 
 scattered, more sausage-shaped, and 
 slightly smaller; droppings of the house 
 mouse are scattered, spindle-shaped, and 
 about 1/16 by 1/4 inch. 
 
 Runways 
 
 Paths regularly used by numbers of 
 rats may become beaten runways on the 
 ground or over piles of refuse, and 
 smoothed routes over board surfaces in- 
 doors. In soft earth, dust, or flour, the 
 foot prints and tailmarks often show 
 clearly ; the fresher the print, the sharper 
 the pattern. Flour, whiting, or other light- 
 colored powder may be dusted from a 
 sifter can on places where rats are be- 
 lieved to be running; then any tracks 
 made during the night can be seen the 
 next day, and one may estimate the 
 number of animals using the runway. A 
 small mirror set at an angle on the end 
 of a stick can be used to search for tracks 
 on overhead pipes or other structures. A 
 flashlight is useful for finding tracks in 
 dark corners. 
 
 Smears 
 
 Grease, soot, and dirt from the belly 
 hairs will leave smears on well-travelled 
 runways. These can be best seen where 
 the animals press their bodies down in 
 changing direction over the sides of 
 beams and on ladders or along pipes or 
 beams. A run along a beam below floor or 
 ceiling joists is often marked by a series 
 of o <^ O shaped marks between the 
 joists (fig. 5). Smears are most evident 
 as dark marks on whitewash or non- 
 
 [11] 
 
Fig. 5.— Smears made by rats traveling along a supporting floor beam. The rats must crawl 
 along the side of the beam under each joist, and thus leave the w w v^ v^ -shaped markings. 
 Courtesy of U. S. Public Health Service. 
 
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 ^6"— 
 
 TOP VIEW 
 
 Fig. 6.— Burrow of Norway rat in a poultry pen. In addition to the open entrance, there are 
 emergency exits or bolt holes, lightly covered by soil, and not easily seen from the surface. 
 (Excavated by Rocci G. Pisano.) 
 
 12 
 
glossy light-colored paint, or as whitish 
 markings where flour is stored. Shiny 
 smears indicate recent use. 
 
 Holes 
 
 Entrances to burrows in open ground 
 or under pavements, foundations, or piles 
 of rubbish are common signs, and, if they 
 show foot prints, are in active use. Not all 
 holes show mounds of earth excavated 
 from burrows. Old holes may last long 
 after disuse. Stopping all holes in an area 
 with plugs of crumpled newspapers or 
 soft earth, and later noting which are re- 
 opened, is an easy way to recognize those 
 being used. 
 
 Nests 
 
 The nests of Norway rats are not easily 
 found, since they are hidden in burrows, 
 under floors or foundations, in piles of 
 
 goods or rubbish, or in disused packing 
 boxes. Nests of roof or black rats may be 
 in similar places (rarely in burrows) , but 
 some are built out-of-doors in dense trees 
 or in bushes and vines. The small nests of 
 house mice are placed in any sort of 
 shelter : under piles of sacks or paper, in 
 loose or baled hay, in boxes, in disused 
 clothing, inside upholstered furniture, 
 and even in office desks. When living in 
 fields, house mice nest in burrows. 
 
 Urine Stains 
 
 These are not usually easy to find, al- 
 though they are present in many places 
 inhabited by rats or mice, even on sacks 
 of food materials. Such stains fluoresce 
 under ultra-violet light. Unfortunately, 
 certain other materials are also fluores- 
 cent, but identification is possible by 
 trained inspectors. 
 
 METHODS OF CONTROL 
 
 The principal means of controlling rats and mice are (1) exclusion, (2) 
 
 traps, (3) poisoned baits, and (4) poisonous gases or dusts. Various other 
 
 control measures have been tried, but many are of little use. Exclusion is 
 
 the real solution, and all other methods give only temporary relief. 
 
 EXCLUSION 
 
 Exclusion of rats and mice means 
 essentially only two things: leaving 
 no opening more than Ys inch in 
 width into a building, or other 
 enclosure, and protecting all food 
 supplies from these animals (see 
 
 figs. 7-10). 
 House mice can pass easily through a 
 crevice % inch wide, and young rats 
 need little more space. If buildings are 
 constructed and maintained with these 
 two principles constantly in mind, no rats 
 or mice can find shelter or food inside. 
 Unfortunately this is not always done, 
 and in some places it is difficult to do. 
 
 In practice, exclusion of rodents re- 
 quires : 
 
 I) Concrete floors and exterior cur- 
 tain walls around foundations. 
 
 2) Solid exterior walls with no holes 
 or large cracks. 
 
 3) Elevation of small buildings 12 
 to 18 inches above ground. 
 
 4) Screens over all openings, both at 
 the ground level and above, of 
 hardware cloth of 1/3- or 1/4-inch 
 mesh, or metal grille with equally 
 small apertures. 
 
 5) Cement or metal sheathing 
 around all holes where pipes or 
 conduits pass through walls. 
 
 6) Tight-fitting doors and window 
 screens. 
 
 Rat proofing alone is not enough ; such 
 construction must be frequently inspected 
 and kept in good repair. Leaving un- 
 screened doors or windows open over 
 night, or failing to replace sheathing after 
 
 [13] 
 
a small plumbing job, can give rodents a 
 chance to reinfest the building. Crates, 
 baled goods, and other packaged ma- 
 terials entering the building should be 
 examined for rodents or young in nests. 
 
 Foundations 
 
 Rats burrow readily under concrete 
 slab floors laid on the ground. Any in- 
 terior openings in such floors, as around 
 sumps, may be used as entrances. Rats 
 also tunnel under the foundations of ex- 
 terior walls, even when the foundations 
 extend 24 inches below the ground sur- 
 face. To prevent such burrowing, a thin 
 curtain wall, L-shaped in section, may be 
 poured around the foundation or slab. It 
 need be only 2 or 3 inches thick. The foot 
 of the L should project outward to dis- 
 courage rats from tunneling down beside 
 the wall. 
 
 To make a curtain wall, dig a trench 
 12 inches wide and 24 inches deep around 
 the edge of the foundation or slab. Brace 
 an upright retaining form (thin boards 
 or even corrugated box cardboard will 
 do) with stakes, so that it stands 2 or 3 
 inches from the inner side of the trench, 
 and fill the space with concrete. Then 
 pour the foot of the L. Later, remove the 
 form and refill the trench with earth. 
 
 Exterior Walls 
 
 Concrete, masonry, or sheet metal walls 
 will exclude rats. When corrugated metal 
 sheets are used, mice may enter at the 
 bottom unless stops are added to close 
 the small semicircular spaces where the 
 corrugations join the foundation or sill. 
 Wooden walls can be kept rodent proof 
 if knotholes are covered, cracks battened 
 tightly, and the bottom boards are nailed 
 down securely. The lower edge should be 
 sheathed with strips of light weight (28 
 gauge) galvanized iron sheeting. 
 
 For outbuildings and farm structures 
 of frame construction on wooden studs, 
 single walls are preferable. Double walls. 
 
 especially if sheathed on both sides with 
 wood or wall board, provide ideal shelters 
 for rats and mice, as well as passageways 
 by which the animals reach upper floors 
 or attics. Old buildings with double 
 wooden walls often may be made rodent 
 proof by tacking strips of sheet iron along 
 the outside base and removing the lower- 
 most inside boards to place stops of one 
 kind or another as shown in figure 7. 
 
 Openings 
 
 Black and roof rats easily climb vines, 
 trees, and rose trellises against outside 
 walls, and can run along electric cables 
 or wires, or even the antenna for a home 
 radio. Rats of all kinds and mice can go 
 up rough stucco or board walls. They 
 seldom climb smooth walls of sheet metal 
 or finished concrete, but may find a foot- 
 hold on outside drains or rain ducts. Rats 
 may cut fly screen, particularly near the 
 ground or roof, although window screens 
 at intermediate levels are rarely damaged. 
 A top-hinged screen frame left unhooked 
 at the bottom may give rodents a chance 
 to get in. 
 
 All outside openings above ground 
 must be screened completely with hard- 
 ware cloth of 1/3- or 1/4-inch mesh. This 
 includes all attic ventilators and louvers, 
 tops of air shafts, elevator wells, rooftop 
 stairway entrances, windows, and vent 
 pipes. 
 
 Gaps left by plumbers and electricians 
 around pipes or conduits entering walls 
 (fig. 7,B) are often the commonest paths 
 of entry for rats and mice, and may be 
 closed in three ways : 
 
 1) In brick, tile, or concrete walls 
 the space is filled with concrete, 
 using reinforcing of old wire 
 screen if necessary (fig. 7,D). 
 
 2) On wooden walls a sheet-metal 
 collar is fitted closely around each 
 pipe, extending several inches be- 
 yond the edges of the opening, 
 and nailed to the wall (fig. 7,G). 
 
 [15] 
 
3) At entrances to steam tunnels, air 
 ducts, or other places where ven- 
 tilation is necessary and where 
 workmen must enter from time to 
 time, close-fitting but removable 
 hardware cloth screens, framed 
 with sheet metal, are bolted or 
 screwed into place. 
 
 Doorways 
 
 Every outside door should fit closely 
 in its frame when closed, and the clear- 
 ance over the floor or sill should never be 
 greater than % inch. A strip may be 
 nailed to the bottom edge if there is a 
 
 wider gap. Doors to restaurants, grocery 
 and feed stores, and granaries or feed 
 rooms should be sheathed along the bot- 
 tom and up each side, for 6 to 12 inches, 
 with a 3-inch width of galvanized sheet 
 iron, securely nailed (fig. 8). An even 
 better practice is to form U -shaped chan- 
 nels of sheet metal, of a width to fit 
 closely over the thickness of the edge and 
 bottom of the door, and fasten them with 
 wood screws. A rat or mouse needs a free 
 edge on a door to start gnawing and can- 
 not begin if the edge is covered with metal 
 or fits closely in the casing. 
 
 Sliding doors to warehouses seldom 
 fit tightly, and rats often enter in the 
 
 Fig. 8.— Permanent rat-proofing for windows and doors. Left, four folded channels of galvan- 
 ized sheet iron are bent to cover edges of hardware cloth (3 meshes per inch); a narrow lip 
 (3/16 to 1/4 inch) is bent out along one margin of each channel to stiffen the frame; the 4 
 channels are riveted together at the corners and the framed screen is fastened to the window 
 casing with screws. An enlarged detail of the channel is shown below. Right, three rectangular 
 channels are bent from galvanized sheet iron to fit closely around the bottom and lower edges 
 of a door and nailed or screwed into place. These channels add strength to the door, they prevent 
 damage to the lower edge, and they will stay in position much better than fiat strips of sheet 
 metal tacked to one side of the door. 
 
 [16] 
 
space between the door and wall. Often 
 strips of wood covered with metal can be 
 fitted to the doors or casings to keep them 
 out. 
 
 At receiving and shipping depart- 
 ments in the rear of stores, the main doors 
 must be open throughout the day and 
 often after dark. An extra pair of half- 
 
 rats and mice. Space below the floor gives 
 shelter for nesting, and the open con- 
 struction above makes food easy to ob- 
 tain. Old wooden cribs should be raised 
 18 inches above the ground, using con- 
 crete, field stone, or wood for supports. 
 The top, walls, and door should be cov- 
 ered with hardware cloth in addition to 
 
 Fig. 9.— Construction for a corn crib to exclude rats. The ends of studs and joists are nailed 
 together to resist lateral pressure when the crib is filled. Spaces between studs are closed by 
 wooden stops at the floor level. Hardware cloth is placed between joists and flooring and between 
 studs and outside slats. The whole crib is elevated on concrete, field stone, or wooden sills so 
 that the floor is at least 12 inches above ground. 
 
 height "Dutch" doors 3 to 4 feet high may 
 be installed to stop rodents. They should 
 be light in weight, sheathed completely 
 on sides and edges with smooth sheet 
 metal, and hung on double-swing spring 
 hinges. They should fit closely over the 
 floor and come together with no more 
 than % inch between the free edges. 
 Loaded warehouse hand trucks or dollies 
 can easily be pushed against and past the 
 doors, and the doors will close at once be- 
 hind the person pushing the truck. No rat 
 can run in at floor level or climb the 
 smooth surface of the doors. 
 
 Corn Cribs 
 
 The ordinary wooden corn crib, set on 
 the ground, provides a good home for 
 
 the wooden slats (fig. 9). New wooden 
 cribs should be built off the ground and 
 lined with hardware cloth. Prefabricated 
 cribs made entirely of sheet metal are 
 available and are used by many farmers, 
 who thereby practically end their rodent 
 troubles in this kind of storage. 
 
 Feed Storage 
 
 Large supplies of livestock feed in piled 
 sacks offer both food and shelter for rats 
 and mice. When feed is stored in ware- 
 houses or barns with open eaves, poorly 
 fitted doors, cracked or warped outside 
 walls, or floors of earth or of wood close 
 to the ground, there is often serious loss 
 of grain and damage to sacks. 
 
 Whenever possible, sacked materials 
 
 [17] 
 
Fig. 10.— Storage of garbage and rubbish. Above, poor; easy o£ access to mice, rats, and flies. 
 Below, good construction; the doors are covered with both hardware cloth and fly screen and 
 provided with spring hinges, thus excluding all of these pests. Courtesy of U. S. Public Health 
 Service. 
 
 [18 
 
should be stored in narrow piles with 
 lanes between, and an alleyway between 
 the outermost tiers and exterior walls. 
 Damage then will be lessened and any 
 rats and mice present can be controlled 
 more readily. 
 
 Rodent proof wooden bins may be 
 built to contain bulk materials. The in- 
 terior should be sheathed with tongue- 
 and-groove boards, or sheet metal. The 
 studs and other framing should be on 
 the outside. The supporting legs can be 
 sheathed with sheet metal to prevent ro- 
 dents from climbing, and the top needs 
 tight covering or screening. 
 
 Smaller stocks of feed for poultry can 
 be kept in large clean garbage cans cov- 
 ered with close fitting lids ; the daily sup- 
 ply can be easily removed with little 
 spilling. Cans holding 2 or 3 sacks of 
 feed are usually available. If the amount 
 of grain given to poultry at each feeding 
 is closely measured so that the birds will 
 clean it up rapidly, little will remain in 
 the troughs to attract rats and mice. 
 
 Temporary stacks of newly threshed 
 grain left in the field sometimes are at- 
 tacked by rodents, and sacks may be dam- 
 aged. Such outdoor stacks in Australia 
 are protected from rodents by a tight 
 "fence" of solid sheet iron about 36 inches 
 wide, of which 6 inches is set into the soil. 
 The corners and the lapping between 
 sheets must be kept tight. 
 
 Hay in bulk or bales is difficult to pro- 
 tect against mice, and often is heavily 
 infested, sometimes also with rats. Nests 
 in baled hay are a common means of 
 carrying infestations of mice into build- 
 ings. No solution for excluding rodents 
 from hay has been found except the sheet 
 iron fence just described. Use of baiting 
 stations (see section on Poisoned Baits), 
 and baits other than the food present, 
 such as barley in a barn containing oat 
 hay, will often reduce the number of ro- 
 dents, especially if such control is con- 
 tinued as the stock of hay is used or grad- 
 ually moved out. 
 
 Human Food Supplies 
 
 Grocery stores may reduce losses by 
 keeping small supplies of grain products, 
 sugar, and similar materials in metal con- 
 tainers. In homes and in store rooms ad- 
 jacent to farm kitchens the same means 
 may be used. Covered tin cans, glass 
 bottles, 2- to 10-gallon crocks, and even 
 small garbage cans are useful for flour, 
 cereals, dried fruits, nuts, etc. Such con- 
 tainers give protection from rodents and 
 make it easy to remove the contents with- 
 out spilling. 
 
 Cabinets and Enclosed Platforms 
 
 The "dead" spaces between walls and 
 floors and built-in cabinets, platforms for 
 refrigerators, and similar conveniences 
 in homes and stores make good hiding 
 places for rats and mice. Often there are 
 entryways into these spaces where work- 
 men have cut holes in fitting pipes and 
 conduits. When such shelters are used by 
 rodents, it may be necessary to remove 
 the false floors or back partitions and stop 
 all entry by tacking tin sheets or hard- 
 ware cloth over the openings. If the space 
 is on the first floor, some of this work may 
 be done in the basement, searching with 
 a flashlight and closing all entry ways. 
 
 Garbage, Boxes, and Trash 
 
 Incorporated cities and towns usually 
 require each residence or business place 
 to provide metal garbage cans for dispos- 
 ing of waste food and trash. But many 
 housewives, restaurant employees, and 
 storekeepers are careless (fig. 10). They 
 fail to cover the cans tightly or they over- 
 load them. Home and farm yards, and 
 the alleys behind both homes and stores 
 often have irregular heaps of boxes, 
 lumber, or trash. The open garbage cans 
 provide food and the piles of debris offer 
 shelter. The practice on many farms of 
 throwing all table scraps into pens for 
 
 [19] 
 
.">^?*^<?^- 
 
 & 
 
 U.m, 
 
 Fig. 11.— Traps for rats and mice. A, wooden snap or breakback rat trap; the same type in 
 smaller size is used for mice; B, steel jump trap size (the leather padded jaws are not neces- 
 sary); C, Schuyler trap (made in both large and small sizes); D, all steel quickset mouse trap; 
 E, 4-hole choker-loop mouse trap; F, wire cage trap, 2 sizes available. 
 
 20 
 
chickens or hogs helps to support both 
 rodents and flies, particularly if the pens 
 are seldom or never cleaned up. To keep 
 down rodents, dispose of refuse promptly 
 and properly. 
 
 Empty boxes and stocks of lumber 
 should be piled compactly and neatly on 
 trestles or other supports that keep the 
 bottom 12 to 18 inches above ground. 
 Trash piles, brush heaps, old prunings 
 from fruit trees, bundles of corn stalks, 
 and all such litter should be burned. 
 Large piles of firewood, if not in separate 
 tiers, may give shelter for rats— and in the 
 country, also for ground squirrels. 
 
 The most effective way to control 
 rats is to build them out and starve 
 them out. The houseowner, store- 
 keeper, or farmer, should inspect his 
 property, keeping in mind the methods 
 of exclusion just discussed, and then 
 repair his premises. In many cases, a 
 little work, using materials already at 
 hand, will result in keeping all rodent 
 pests out of buildings. 
 
 TRAPS 
 
 Trapping is the preferred method 
 of control in homes and office 
 buildings, because animals killed 
 in this way may be easily removed, 
 whereas poisoned rodents may die 
 within walls or in other inacces- 
 sible places and create bad odors. 
 Almost any trap will catch some 
 rats, but long experience proves 
 that the ordinary spring snap trap 
 is best, and cheapest. 
 
 The many kinds of traps designed and 
 manufactured comprise four main types: 
 wooden or steel "breakback" or guillo- 
 tine traps, steel jaw traps, cage traps, and 
 automatic traps (fig. 12) . Results depend 
 more on the number of traps used and 
 on the way in which they are placed 
 than on the type of trap used. Contrary 
 to popular opinion, human odor from 
 handling traps will not keep rats or mice 
 
 away; it is not necessary to wear gloves 
 or to boil, wash, or smoke the traps. It is 
 important, however, to keep traps in good 
 working order and to set each trap with 
 a "hair trigger" so that the least touch 
 will release the spring. 
 
 Trapping House Mice 
 
 The small breakback arid choker-loop 
 traps are used where there are signs of 
 mice: tracks, droppings, holes, or dam- 
 age. In buildings, place traps along the 
 wall, with trigger end toward the base- 
 board, or against boxes or other objects, 
 at intervals of 2 to 3 feet. Many traps 
 should be used if mice are abundant. If a 
 mouse hole is found, put a trap about 9 
 inches on either side. 
 
 Rolled oats are almost always effective 
 as bait (cheese is not!). Bread crumbs, 
 oatmeal, or other cereals may be tried. 
 Meat or fish baits are seldom necessary. 
 A little loose bait should be placed under 
 the trigger, to catch mice skillful at rob- 
 bing traps. The "set" of the trigger should 
 be delicate. A string or light wire from 
 the trap to some nearby object will pre- 
 vent a prowling cat from carrying the 
 trapped mouse away, and will also keep 
 a mouse caught by the tail or foot from 
 crawling into hiding. 
 
 Trapping Rats 
 
 Rats are considered somewhat "wiser" 
 than mice, yet many are taken with 
 wooden snap traps or steel traps. Trap- 
 ping is most useful in homes, offices, and 
 generally in places where there are few 
 rats. If there is a heavy infestation, other 
 control methods should be used first, and 
 traps placed to kill those remaining. 
 
 The wooden-based spring trap is gen- 
 erally used, but some trappers employ 
 the No. steel trap. If traps are set and 
 baited at once, some rats will be caught. 
 However, better results will be had if the 
 traps are placed and baited, but left unset 
 for 3 to 5 nights, then baited and set. This 
 
 [21] 
 
practice overcomes the rats' fear of a new 
 object. 
 
 Traps should be placed along runs or 
 where there is other evidence of rats, 
 spaced at intervals of 6 feet or more, with 
 the trigger ends close to the wall. Each 
 trap should be fastened to some nearby 
 object by a cord or light wire about 2 
 feet long. 
 
 In places where regular rat runs are 
 found, baited traps are usually set 2 or 3 
 feet off the trails. If no rats are taken, then 
 traps, baited or unbaited, are set with the 
 trigger directly across the runway. 
 
 When rats are using overhead pipes 
 or beams, traps are braced or fastened 
 on the runs with the trigger across the 
 line of travel. The best method to support 
 such traps is to drill a hole in the trap 
 bottom near the trigger, then drive a 
 finish nail into the beam or twist a wire 
 around the pipe, leaving a short upstand- 
 ing end of wire. The hole in the trap is put 
 over the nail or wire end. A soft wire from 
 the other end of the trap is fastened to 
 some object below the level of the run- 
 way. When sprung, the trap and rat will 
 bounce off the support and hang from 
 the wire, leaving the runway free for other 
 rats to find other traps similarly placed. 
 
 When a trap is plaped on the ground, 
 the base may be countersunk in the soil 
 so that only the trigger projects above 
 the surface. Indoors or out, for especiaTy 
 wary rats, it is often helpful to cover a 
 trap with sawdust, fine soil, or other cam- 
 ouflage that does not interfere with the 
 trigger or action. 
 
 To increase the chance of catching anv 
 rat that crosses a trap, some workers add 
 a trigger card, a piece of corrugated card- 
 board, about 2 X 2% inches, forced over 
 the trigger to increase its effective sur- 
 face. 
 
 Rats that ignore an exposed trap will 
 often be caught if a board or box is placed 
 to shield the trap, leaving a space for 
 them to enter and for the trap spring to 
 operate. 
 
 Rat baits may be of almost any human 
 food. In many places these rodents de- 
 velop special preferences for the kinds of 
 food most readily available. In general 
 the Norway rat seems to prefer protein 
 and fatty foods, whereas the roof and 
 black rats seem to have greater liking for 
 cereals and fruits. 
 
 Sometimes a trial with several kinds 
 of baits will indicate what the local rats 
 prefer. Rolled oats and other cereals, 
 corn meal, and bread crumbs may find 
 favor; ground meat, bacon, fish, apples, 
 and raisins also are used. When any finely 
 divided bait is used, some should be scat- 
 tered under the trigger as well as on top. 
 Solid materials should be pressed onto or 
 tied to the trigger. If the material spoils 
 or dries, baits not taken should be re- 
 placed the next day, but dry cereals can 
 remain longer. 
 
 Traps should be examined at least once 
 each day, preferably in the morning. 
 Dead rats should be removed promptly 
 and buried at least 2 feet deep, burned in 
 an incinerator, or placed in a tight gar- 
 bage can for early removal. Since fleas 
 and mites will leave dead rats and may get 
 onto people, the trapper should handle 
 dead rats as little as possible, and should 
 wear gloves. 
 
 Cage and box traps that capture one 
 rat at a setting are provided with 1 or 2 
 drop doors. Some have an overhead trig- 
 ger on which the bait is fastened and the 
 floor is released when a rat works at the 
 bait. Others have a treadle in the floor on 
 which the rat steps to drop the door. The 
 "automatic" traps are intended to catch 
 more than one rat. When one enters, a 
 balanced door closes behind the animal, 
 preventing its escape. As the rat moves 
 into another compartment the outer door 
 reopens. Several types of electric or 
 "electronic" traps have been offered for 
 sale or lease. None of these special traps 
 has proved more efficient than a good 
 supply of snap traps, and all are much 
 more expensive. 
 
 [22] 
 
Fig. 12.— Placing traps and ANTU dust for rat control. The steel trap (unbaited) is set with its 
 trigger pan on the same level as the jaws, the spring is turned at a slight angle with the trap 
 axis, the trap chain is nailed to some solid object, and the trap is placed where rats have been 
 running, as indicated by black smears on the pipes. One wooden trap, baited with cut apple, is 
 set near the run on a flat surface used by rats, as shown by the droppings. Other kinds of bait 
 may be used. A thick layer of white dust containing ANTU has been sprinkled on the edges of 
 timbers traveled by rats; any Norway rat that evades the traps but walks through the dust is 
 likely to be poisoned. It is usual practice either to set traps or put down dust patches but not to 
 combine these two methods at one site. Courtesy of U. S. Public Health Service. 
 
 POISONED BAITS 
 
 Poisons 
 
 The chief method of routine control 
 by all government agencies, commercial 
 pest control operators, and many others 
 is use of poisoned food or baits. The 
 principal poisons are red squill, zinc 
 phosphide, and ANTU, although ar- 
 senic, barium carbonate, phosphorus, and 
 strychnine have been used. Many other 
 substances have been tried with little or 
 no success. Thallium sulfate and Com- 
 pound 1080 are both effective poisons, 
 but they are too dangerous to be used by 
 the general public. 
 
 Except for red squill, all these poi- 
 sons are dangerous to man, pets, and 
 domestic animals. Stocks of poison and 
 
 poisoned bait and all equipment used 
 with them must be labeled POISON 
 and must be securely locked up when 
 not in use. Care also must be used in 
 handling and placing poisoned baits 
 and in the disposal of unused baits and 
 of poisoned animals. 
 
 Many types of commercial rat and 
 mouse poisons are on sale. In California, 
 all poisons and poisoned baits (economic 
 poisons) for pest control and for sale to 
 the public must first be submitted for 
 registration to the State Department of 
 Agriculture (Calif. Agric. Code, 1945, 
 Sees. 1061-1079), and only those which 
 are satisfactory are registered for sale 
 within the state. The printed label on each 
 package must include the name and per- 
 centage of active ingredients, a statement 
 
 [23 
 
about the poison, and information about 
 antidotes. The Federal Insecticide, Fung- 
 icide, and Rodenticide Act of June 25, 
 1947, provides that all rodent poisons in- 
 tended for interstate commerce must be 
 similarly registered and labelled. Licens- 
 ing does not guarantee that a material will 
 always provide effective control, but it 
 eliminates some ineffective rodent poisons 
 formerly sold, such as those sometimes 
 advertised in magazines for direct sale 
 to users. 
 
 Baits 
 
 Rolled oats and other cereals, corn 
 meal, bread crumbs, and diced bread are 
 the commonest baits. In England dry 
 stale bread soaked in water until mushy 
 is used successfully. Wheat soaked over- 
 night in water and then drained has 
 served with some poisons. Addition of 
 mineral oil, corn oil, molasses, peanut 
 butter, or tomato puree to bread or cereal 
 has been advised by the U. S. Fish and 
 Wildlife Service (Silver and Garlough, 
 1941). 
 
 Trials on the relative attractiveness of 
 different baits in the northeastern states 
 showed preferences in decreasing order 
 to be: raw meat, raw fish, rolled oats, 
 whole wheat, corn meal, bread crumbs, 
 canned fish, canned meat, cooked cereals, 
 cheese, meat scrap, powdered milk, fish 
 meal, fresh vegetables, cooked vegetables, 
 and fresh fruits (O'Conner et at, 1935). 
 Rats in different localities and in places 
 where one or another kind of food is most 
 available often definitely prefer particu- 
 lar foods. Several kinds of bait should be 
 tried to determine what the rats like best. 
 Wet baits must be prepared and used 
 fresh each day. Meat or fish in dry or hot 
 places will soon gloss over and spoil. 
 
 Prebaiting 
 
 For many years it has been the general 
 practice simply to put down baits in places 
 frequented by rats, leave them for one 
 or more nights, and then remove the un- 
 
 eaten baits. Some rats were killed, but 
 results were usually unsatisfactory. A re- 
 cent major improvement is the use of 
 prebaiting (Bartlett, et al, 1946). Small 
 piles of clean unpoisoned baits are placed 
 at selected sites or in special bait con- 
 tainers for several nights, the supply be- 
 ing replenished whenever rats take the 
 material. The prebait is then replaced by 
 poisoned food of the same kind, which is 
 left 1 to 3 nights, and then the remainder 
 removed. 
 
 Prebaiting takes account of an impor- 
 tant feature in rat behavior— a suspicious 
 fear and avoidance of new objects, even 
 a new food. If clean prebait is offered, 
 they will, after one or more nights, over- 
 come their fear and usually accept the 
 new food. Also, when sites are prebaited 
 for several nights in succession, increas- 
 ing numbers of rats will come to feed at 
 these places. Then when poisoned bait is 
 substituted, a larger number will be 
 killed. 
 
 A convenient program is to prebait on 
 the first, third, and fifth days, replace 
 with poisoned bait on the sixth day, and 
 remove all remaining poison and dead 
 rats on the eighth day. If acceptance of 
 the prebait is poor, the prebaiting period 
 should be extended for several days. The 
 largest take of poisoned bait will be on the 
 first night it is exposed; a little may be 
 taken the second night, but practically 
 none thereafter. 
 
 Prebaiting may provide a crude 
 method of estimating the number of rats 
 using bait stations. A given weight of 
 prebait— say 2 ounces, which may be 
 measured by bulk with a suitable spoon- 
 is put out at each station. On succeeding 
 days, if much is eaten, the station is re- 
 filled, doubling the amount each time 
 (4, 8, 16 ounces, etc.), and this practice 
 is continued until the amount taken 
 "levels off," when a maximum number of 
 rats will be eating at the station. 
 
 Assuming each animal eats about 1 
 ounce (25 to 30 grams), one may esti- 
 mate the number of rats. Then after 
 
 [24] 
 
poisoned bait has been substituted, a 
 guess as to the number killed can be 
 made, on the basis of 1/3 ounce (10 
 grams) per rat, since the animals eat a 
 smaller amount of poisoned food. A week 
 or two after poisoning, an estimate of 
 those remaining can be made by using a 
 new bait (unpoisoned) and determining 
 the amount removed. 
 
 Placing Baits 
 
 Both prebait and poisoned bait should 
 be carried in a can or bag and placed with 
 a long-handled spoon. Baits may be put 
 directly into rat burrows, in rat-infested 
 spaces under buildings, or in crevices 
 about foundations or walls. In buildings, 
 baits may be placed directly on floors, but 
 should not be set out on boxes, packages, 
 or sacks containing human food or ani- 
 mal feed. To avoid spilling and waste, 
 baits sometimes are placed on small sheets 
 of paper or in shallow paper or metal 
 cups. The baits should be put down near 
 walls, beside piles of boxes or sacks, and 
 in any other places where there is evi- 
 dence of rats. It is important to distribute 
 enough baits so that all rats will have 
 access to one or more. 
 
 Several kinds of special containers have 
 been designed (figs. 13, 14) for safer 
 exposure of poisoned baits. Most of these 
 are built so that only rats and mice may 
 enter. If one of the more complex types is 
 used, the containers must be placed, pre- 
 baited, and left for some days, even for 2 
 weeks, until rats enter freely. Then a regu- 
 lar schedule of prebaiting may be started. 
 
 Be sure to keep poisoned material 
 away from children, pets, and poultry. 
 
 If the first poisoning is successful, it 
 need not be followed by another for about 
 3 months, depending on the amount of 
 new "sign" of rats that appears. When 
 the number remaining is small, trapping, 
 with a new bait, may be tried. 
 
 Should one campaign fail to kill 
 enough rats, a follow-up campaign may 
 be started about 2 weeks later. A new 
 
 bait and new poison must be used, be- 
 cause any rats made ill but not killed by 
 the first attempt are likely to be "preju- 
 diced" against the bait and poison first 
 used. After a good cleanup, rat proofing 
 is in order. 
 
 Dead animals and uneaten bait 
 should be burned, or buried at least 
 2 feet deep. They should not be put 
 into garbage^ since garbage often 
 serves as feed for hogs, which would 
 thereby be poisoned. 
 
 The real measure of success is not the 
 number of rats found dead after a poison 
 campaign, but the number that escape. 
 For good results, 90 per cent or more 
 must be killed; otherwise the young in 
 nests and those developing in surviving 
 females will soon repopulate the area 
 treated. 
 
 Formulas 
 
 Some poisoned baits may be bought 
 already prepared, but if these are used, 
 prebaiting is not possible. Poisons are 
 sold by dealers in chemicals and in garden 
 and orchard supplies, and by some drug 
 stores. Bait materials can be purchased 
 from groceries and feed stores. To use 
 the prebaiting method it is necessary to 
 buy the bait and poison separately and 
 prepare the poison mixture according to 
 one of the following formulas. 
 
 Red Squill 
 
 This rat poison is obtained from an 
 onion-like plant {Urginea maritima) na- 
 tive to lands about the Mediterranean Sea. 
 At present there are some experimental 
 plantings in this country. The large (5 to 
 6 pound) bulbs are sliced, dried, and 
 ground to a fine reddish powder. This 
 powder^ keeps well when stored in a 
 tightly-capped can or bottle. 
 
 ^ Different batches of squill bulbs yield powder 
 varying widely in toxicity. For effective control, 
 rats should be killed by 500 to 600 milligrams of 
 red squill per kilogram of live rats as determined 
 by laboratory test (bioassay) . Most or all red 
 squill powder now sold in the United States 
 meets this specification, since squill of lower 
 toxicity is fortified until of proper strength. 
 
 [25 
 
Fig. 13.— For legend, see facing page. 
 
Of all rat poisons, red squill is the 
 safest for use by the general public, but 
 is not always the most efficient. Experi- 
 ments show that cats, dogs, chickens, 
 pigeons, and hogs usually will not eat 
 squill preparations, or will quickly vomit 
 any they do eat. Poisoned baits, however, 
 should not be put where children or pets 
 can find them. 
 
 Formula 1 
 
 Bait (cereal, bread crumbs, fish, or ground 
 
 meat, etc.) 9 parts by weight 
 
 Red squill, powdered 1 part by weight 
 
 To measure dry materials by weight, 
 use 1 level tablespoon of red squill to 1 
 measuring cup of bait. Sift the dry powder 
 over the bait, turning and mixing until 
 the poison is well distributed. 
 
 Commercially prepared red squill baits 
 are available as a dry mix of poison and 
 bait, either loose or twisted in small 
 squares of paper as "torpedos." Other 
 squill preparations are baked as small 
 
 biscuits or wafers. Such products often 
 are issued by city health departments to 
 citizens for rat control about homes; sat- 
 isfactory prebaiting is not possible with 
 these. 
 
 A liquid red squill extract is also avail- 
 able, but generally it seems less efficient 
 than the powder. One pint of liquid serves 
 to moisten % pound of bait (meal, or 
 stale bread cut in V2-iiich cubes) . 
 
 Zinc Phosphide 
 
 This chemical (ZngPg) is a heavy dark 
 gray powder with a faint odor of phos- 
 phorus resulting from the slow release of 
 phosphine (PH3). Both the powder and 
 gas are serious poisons. Zinc phosphide 
 should be weighed, mixed, and handled 
 only out-of-doors or in a well-ventilated 
 room, and workers should wear leather, 
 or rubber, gloves when mixing or dis- 
 tributing poison baits. 
 
 Fig. 13.— Containers for safe exposure of baits used to poison rats. 
 
 A. Leaning board. Provides tunnel under which rats may eat; if space beneath is narrow, large 
 cats cannot reach the bait; also may be placed over sodium fluosilicate dust (p. 31) to increase 
 chance of rats running through the dust, and prevent them from scattering the material. 
 
 B. Box shelter. Narrow box, 2 sides removed and ends cut to make 3 x 3-inch opening; will 
 shelter either bait or trap. 
 
 C. Sheet metal hood. Useful outdoors to protect bait from rain. Board about 8 x 14 inches with 
 half cylinder of sheet metal (14 x 18 inches, or sides of two No. 10 cans); small can 3 or 4 inches 
 in diameter by 1 inch high holds the bait; used by Navy in Pacific Ocean areas (T. B. Murray, 
 U. S. Naval Med. Bui., 46: 1318, 1946). 
 
 D. Bait box. Of M-inch wood, open at both ends with door in middle of top on leather hinge; 
 used on garbage dump; too small for cats to enter {Coogan, Public Works, 75:25, Sept., 1944). 
 
 E. Bait box. Of wood; about 12 inches on each side, but dimensions may be varied; entrances 
 about 3x3 inches; diagonal cleat on inside floor forms bait enclosure; slat at back for nailing 
 on wall; not easy to remove prebait (/. Silver and F. E. Garlough, 1941, p. 15). 
 
 F. Metal bait box. Made of 28-gauge galvanized iron; rear door hinged at bottom to insert bait 
 and clean; about 12 inches long and 8 inches on side (W. S. Mangold and George King). 
 
 G. Drain tile or sewer pipe. Useful on garbage dumps; also may be used where poultry are 
 present, if open end is faced within 3 inches of a wall; pipe is 3 inches in diameter and 24 inches 
 or more in length; one end plugged with wood or concrete; closed end may be partly buried in 
 garbage dump or embankment; bait should be placed well inside by use of long spoon (A. D. 
 Middleton, Rat control on farms. Great Britain Ministry of Fisheries and Agriculture, 1945). 
 
 H. P3 {protected poison point). Special box of wood, Ys- to i^-inch thick (or of galvanized sheet 
 iron); removable cover (not shown) fits over top; box when placed against a wall forms a tunnel 
 without floor; rat must travel in 3 directions to reach bait inside on bottom, below small ledge; 
 large enough for several rats to feed at one time; poultry or pets cannot reach into bait com- 
 partment; lid may be fastened by inside spring to prevent opening by children, and box may 
 be bolted to floor or wall; bait not easily spilled if box is overturned, yet prebait can be removed 
 easily (British Patent, No. 541,844; design by Charles Elton and others. Bureau of Animal Popu- 
 lation, Oxford, England; see also S. A. Barnett, et al. Rats and Mice, Great Britain Ministry of 
 Food, pp. 13-17, 1946). 
 
 [27] 
 
ANTU 
 
 Fig. 14.— Poison station lor house mice (using 
 drinking fount for baby chicks). Poisoned 
 grain or seeds in the inverted jar fiow into the 
 open-topped metal base, where mice can feed. 
 The jar should be labelled POISON. This 
 device should be used only in an attic or store- 
 room securely locked to prevent entry by chil- 
 dren, unauthorized persons, or animal pets, 
 and where the odor of dead mice will not be 
 objectionable. 
 
 Bait 
 
 Zinc phosphide, 
 powdered 
 
 Formula 2 
 
 96 to 100 parts by weight 
 1 part by weight 
 
 Corn oil, mineral oil, or glycerin, at 12 
 to 24 fluid ounces per 100 pounds of bait, 
 is often added to any dry cereal to make 
 the poison adhere. The bait and poison 
 are first mixed dry until the zinc phos- 
 phide becomes an evenly distributed coat- 
 ing. Then the oil, well warmed, is added 
 slowly and the mixture is stirred again. 
 Baits may be cereal or bread crumbs alone 
 or may include some fresh fish, horse 
 meat, or hamburger. Ground apple or 
 carrot are sometimes used. Stale bread, 
 without an oil, but mixed with an equal 
 amount of water, has been used with 2% 
 to 5 per cent zinc phosphide in England. 
 
 This new poison (alphanaphthyl- 
 thiourea), has proved useful for control 
 of Norway rats. It is much less toxic for 
 black or roof rats, house mice, or field 
 rodents, and other poisons should be used 
 for those animals. ANTU will not seri- 
 ously poison poultry and man, but may 
 kill cats, dogs, and pigs, and should be 
 handled carefully (Richter, 1945; Ward, 
 1946). 
 
 ANTU is a fine bluish-gray powder, 
 insoluble in water, that keeps well when 
 stored dry. It may be mixed with wet or 
 dry foods, and adheres well when dusted 
 from a sifter on cut moist baits of apple, 
 sweet potato, cantaloupe, watermelon, or 
 on ground meat, ground fish, or chicken 
 heads. 
 
 Formula 3 
 
 Bait 
 ANTU 
 
 100 parts by weight 
 2 or 3 parts by weight 
 
 Arsenic 
 
 This heavy white powder (also known 
 as arsenic trioxide, arsenious acid) has 
 often been used for rat control. It is now 
 less in favor in the United States than in 
 England and elsewhere. The arsenic must 
 be of very fine particle size (6 to 9 mi- 
 crons) for good results, since the larger 
 particles are less toxic. It should be well 
 refined because impurities make arsenic 
 less acceptable to rats. The laws of some 
 states and countries require that arsenic 
 be colored blue or green to distinguish it 
 from flour, soda, and other harmless or 
 edible white powders, but this does not 
 affect its toxicity. 
 
 Formula 4 
 Bait 90 parts by weight 
 
 Arsenic trioxide 10 parts by weight 
 
 Arsenic has been used with various dry 
 and moist baits. In preparing and dis- 
 tributing baits, workers should be care- 
 ful to keep arsenic off the skin, which is 
 irritated by the chemical. 
 
 [28] 
 
Barium Carbonate 
 
 This heavy, fine white powder has been 
 largely discarded in favor of more recent 
 discoveries. It is cheap but not always 
 easy to buy. 
 
 Formula 5 
 Bait 4 parts by weight 
 
 Barium carbonate 1 part by weight 
 
 Corn meal, rolled oats, bread, and 
 ground meat or fish have served as baits. 
 Dry materials are first mixed with the 
 barium carbonate, then moistened. Bread 
 baits may be moistened with milk; 
 "sloppy" baits may be more effective in 
 hot weather. Fresh foods should have the 
 dry barium carbonate sifted over them, 
 and should be stirred until evenly coated 
 or mixed with the poison. 
 
 Strychnine 
 
 Both alkaloid and sulfate strychnine 
 have been tried for rat control, but results 
 are often unsatisfactory, since some rats 
 will not take baits with this poison and 
 others eat but little. House mice will often 
 take strychnine-coated wheat, and this 
 bait has been used especially for mice in 
 outbuildings. In residences, trapping 
 mice is better practice. 
 
 Yellow Phosphorus 
 
 This is the basis of many commercial 
 rat and mouse poisons, and is often sold 
 in a sirup to be spread on baits of bread 
 and other foods. Some of these prepara- 
 tions seem quite effective. 
 
 Phosphorus has a distinctive, garlic- 
 like odor, and is luminous in the dark, 
 but these features do not prevent rats 
 from eating phosphorus baits. There is 
 a fire hazard from phosphorus probably 
 only when the particles are larger than of 
 microscopic (colloidal) size. 
 
 No directions for making phosphorus 
 preparations are given here because the 
 process is too complicated. Children and 
 even adults sometimes eat phosphorus- 
 poisoned sirup by mistake, with serious 
 
 or fatal results. Hence this rodent poison 
 is more of a hazard in homes than other 
 types. 
 
 Special Poisons 
 
 Two other rodent poisons are not avail- 
 able to the general public because of the 
 great hazards they involve. Baits poisoned 
 with these chemicals have no distinctive 
 appearance, taste, or smell. Both are toxic 
 to rodents, other wild animals, domestic 
 livestock, and man. 
 
 Thallium sulfate (thallous sulfate, 
 TI2SO4) is a "heavy metal" poison ob- 
 tained from smelters. It has been used by 
 government agencies for control of both 
 rats and field rodents. Thallium poisoned 
 grain has several times improperly come 
 into the hands of persons who, not know- 
 ing it was poisoned, used it for food. A 
 number of illnesses and several deaths re- 
 sulted. Sale or possession of thallium for 
 rodent control in California is restricted 
 by law to federal, state, county, and mu- 
 nicipal officers and employees (Calif. 
 Agric. Code, 1945, Sec. 1066.5) . 
 
 Since 1945, a new wartime discovery, 
 "1080" (sodium fluoroacetate), has been 
 used by military and government agencies 
 for rodent control (Kalmbach, 1945; 
 Ward, 1946) . It is very effective, but ex- 
 tremely dangerous. Pet dogs and cats may 
 be poisoned by eating rats or mice killed 
 by "1080." 
 
 These poisons can only be used by 
 properly trained persons and in places 
 where there is little chance of secondary 
 poisoning. 
 
 There is no certain antidote for either 
 of these poisons. Thallium is a slow- 
 acting poison, but "1080"' works so rap- 
 idly that it has been impossible to save 
 experimentally poisoned animals even 
 with prompt first aid under the best of 
 laboratory conditions. 
 
 Deodorants for Dead Rats 
 
 It is often claimed by manufacturers 
 of commercial rat poisons that their prep- 
 
 29 
 
arations will "embalm the dead rats," or 
 cause them to have no odor, or that rats, 
 when poisoned, will leave a building to 
 find drinking water. None of these claims 
 has been proved. 
 
 Rats dying within a wall often produce 
 persistent offensive odors. Powdered ac- 
 tivated charcoal blown into the place is 
 reported to be the most effective deodor- 
 ant. Other substances recommended are 
 chloride of lime and solutions of lead 
 nitrate, lead acetate, or formaldehyde. A 
 commercial product, Hyamine 1622, is 
 reported to reduce such odors. 
 
 POISONOUS GASES 
 
 Many rats in burrows or hidden in 
 ships, buildings, and other en- 
 closures can be killed by use of 
 poisonous gases. Any careful per- 
 son may use this method out-of- 
 doors, but within buildings gas is 
 more dangerous, and should be 
 used only by a trained official or 
 licensed pest control operator 
 wearing a suitable gas mask. All 
 of the gases used are poisonous to 
 man and domestic animals. Take 
 care to avoid accidents. 
 
 Calcium Cyanide 
 
 This compound, Ca(CN)o, is the com- 
 monest material used for gassing. In the 
 presence of moisture (water in the air or 
 soil of rat burrows) , this chemical forms 
 hydrocyanic acid gas (HCN). Both cal- 
 cium cyanide and the gas are deadly 
 poisons for animals, insects, and man. In 
 parts of the arid West and Southwest dur- 
 ing the dry season there may not be 
 enough moisture for successful use of 
 cyanide, and cracks in the ground may 
 permit quick escape of the gas, which is 
 lighter than air. 
 
 Calcium cyanide is available both as a 
 dust (Cyanodust A) and in granular 
 form (G Fumigant). The dust is applied 
 with a special pump (fig. 15). Air is 
 
 forced through a glass jar which contains 
 the dust, and the dust-laden air passes 
 through a hose into the burrow. The tip 
 of the hose is placed 10 to 12 inches in- 
 side the burrow, the entrance is closed 
 with earth, and several strokes are made 
 with the pump. If dust is seen coming out 
 of hidden "bolt holes" (fig. 6) or through 
 cracks in the ground, such openings 
 should be covered with earth. The dust 
 pump produces a high concentration of 
 cyanide gas in the burrow, but only for 
 a short time. 
 
 Corn cribs may be temporarily rid of 
 rats by use of a dust pump. There is no 
 danger in using corn from fumigated 
 cribs after several days of damp air, 
 since the poisonous gas is generated 
 rather quickly and the residue consists 
 only of lime. 
 
 When using granular cyanide, place 1 
 tablespoon (one ounce) of the material 6 
 to 8 inches inside the burrow with a long- 
 handled spoon, and block the entrance 
 tightly, taking care that the cyanide is 
 not covered. If there is moisture within 
 the burrow, a strong concentration of 
 cyanide gas will develop just inside the 
 blocked entrance, where it will kill any 
 rat trying to dig out. In damp weather 
 the cyanide may be placed during the 
 afternoon. When the temperature is low, 
 the material should be put into burrows 
 during the morning to give time for the 
 gas to form. This method is not effective 
 on garbage dumps, under piles of trash, 
 or in places where the ground is cracked, 
 as the gas will leak away too rapidly. 
 
 Cyanide in any form must be handled 
 carefully. The material should not be used 
 indoors, or for burrows under or near a 
 building where people, livestock, or poul- 
 try are present. Control should not be at- 
 tempted during a strong wind. In opening 
 cyanide cans, loading a pump, and using 
 the pump or spoon, the operator should 
 stand to the windward and should avoid 
 exposure to dust or fumes. Cyanide read- 
 ily dissolves in water; it can be used when 
 the air is damp, but not during a rain or 
 
 [30] 
 
when the ground is wet. The supply can 
 should be covered tightly except when 
 cyanide is being removed. The chemical 
 should always be kept off the operator's 
 hands and clothing. 
 
 Other Gases 
 
 No other gas is so widely used as cya- 
 nide for rat control. Carbon disulfide 
 (CS2) applied with a pump or on waste 
 balls, as for control of ground squirrels, 
 can be used against rats. Fluid carbon 
 disulfide burns readily, and the vapor is 
 highly explosive. It should not be used 
 where there is any fire hazard from open 
 flames, cigarettes, electric sparks, or other 
 sources. Details on the use of carbon di- 
 sulfide are given in California Agricul- 
 tural Extension Circular 138, "Control of 
 Field Rodents in California." 
 
 Sulfur dioxide (SOg), produced by 
 burning sulfur in air and forcing the 
 fumes into burrows, is of some value, but 
 now is seldom used. 
 
 Methyl bromide (CH,Br) is a highly 
 volatile fumigant occasionally used for 
 burrows, especially where killing fleas 
 for disease prevention is necessary, but 
 it requires special equipment for use. 
 
 Carbon monoxide (CO) from an auto- 
 mobile exhaust may be forced through a 
 hose into rat burrows. The pressure from 
 the engine forces the gas into all parts of 
 a burrow. Unlike some other gases, car- 
 bon monoxide remains in a burrow some 
 time, not being absorbed in the soil or 
 soil water; hence it has a long period of 
 effectiveness. This gas may be used for 
 burrows under cement-floored farm build- 
 ings where cyanide would be dangerous 
 to livestock. 
 
 POISONOUS DUSTS 
 
 If sodium fluosilicate (NaoSiFg) is 
 spread on floors where rats and mice run, 
 some of the dust gets on to their hair and 
 feet, proves irritating, and is licked off 
 and swallowed (Mackie, et al, 1934) . The 
 
 f4. 
 
 'i^^'^F' 
 
 Fig. 15.— -Use of pump with calcium cyanide 
 dust to gas rat burrows. Courtesy of U. S. Pub- 
 lic Health Service. 
 
 rodents die in 3 to 6 days. This chemical 
 contains fluorine and must not be used 
 where it might contaminate food. The 
 dust should not be stirred up, and under 
 no circumstances should kernels of cereals 
 on floors dusted with fluosilicate be swept 
 up and used as food for man or animals. 
 
 The law states that no food product 
 may carry a residue of fluorine in excess 
 of 0.019 grain per pound. Products found 
 to contain more may be seized by govern- 
 ment authorities, and the owners may be 
 prosecuted. 
 
 In some places sodium fluosilicate may 
 be put down in a narrow strip on the floor 
 close to the walls in a room or building. 
 Then boards may be leaned along the 
 floor and against the wall to prevent the 
 dust from being scattered to nearby ob- 
 jects. Rats and mice usually run along 
 the bases of walls, and tend to go beneath 
 boards so placed. 
 
 31 
 
ANTU, up to 20 per cent, when mixed 
 in flour, pyrophyllite, or talc, may be 
 dusted heavily (% inch or thicker) on 
 rat runs and entrances to burrows for 
 control of Norway rats. It has little value 
 for roof or black rats. This practice is not 
 a major means of control, but has been 
 used to dispose of a few rats that escape 
 death by poison, traps, or gas. 
 
 In areas where murine typhus is a haz- 
 ard, DDT dust (5% to 10%) is placed 
 on runways to catch on the feet and fur of 
 passing rats and kill many of their fleas. 
 When the number of fleas, which carry 
 endemic typhus to man, is reduced, there 
 is less danger of spreading the disease 
 when the rats are killed off. 
 
 Any of these dusts may be applied with 
 a sifter can. When much dusting is to be 
 done, however, it is helpful to have a 
 large (1 gallon) can with fly screen sol- 
 dered on one end and a carrying handle 
 on the side. For dusting rat runs on beams 
 and pipes, a small sifter can fastened at 
 right angles to the end of a 3- or 4-foot 
 stick is convenient. 
 
 OTHER MEANS OF 
 CONTROL 
 
 Destruction of Burrows 
 
 When rat burrows are numerous in 
 fields or poultry pens, the first efforts at 
 control should be by methods already 
 described. Then the area should be 
 plowed to a depth of 18 inches with a 
 subsoiler or "chisel" to destroy most of 
 the burrows. With fewer safety retreats, 
 it will then be easier to hold down the 
 number of rats. 
 
 Burrows in yards or about buildings 
 can be collapsed by use of a pick, crow- 
 bar, or shovel. 
 
 Flooding 
 
 Rats may sometimes be killed by flood- 
 ing their burrows, especially on poultry 
 or pigeon farms. Rat burrows are shorter 
 
 than those of field rodents, and the ani- 
 mals may escape the water; results there- 
 fore are not always satisfactory. 
 
 Blocking 
 
 In some places it is possible to close 
 all rat entrances to a building and then 
 kill most or all of the rats inside. This 
 practice is called blocking. A careful pre- 
 liminary search is made and all but the 
 one or two entrances in most active use 
 are blocked securely with sheets of tin, 
 large rocks, or stout boxes filled with 
 earth. On the night selected for opera- 
 tions, the remaining entrances are quietly 
 blocked about an hour after dark. Then 
 two or more persons, equipped with 
 strong flashlights, enter and kill the rats 
 with clubs. Boxes and other objects are 
 moved when necessary to expose all the 
 rats. Rats may even be prebaited to con- 
 centrate in a building, so that a larger 
 number can be destroyed. Two or three 
 dozen rats have been reported killed at 
 one time in a poultry house or similar 
 structure. 
 
 Dogs, Cats, and Ferrets 
 
 A rat terrier often is helpful in block- 
 ing, or when stored goods are being 
 moved in a room or warehouse. The dog 
 will catch rats that escape into corners or 
 avoid human efforts. 
 
 Some domestic cats are good mousers, 
 and cats are kept about barns or ware- 
 houses to help hold down the population 
 of house mice. Cats seldom kill large rats, 
 but when a property has been largely 
 cleared of rats by other means, cats may 
 capture incoming migrants. On the whole, 
 however, they are of minor value. 
 
 Ferrets sometimes are trained to pur- 
 sue rats in their burrows and force the 
 rodents out so that they may be shot. 
 Sometimes the ferret kills and eats the 
 rat, then remains below ground, and the 
 owner must dig to recover his pet. Ferret- 
 ing is a sport, not a serious means of rat 
 
 32 
 
control. In California, possession of fer- 
 rets is illegal save for research and testing 
 of certain biologies used in public health 
 work (Calif. Admin. Code, Title 14, Sec. 
 700, 1945; Chap. 76, Calif. Statutes, 
 1933). 
 
 Rat Virus 
 
 Several kinds of "rat viruses," contain- 
 ing cultures of bacteria presumed to 
 spread disease through a population of 
 rats, have been exploited. Their use is 
 commoner in continental Europe than 
 elsewhere. Unbiased studies by rodent- 
 control officials and bacteriologists in the 
 United States have shown that such cul- 
 tures spread on baits give far poorer re- 
 sults than the manufacturers claim. The 
 bacteria used belong to the food-poisoning 
 group (Salmonella) and may contami- 
 nate human food. In such institutions as 
 hospitals, use of bacterial cultures for rat 
 control has been followed by outbreaks 
 of food poisoning, both in this country 
 and in Europe. Their sale or exposure for 
 sale has been prohibited in California 
 since 1926 (Calif. Admin. Code, Title 17, 
 Sec. 7925, 1945). 
 
 OTHER ASPECTS OF 
 CONTROL 
 
 Legal Requirements 
 
 In many communities, rat control is 
 dealt with in certain ordinances or in the 
 public health laws, such as the California 
 Health and Safety Code (Division 3, 
 Sees. 1800-1813, 1945). The burden for 
 control is placed on the owner or occupier 
 of a premises, and failure to carry on 
 proper control is declared a misdemeanor 
 that may be made the subject of court 
 action. 
 
 In increasing degree, many cities are 
 also prescribing the detailed requirements 
 for proper rat proofing of buildings, both 
 new construction and alterations. In prac- 
 tice, many details of construction and 
 maintenance are ignored, because owners, 
 
 contractors, and workmen are seldom 
 "rat conscious." If rodent-borne disease 
 flares up in a community, a wave of pub- 
 lic interest in rat proofing and a program 
 of rat riddance may arise, but unfortu- 
 nately it soon declines as the threat of im- 
 mediate danger passes. 
 
 Pest Control Operators 
 
 When the owner of a residence or busi- 
 ness establishment does not want to at- 
 tempt rat control himself, he may engage 
 a trained pest control operator for the 
 work. The latter will do a premises as a 
 single job for a specified charge, or will 
 agree to do periodic inspection and con- 
 trol as needed under a contract for a stip- 
 ulated monthly or annual fee. The degree 
 of efficiency will vary with the individual 
 operators or companies. 
 
 Several states now require that any per- 
 son engaging for hire to do pest control 
 must first pass an examination and be 
 licensed. California provides for two 
 types: structural pest control operators, 
 who work in and about buildings; and 
 agricultural pest control operators, who 
 deal with pests of crops and farm lands. 
 (See Calif. Structural Pest Control Act, 
 Sept. 15, 1945, a part of the Business and 
 Professions Code, Div. 3, Chapt. 14, Sec. 
 8500-8677; and Calif. Agric. Code, Sec. 
 150). State and national organizations 
 of the operators hold schools and confer- 
 ences for improving the technical and 
 practical qualifications of their members. 
 
 WHITE-FOOTED MICE 
 
 The commonest rodents in Cali- 
 fornia, outside cities and culti- 
 vated areas, are the small "deer 
 mice" or white-footed mice (genus 
 Peromyscus). There are several 
 kinds in various parts of the state, 
 but one or another is present in 
 every sort of wild environment 
 from the humid coast belt to the 
 
 [33 
 
driest desert areas, and from sea 
 level to the high mountains. Ex- 
 clusion and trapping are the best 
 methods for control of these mice. 
 
 The head-and-body length of adults is 
 3 to 5 inches, and the tail is 21/^ to 5 2/3 
 inches long in different species. The eyes 
 are large and black, the ears thin and 
 
 mice, 
 
 often invaded by white-footed 
 which may cut clothing or bedding to 
 obtain nest material, and eat any human 
 food not in mouse-proof containers. They 
 are particularly undesirable because in 
 some parts of California they are infected 
 with plague. 
 
 The owner of a cabin must make special 
 effort to build out these small rodents. 
 
 Fig. 16.— The white-footed mouse, or deer mouse (Peromyscus). Its head-and-body 
 length is 3 to 5 inches and the tail about the same. The large black eyes are conspicu- 
 ous, the ears are large, the upper surface of the body is usually pale brown (bluish 
 gray in young), and the feet and under parts are white. 
 
 prominent; the upper surface of the body 
 is pale brown to gray, and the under parts 
 and feet white (fig. 16) , 
 
 White-footed mice are strictly night- 
 time foragers, but they are active through- 
 out the year. They make no runways. 
 They hide and nest in cavities of old 
 stumps or downed logs, in rock piles, and 
 probably in burrows. From 2 to 6 young 
 are produced in a litter, and several 
 broods may be borne during the year by 
 one female. In nature these mice feed on 
 a wide variety of plant materials. 
 
 About homes, cabins, and barns where 
 no house mice {Mus musculus) are pres- 
 ent, they will come indoors and become 
 household pests. Mountain cabins that 
 are closed after the summer vacation are 
 
 The detailed directions for exclusion of 
 house rats and mice (pp. 13-21) should 
 be adapted as necessary to make cabins 
 rodent proof. 
 
 Old and loosely built structures, and 
 those of "rustic" design with bark slabs 
 and other rough or irregular materials 
 will need special attention. Buildings of 
 natural stone combined with wood afford 
 many irregular crannies where mice may 
 hide, especially after parts of the struc- 
 ture have settled. Foundations of loose 
 stones not solidly cemented, or of partly 
 rotted logs, give shelter to the mice. 
 
 Besides openings on the first floor, the 
 roof, upper windows, and chimney top 
 are paths of entry to mice in winter when 
 snow blankets most or all of a building. 
 
 [34 
 
The chimney should be capped tightly 
 with hardware cloth of 1/3- or 1/4-inch 
 mesh. Loose shingles or shakes must be 
 fastened down, and all crevices between 
 boards covered with battens, well nailed. 
 Sheet tin or hardware cloth serves to cover 
 cracks or holes wider than 1/3 inch. Ir- 
 regular crevices may be wedged with 
 folded strips of fly screen. Pipes entering 
 
 Strychnine-coated grain sometimes will 
 serve for control, but may be refused by 
 the mice. If the owner cannot control 
 these mice by the methods described, he 
 should get help from some government 
 agency (p. 3). 
 
 Dead mice in mountain cabins should 
 be carefully handled to avoid the chance 
 of live fleas or ticks getting on to a per- 
 
 
 Fig. 17.— The wood rat, or pack rat (Neotoma). Its head-and-body length is 7 to 8 
 inches and the tail 6I/2 to IVi inches. The fur is soft, the upper surface is warm brown 
 (bluish gray in young), the ears are large and scantily haired, the nose is blunt, and 
 the tail is hairy. 
 
 walls or floors should be surrounded by 
 metal collars (fig. 7) . 
 
 Bedding, mattresses, and clothing not 
 in use in summer, or stored during the 
 winter, are safe only in a tight closet 
 lined with sheet metal or hardware cloth. 
 All foods should be in tightly covered 
 containers of metal, glass, or crockery, 
 or in cabinets fully lined with metal. 
 
 Killing of white-footed mice in build- 
 ings is best done with mouse traps, using 
 a number of traps and persisting until all 
 are caught. Rolled oats make good bait, 
 but other foods should be tried if neces- 
 sary. Some owners leave a number of 
 traps baited and set when closing their 
 buildings for the winter. Dusting the 
 floor with napthalene flakes or sulfur 
 is reported to reduce mouse damage. 
 
 son's clothes or skin. Gloves should be 
 worn, and the clothing sprayed with a 
 pyrethrum spray immediately after pick- 
 ing up any mice. The dead rodents should 
 be completely burned, out-of-doors, or 
 buried to a depth of 2 feet. 
 
 WOOD RATS 
 
 The native wood rats, commonly 
 known as "pack rats" or "trade 
 rats," are sometimes a nuisance in 
 homes, summer cabins, and other 
 buildings outside cities and towns. 
 Exclusion and trapping are the 
 best means of control. 
 
 The brown-footed wood rat {Neotoma 
 juscipes, fig. 17) of the foothills and 
 lower mountains, has a head-and-body 
 
 [35 
 
length of 7 to 8 inches, and the tail is 6^^ 
 to 7Y2 inches long. The nose is blunt, the 
 medium-sized ears are slightly haired, 
 and the body coat is warm brown on the 
 upper surface. The coat is softer than that 
 of alien house rats, and the tail is mod- 
 erately haired. 
 
 This wood rat builds large conical 
 nests of sticks and forest litter, either on 
 the ground or on horizontal branches in 
 oaks and other trees and, if given a 
 chance, will nest in buildings. These ro- 
 dents are active throughout the year, 
 mainly at night. They climb readily, and 
 eat a wide variety of plant materials. They 
 may also be attracted to human food sup- 
 plies in buildings or in outdoor camps. 
 Wood rats often remove small bright ob- 
 jects, such as spoons, forks, knives, or 
 mirrors, sometimes leaving sticks or other 
 items "in trade." 
 
 In the higher mountains there is a 
 larger gray, bushy-tailed wood rat (Neo- 
 toma cinerea) that makes a nest of piles 
 of rough sticks in rock slides. It also in- 
 vades cabins for food or shelter. Since 
 wood rats at times may carry diseases 
 transmissible to man, they should be ex- 
 cluded from homes and buildings. 
 
 The directions for excluding alien rats 
 and white-footed mice apply to making 
 foothill cabins, mountain resorts, and 
 similar places proof against wood rats. 
 No double walls or other spaces should be 
 left to provide nesting places for these 
 and other rodents. Means just described 
 to protect food and bedding from white- 
 footed mice will serve also against wood 
 rats. Rock piles near cabins or crevices 
 in foundations are other common shelters 
 for these rodents. 
 
 Wood rats are less wary than alien rats, 
 and may be caught in wooden-based 
 spring rat traps baited with rolled oats, 
 peanut butter, raisins, or prunes. They 
 are not readily attracted by other grain 
 baits, and are not easily poisoned with 
 strychnine. 
 
 For safety in respect to disease, besides 
 clearing buildings of these rodents, those 
 in nearby nests, logs, or rock slides should 
 be trapped. Dead wood rats should be 
 handled with the precautions stated for 
 white-footed mice. When a building is 
 closed for the winter, several baited rat 
 traps should be placed to capture any 
 wood rats that gain entrance during the 
 owner's absence. 
 
 In order that the information in our publications may be more intelligible it is sometimes necessary 
 to use trade names of products or equipment rather than complicated descriptive or chemical iden- 
 tifications. In so doing it is unavoidable in some cases that similar products which are on the market 
 under other trade names may not be cited. No endorsement of named products is intended nor is 
 criticism implied of similar products which are not mentioned. 
 
 36 
 
LITERATURE CITED 
 
 Bartlett, S. a., and others 
 
 1946. Infestation control: rats and mice. Min. of Food, London, iv + 1-36. 41 figs. 
 
 Beck, M, Dorothy and Alwine Van Allen 
 
 1947. Typhus fever in California, 1916-1945, inclusive. Amer. Jour. Hyg. 45:335-54, 1 fig. 
 
 California, State of 
 
 1945. California Agr. Code. California State Printing Office, Sacramento. 476 p. 
 
 GuNDERsoN, Harold 
 
 1943. Clean out the rats! Farm Science Reporter (Iowa State College) 4(1): 10-12. 
 
 Hall, E. Raymond 
 
 1927. An outbreak of house mice in Kern County, California. Univ. California Publ. Zool. 
 30:189-203. 
 
 Hull, Thomas G. 
 
 1941. Diseases transmitted from animals to man. 2d ed. Charles C. Thomas, Springfield, 
 111. 403 p. Illus. 
 
 Kalmbach, E. R. 
 
 1945. "Ten-eighty," a war-produced rodenticide. Science 102:232-33. 
 
 Mackie, D. B., W. C. Jacobsen and W. B. Carter 
 
 1934. Rodent control with sodium fluosilicate by a new method. California Dept. Agr. 
 Monthly Bui. 23:192-96. 
 
 O'Connor, M. G., R. E. Buck and C. R. Fellers 
 
 1935. Properties, toxicity and palatability of red squill and powder baits to rats. Indus, and 
 Engin.Chem. 27:1377-80. 
 
 Piper, Stanley E. 
 
 1928. The mouse infestation of Buena Vista Lake basin . . . 1926 . . . 1927. California Dept. 
 Agr. Monthly Bid. 17:538-60. 
 
 RicHTER, Curt P. 
 
 1945. The development and use of alpha-naphthyl thiourea (ANTU) as a rat poison. 
 Amer. Med. Assoc. Jour. 129:927-31. 
 
 Silver, James, and F. E. Garlough 
 
 1941. Rat control. U. S. Dept. Interior, Fish and Wildlife Service. Conserv. Bui. 8:1-27. 
 10 figs. 
 
 Storer, Tracy I. 
 
 1931. A minor plague of house mice at Davis, California, in 1926. Jour, of Mammal. 12: 
 
 316-17. 
 1947. Control of field rodents in California. California Agr. Ext. Circ. 138:1-51. 23 figs. 
 
 Ward, Justus C. 
 
 1946. Rodent control with 1080, ANTU, and other war-developed toxic agents. Amer. Jour. 
 Pub. Health 36:1427-31. 
 
 35m-4,'48(A7854) 
 
 [37 
 
DOYOU HAVE QUESTIONS 
 
 about crops . . . soils . . . fertilizers 
 irrigation . . . or special problems 
 affecting your own farm operation q 
 
 YOUR FARM ADVISOR CAN GET THE BEST INFORMATION 
 
 He is ready to talk over your farming 
 problems with you and help you solve 
 them. There is no charge for this service. 
 
 You will find the Form Advisor for your district in the following list: 
 
 Alameda County: 
 
 Post Office Bldg., Hayward 
 Butte County: 
 
 Federal Bldg., Oroville 
 Colusa County: 
 
 Federal Bldg., Colusa 
 Contra Costa County: 
 
 Cowell 
 El Dorado County: 
 
 Post Office Bldg., Placerville 
 Fresno County: 
 
 Room 20, Federal Bldg., Fresno 
 Humboldt County: 
 
 Post Office Bldg., Eureka 
 Imperial County: 
 
 Court House, El Centro 
 Kern County: 
 
 2610 M Street, Bakersfield 
 Kings County: 
 
 131 E. 8th St., Hanford 
 Lake County: 
 
 Kelseyville 
 Lassen County: 
 
 Memorial Bldg., Susanville 
 Los Angeles County: 
 
 908 N. Spring St., Los Angeles 12 
 Madera County: 
 
 Post Office Bldg., Madera 
 Marin County: 
 
 Post Office Bldg., San Rafael 
 
 Mendocino County: 
 
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 Napa County: 
 
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 Nevada County: 
 
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 Orange County: 
 
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 Riverside County: 
 
 Post Office Bldg., Riverside 
 Sacramento County: 
 
 315 Federal Bldg., Sacramento 2 
 
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 Court House, Hollister 
 
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 San Diego County: 
 
 404 U. S. Customs Bldg., 
 San Diego 1 
 
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 145 S. American St., Stockton 7 
 
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 Post Office Bldg., San Luis Obispo 
 
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 Half Moon Bay 
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 Santa Clara County: 
 
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 Court House Annex, Santa Cruz 
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 52 N. California St., Ventura 
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 Court House, Woodland 
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 Federal Bldg., Marysville 
 
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 UNIVERSITY OF CALIFORNIA . BERKELEY