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 \ ". •^m 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. ^.^.„, bolt holes x,rv. entrance nest vS ■ , » ^. I ,\t^. . \ ' j^. ,\\t . . ' . I < . ,\\i, . ,\ t ^, . ,\ ' . I . ' . ' ,• . .\\' . . ' . ■ . . .v . . 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