The Rat and Its Relation to the Public Health

Volume Vi, No. 4; Volume Vii, No. 6; Volume Viii, No. 2; Plague Among

the Ground Squirrels of California, W. B. Wherry, Journal Infectious Diseases, Volume V, No. 5; California Mammals, Frank Stephens.

RODENT EXTERMINATION.

By Passed Asst. Surg. WILLIAM COLBY RUCKER,

United States Public Health and Marine-Hospital Service.

It should be remembered that rodents are extremely wily creatures and that any campaign against them is a contest between the wit of man on the one hand and acute animal instinct on the other. The rat, by his constant association with man, has become extremely wary, and is frightened by anything in the least out of the ordinary. They will eat the bread on which poison is spread so carefully that they will leave behind the poison and take practically all the bread; they will open traps by pressing down the pan, and they have been known to repeat this operation several times within an hour, entering the trap, eating the bait, and then liberating themselves. At other times they will enter the trap and stand on the pan with their hind legs, eat the cheese, then carefully turn around and back out. This, of course, is not possible with snap traps, but they have been known to spring them by causing pieces of wood to fall upon them, after which the bait would be eaten. Rats are found wherever food exists in abundance or where they can find suitable breeding and nesting places.

Rodent extermination is a problem, with difficulties arising from the animal’s highly developed regard for self-preservation. In main, the rat requires two conditions for life. He needs plentiful food and places suitable for nesting and breeding. Eliminate either of these elements and you drive away your rats. Yet the problem remains far more difficult than shown in the simple terms of the above equation. The fabulous speed at which rats multiply will baffle all but the most determined and efficient efforts to exterminate them. Under normal conditions each female bears 3 litters a year and each litter produces 10 young. Under conditions ideally favorable, it has been computed that 1 pair of rats will in five years, providing all can live so long, increase to 940,369,969,152. Such a result is, of course, impossible in nature, for it means that every rat born of the original pair survive five years; that every litter of 10 contains 5 males and 5 females; and that the ideally favorable conditions persist. On the other hand, rodent existence is an unending struggle in which an enormous percentage succumbs; the ratio of half males and half females does not hold; and ordinary conditions of life are hardly even favorable. Nevertheless, the above proves emphatically that no rat eradication can be effective unless the breeding is curtailed. Any campaign against rodents must aim (a) to slaughter the greatest possible number of those already living and (b) to prevent the possibility of further breeding.

The existing rats are best attacked by trapping, by poisoning, and by their natural enemies. Traps and poisons alone have been found insufficient to keep pace with the rat’s speed of multiplication. The surest of the rat’s enemies are his natural ones, and once they have been loosed upon him his chance of escape is reduced. The cat, dog, skunk, and other rodent foes, given a fair chance, quickly drive out rats. But these animals do not eradicate the pest. The rats will probably migrate to some other shelter, returning when their natural enemies have quieted down. Absolute extermination is reached only when conditions make the continuation of species impossible for the rat.

The size and frequency of rodent litters decreases proportionately with every cutting off of food supplies. Separate the rat from his pabulum and he will not breed so freely nor so often as when he is well fed. Destroy rat habitations and make it impossible for them to find new nesting places, and breeding will virtually cease, since the unsheltered progeny can no longer survive, and since the starving parent rats are driven to cannibalism in the struggle for existence.

Campaigns against rodents must cover five directions: (1) Trapping, (2) poisoning, (3) exposing them to natural enemies, (4) cutting off food supply, and (5) destroying existing nests at the same time that the making of new ones is prevented.

Parenthetically, it may be noted that while these principles apply equally to the extermination of rats in cities and in country districts, their application must vary according to the place.

TRAPPING.

The kind of traps to be used varies with the rodent to be captured and the locality which it infests.

CAGE TRAPS.

The large 19-inch French cage trap gives good results where rats are plentiful. It should be made of stiff, heavy wire and well reenforced, as a large, strong rat will force his head between the wires in a weak trap and thus escape. Before setting, the lever on the trap should be tested to see that it works properly. The trap should be placed on a hard surface, with the rear end a little higher than the entrance, so that the trap will close promptly. When setting the trap in the open it should be fastened to a board on which about an inch of soft dirt has been spread. Place the trap where the rat usually goes for food or in a runway and disturb the surroundings as little as possible. It is sometimes well to place the trap near where there is dripping water, as the rats come there to drink. If the trap is set in hay or straw or wood it should be covered (with the exception of the entrance) with this material. When this is not possible it should be covered with a piece of sacking or placed in a dark corner or beneath the floors. When setting the traps in the sewer a dry place should be chosen.

The rat is more or less of an epicure, therefore the bait should be changed at frequent intervals. Also he should be given food which he is not in the habit of getting. For example: In a meat market vegetables are the best bait, while in a location where vegetables are plentiful fresh liver and fish heads, or a little grain, are best. The following may be suggested as good bait to be used: Fish, fish heads, raw meat, cheese, smoked fish, fresh liver, cooked corn beef, fried bacon, pine nuts, apples, carrots, and corn. When trapping in chicken yards a small chick or duckling is remarkably good. When a large number of rats are caught in one trap, search for the female and leave her alive in the trap, as she may call in the young or the males. The bait should be fastened to the inner side of the top of the trap with a piece of fine wire so that the first rat in can not force the bait underneath the pan and thus prevent the entrance of other rats. A few grains of barley should be scattered near the entrance of the trap and a small piece of cheese or meat fastened to the pan with a piece of wire. It is often well to touch the pan with a feather which has been dipped in oil of anise or oil of rhodium. Before leaving the trap it should be smoked with a piece of burning newspaper to kill the smell of the human hands or the rats which have been in it. Do not handle the trap after burning it out. When trapping in a neighborhood where rats are known to exist the traps should not be moved for three or four days unless they have rats in them, as it is well for the rats to become accustomed to seeing them and thus careless about entering. It is not wise to kill rats where they are caught, as the squealing may frighten the other rats away.

SNAP TRAPS.

Snap or spring traps are best for use in houses and stores, with the exception of fish and meat markets. Snap traps are best for use in runways, beams, and shelves. It is sometimes well to disguise the trap by covering its floor with a little sawdust or dirt. They should be first tested to see that they work properly and that the staples are secure. New traps should be smoked or stained to render them an inconspicuous color.

The bait should consist of some firm material, such as fried bacon or tough meat, and should be tied on so that the rat will be obliged to pull on it and thus spring the trap. The trap should be placed in a corner or close to the wall on a flat, hard surface, so that the rat can not spring it with his tail or by walking on it.

BARREL TRAPS.

In warehouses and granaries large numbers of rats may frequently be trapped by using a barrel or garbage can having a metal top which is carefully balanced. Large pieces of strong cheese are placed in the middle of the cover and a plank laid from the floor to the edge of the barrel. The rat runs up the plank onto the smooth metallic lid which tips and the rat is precipitated into the barrel.

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