Advances in Rodent Control by E

Home , Barium carbonate, Sodium fluoroacetate, Zinc phosphide

Advances in Rodent Control by E. R. KALMBACH

B,'EFORE THE WAR our three most useful rodent poisons were imported. Red squill came from the Mediterranean Sea area. Strychnine came from India and French Indochina. Thallium sulfate came from Germany, France, and Belgium. Hostilities cut off almost all those sources at the time that the armed forces were planning and exe- cuting the Pacific campaigns that sent our troops into areas where many strange rat-borne diseases existed. Other factors added to the urgency: Rodents destroy crops, and there was great need of food ; in many theaters of war we had concentrations of men and supplies, which were prey to attack by rats and mice; there were contacts with species and populations of rats not previously encountered by great numbers of human beings. The shortage of effective rat poisons was extremely serious. A start had been made in the search for substitute materials. Two independent groups had intensified their efforts and each had made progress. Curt P. Richter, of Johns Hopkins University, had discovered the toxicity to laboratory rats of ANTU, or alphanapthylthiourea. The Fish and Wild- life Service, of the Department of the Interior, at its Wildlife Research Laboratory in Denver, studied a number of promising compounds. When the emergency appeared to be acute, the value of the research programs was recognized by the Office of Scientific Research and Development, and funds were provided to put them into high gear. The accomplish- ments that resulted were distinctly a product of war, but they are now available to enhance the peacetime welfare and economy. ANTU soon was produced in quantity adequate for widespread test- ing, and, at Dr. Richter's request, the Fish and Wildlife Service entered into cooperation with his group to determine the value of the new poison for the control of both rats and field rodents. Although ANTU killed S90 ADVANCES IN RODENT CONTROL 891

Norway rats readily, black and Alexandrine rats were less susceptible, and house mice and most field rodents were relatively unaffected by it. It was found to be dangerous to dogs, cats, hogs, and chickens, but almost nontoxic to monkeys. These findings indicated that ANTU should be useful in areas where the Norway rat predominated, that it should be used with a degree of caution where pets are present, but that it appeared to resemble red squill in the matter of safety to human beings. The search for new poisons led to the demonstration of compound 1080, or sodium fluoroacetate, as a rodenticide. In this discovery the Patuxent Wildlife Research Refuge in Maryland accumulated and tested many potential rodenticides, among which was 1080. The Denver Wild- life Research Laboratory confirmed the findings and demonstrated the utility and hazards of the material under field conditions. Compound 1080 differs from ANTU in that it is toxic to all forms of life on which it has been tested. It has been found to be an effective poison for the control of all types of rats, mice, ground squirrels, and prairie dogs. It is highly soluble in cold water, and consequently one of the best methods of using it against rats and mice in buildings is to ex- pose it as water solutions at frequent intervals along the rodent runways. The indiscriminate toxicity of compound 1080 is its one serious disad- vantage. So susceptible are dogs, cats, and hogs to it that they may be killed by eating sick or dead rats that have been poisoned by it. This secondary hazard is in addition to the danger to animals that might pick up baits or drink the poisoned water. There is no known antidote for 1080, so it is essential that this new poison be used with utmost caution. It is recommended by the Fish and Wildlife Service that compound 1080 be used only by experienced persons who are trained to handle poisons and who will treat it with the respect that it deserves. Compound 1080 was first employed in the control of field rodents in October 1944, when it was used with signal success against California ground squirrels. Its performance in the control of these and other field rodents has rarely been equalled, and, if baits prepared with it are pro- tected by color, it appears to be a reasonably safe poison from the stand- point of bird mortality. There arc, however, certain objections to compound 1080 in field use. Its toxicity is high for the mammals teste'd, especially for certain carni- vores. Domestic dogs and cats, coyotes, bobcats, foxes, and badgers, that are likely to feed on surface-killed rodents, are greatly endangered. A 20-pound coyote or dog can be killed by a dose of compound 1080 that would not kill an 8-ounce rat. It is for that reason that the Fish and Wildlife Service has emphatically recommended that this rodenticide not be released to the public for unskilled use until adequate safeguards have been evolved that will protect both human life and beneficial creatures. Both ANTU and 1080 arc being produced in the United States from 892 YEARBOOK OF AGRICULTURE raw materials that are readily available. No future disruption of world trade can now cause a serious shortage of effective rodenticides in this country. But since ANTU and 1080 may still not be the ideal rodent killers, research is continuing to develop even more satisfactory ones and to learn better and safer ways to use them. While in no sense a new discovery during the war, another poison, zinc phosphide, was used rather extensively in place of the rodenticides that had become scarce. Previously developed as a control agent for meadow and pine mice, zinc phosphide was found to give excellent control of house rats and mice, and even of ground squirrels and prairie dogs. Be- cause of its black color and marked odor and taste, it is not readily eaten by game and other seed-eating birds. The secondary poisoning hazard to carrion-feeding birds and mammals also is low. Zinc phosphide, however, is toxic to all forms of life, and the preparation of baits containing it and their exposure in the field should be entrusted only to personnel trained in rodent control. The use of zinc phosphide to control range rodents takes more skill than is commonly used in such programs. Prcbaiting, a practice of expos- ing unpoisoned grain at rodent burrows several days before the exposure of the poisoned grain, is necessary. This poison is unstable and loses its toxicity with exposure. There also are seasons when it cannot be used effectively. Thus, although zinc phosphide proved to be a valuable emergency tool, it will eventually be replaced in general rodent control by other poisons as they become available.

The Story of Red Squill

Red squill {Urginea maritima)^ a liUaceous plant, the bulbs of which are harvested as a wild crop in the Mediterranean region, long has been used as a lethal agent in rat control. The distastefulness of red squill to human beings and to most domestic animals and its inherent strong emetic action to animals other than rats combine to make it the most specific raticide. It can be used with comparative safety by the general public to combat rats. With the outbreak of war in Europe, the supply of red squill from the Mediterranean sources became a matter of immediate concern, and in 1939 drug dealers imported 889,664. pounds of bulbs and dried bulb slices. This was in marked contrast with a total import of 121,027 pounds in 1938 and reflects the effort put forth to increase inventories of this item. To procure all the red squill possible, little regard was given to the quality of the material obtained; as a result, there was on hand in this country a large supply of red squill having a potency so low that it was of litüe value in rat control. To cope with the situation, the Wildlife Research Laboratory at ADVANCES IN RODENT CONTROL 893

Denver developed a method of fortifying this poorer quality squill. The method consists essentially of preparing concentrated extracts of the rat-killing principles present in small quantities in the weaker squills and impregnating with them a predetermined amount of similar unex- tracted material in such a proportion that the final product is ade- quately toxic. Standardized powders of this type became generally known as fortified red squill powder. Since there is no chemical method known for determining the potency of red squill, all toxicity determina- tions were carried out by feeding known quantities to captive rats. After fortified red squill powder had been produced in some quantity in a small pilot plant erected and operated at the Denver laboratory, a larger plant for processing this material was constructed under the laboratory's supervision for the Louisiana Health Department at New Orleans. This unit turned out a considerable quantity of fortified red squill, which was used to combat the rats causing typhus in that city and surrounding territory. Along with research designed to increase the effectiveness of squill preparations sold to the public, studies were undertaken to propagate red squill in the Western Hemisphere. Southern California and Baja California, Mexico, where the climate resembles that of North Africa, were found to be suitable areas for growing the plant. Highly toxic bulbs have been produced on an experimental basis in these localities. Later the Bureau of Plant Industry, Soils, and Agricultural Engineering of the Department took over the propagative aspects of this program, in charge of D.. M. Crooks. The Fish and Wildlife Service collaborates in determining the toxicity of the bulbs produced.

Development of Deterrents Against Mammals

Beginning even before the war, increasing importance was placed on the use of deterrents as a means of reducing damage by rodents. The early work emphasized the reduction of damage by field mammals through repellent sprays and paints. The animals concerned included principally deer, rabbits, mice, and tree squirrels—creatures that damage seed and seedlings in reforestation projects, sheltcrbelt plantings, farm wood lots, and orchards. Hundreds of compounds were studied. In cooperation with the Forest Service, Soil Conservation Service, State game and fish departments, and other organizations, tests were con- ducted in Montana, Louisiana, Texas, the Lake States, the Great Plains shelterbelt area, New York, and the New England States. A rabbit repellent paint known as "96a" was then developed. It consists of copper carbonate, copper sulfate, and dry lime-sulfur as the active ingredients and a synthetic resin and asphalt emulsion dissolved in ethylene dichloride as the adhesive. When applied to the bark of 894 YEARBOOK OF AGRICULTURE

dormant trees, ''96a" is effective in preventing damage by rabbits. It is now being manufactured by the Fish and Wildlife Service at its Sup- ply Depot at Pocatello, Idaho, where it is available to the public. As the war progressed, the study of repellents for rats, mice, pocket gophers, and other animals that damage stored foods, communication lines, and other vital war goods was stressed.'A search was made for suitable repellent materials, which, when applied to such army subsistence packs as 10-1 ration and K-ration, would minimize damage done by rodents, both in this country and overseas. The work was carried out in •cooperation with the Quartermaster Corps Subsistence Research Labo- ratory, and seventy-odd materials were evaluated. Microcrystalline waxes, used by the Army to waterproof packaged food, were found particularly effective in preventing rodent damage. Only where adverse storage conditions prevailed did rats gnaw through this protective barrier, and then only after protracted exposure of the wax-dipped boxes to almost constant dripping of water. Under dry or moderately dry storage conditions, such boxes were completely undam- aged regardless of the contents. Other substances found to be of particular value in minimizing rodent damage when applied to food packages were water glass (37-percent sodium silicate) and prepared tung oil. Ammonium sulfate (approximately 20 percent) was found useful in such insulating materials as cotton, ground paper, and sawdust, in preventing nesting of mice and rats. A commercial insulating material consisting of porous glass brick was also found to be impervious to rat damage. When the material was placed so as to obstruct the movements of wild rats, the animals either abandoned that route or gnawed into adjoining sections of the wall. Com- mercial installations of this product in refrigeration rooms have further demonstrated its value as a rodent-proofing material. Contributions were also made to the problem of protecting overhead and underground tele- phone cables from tree squirrels and pocket gophers, and synthetic tires and tubes from porcupines, rats, and mice. Despite the progress made, there is much yet to be done in the field of animal deterrents. What appears to be of value under one set of conditions often fails under another. The cumulative effect of several animals, also, may enable them to break through a protective barrier, whereas one animal alone could not do so. The ultimate answers will have to be obtained under the exacting conditions presented by a host of varied field conditions. The outlook is not overly promising, but it does present a challenge to painstaking and persevering research. An outstanding attribute of red squill as a raticide is its emetic property, which serves as a protection against the fatal poisoning of domestic animals. Rats, being unable to vomit, are, on the other hand, subjected to its toxic action. In periods when effective squill is not obtainable or under ADVANCES IN RODENT CONTROL 895 conditions where squill has not given effective rat control, other poisons have had to be used. These do not as a rule possess marked emetic prop- erties, and accidental poisoning of dogs and cats often follows their use. As early as 1937 the Biological Survey undertook a study of modi- fying poisoned baits exposed for rats so as to make them relatively harmless to other animals. Various emetics, including copper sulfate, zinc sulfate, and tartar emetic ( antimony-potassium tartrate ) were used in food baits containing thallium sulfate or zinc phosphide as the toxic principles. Although certain combinations gave a reasonable degree of protection to dogs when the concentration of the emetic principle was adequate, difficulties were encountered with the acceptance by field mice of such baits. The subject later was dropped when it was learned that dogs were not likely to be poisoned in field-mouse control. In 1942, however, the study was resumed in view of the need of protecting domestic pets in cities where intensive rat control was being con- ducted through the use of such poisons as thallium sulfate, zinc phosphide, and barium carbonate. As a result of this work, the following conclusions were reached. The primary toxic action of one to five lethal doses of zinc phosphide, thallium sulfate, and barium carbonate can be appreciably reduced or nullified in dogs and cats that might feed on rat baits by including tartar emetic in the following proportions: Zinc phosphide, 8 parts; tartar emetic, 3 parts; thallium sulfate, 7 parts; tartar emetic, 4 parts; barium carbonate, 140 parts; tartar emetic, 3 parts. There is no assur- ance, however, from these experiments that similar benefits would accrue to human beings who accidentally ingest such materials.

Use of Color in Protecting Seed-Eating Birds

To make poisoned grain baits exposed for the control of field rodents less dangerous to sccd-cating birds, including valuable game species, use is now being made of the rather simple physiological fact that birds perceive and react to color, while rodents, being almost if not completely color blind, do not. After the announcement of the first experiments in 1943, field tests have confirmed the utility of the practice, and it is used in the preparation of all grain baits by the Fish and Wildlife Service in which the highly toxic compound 1080 is employed. The aversion of birds to feeding on unnaturally colored food items is a fact known for many years and one that was employed in preventing birds from feeding, on newly sown grain. The procedure originated in Europe, where a brilliant blue pigment was used as a protective material. In the United States red lead has been used to a limited extent for the same purpose. Using the principle to safeguard birds that might otherwise feed on poisoned grain exposed for rodents is, however, a development that was timed with the war effort. Although much re- 896 YEARBOOK OF AGRICULTURE mains to be done, sufficient progress has now been made to warrant the assertion that the addition of color to poisoned baits has definitely in- creased their safety to birds and permitted their use under conditions where formerly they were hazardous. On the basis of work so fa^ completed, the most effective deterring colors for birds are those near the center of the humanly visible spec- trum, the yellow and green bands. There are reasons for believing, however, that future work will disclose variations in this concept that may even involve color differences attributable to particular species. In regard to rodents, v/hich do not have an aversion to color, several problems are still to be solved. These involve not so much the visual acceptance of colored baits as the preparation of baits with dyes that do not have an odor or taste objectionable to rodents. At present, baits readily acceptable to rodents are being prepared with yellow dyes, which are considered to be less effective in deterring birds than green dyes. Eventually we hope to develop a dye that is perfect in keeping birds away but at the same time is thoroughly acceptable to rodents.

THE AUTHOR

E. R. Kalmbach, a senior biologist in the Fish and Wildlife Service, United States Department of the Interior, is in charge of the Wildlife Research Laboratory at Denver, Before joining the Biological Survey in 1910 he was engaged in museum work in Michigan.

FOR FURTHER READING Garlough, F. E.j and Spenccrj Donald A.: Control of Destructive Mice, U. S. Depart- ment of the Interior, Fish and Wildlife Service, Conservation Bulletin 36, 1944. Garlough, F. E., Welch, J. F., and Spencer, H. J. : Rabbits in Relation to Crops, U. S. Department of the Interior, Fish and Wildlife Service, Conservation Bulletin 11, 1942. Kalmbach, E. R. : Birds, Rodents, and Colored Lethal Baits. Transactions, Eighth North American Wildlife Conference, American Wildlife Institute, pages 408-416, 1943. Kalmback, E. R., and Welch, J. F.: Colored Rodent Bait and Their Value in Safe- guarding Birds, Journal of Wildlife Management, volume 10, pages 353-360, October 1946. Silver, James, and Garlough, F. E.: Rat Control, U. S. Department of the Interior, Fish and Wildlife Service, Conservation Bulletin 8, 1941. Spencer, H. J. : Emetic Agent in Toxic Rat Bait, a Safeguard for Dogs and Cats, U. S. Department of the Interior, Fish and Wildlife Service, Wildlife Leaflet 264, 1944 (processed).