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, shelterbelt 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 conducted 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 dormant trees, "96a" is effective in preventing damage by rabbits. It is now being manufactured by the Fish and Wildlife Service at its Supply 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 Laboratory, 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 undamaged 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 20percent) was found useful in such insulating materials as cotton, ground d 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. Commercial 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 telephone 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 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 properties, and accidental poisoning of dogs and cats often follows their use.

As early as 1937 the Biological Survey undertook a study of modifying 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 conducted 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 assurance, 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 seed-eating 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 remains 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 far completed, the most effective deterring colors for birds are those near the center of the humanly visible spectrum, 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, which 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.