FIGURE 195.—Red-squill bulb (Urginea maritima). About one-fourth natural size
RAT Control Aided by Development of Effective New Poisons
Three useful rat poisons that were practically unknown in this country five years ago have now come into wide use. These are red squill, thallium sulphate, and calcium cyanide. Of these the most important is red squill, as it has the widest application and is attended with the least danger to human beings and other animal life.
Red squill is a perennial plant that grows wild along the Mediterranean coast. It has a large pear-shaped bulb, usually measuring from 3 to 6 inches in diameter and weighing up to 10 or more pounds. (Fig.195.) Another commercial variety, white squill, is used in medicine as a heart tonic, emetic, diuretic, and nauseant expectorant. Red squill, however, has all these properties and in addition possesses an active principle not yet definitely isolated and identified, but found to be very poisonous to rats.
Red squill has been used as a rat poison in Europe for many centuries, but the variability in poisonous action (toxicity) and the uncertain results attending its use have kept it from becoming popular. An investigation started in 1923 by this department, however, resulted in the perfection of a process for manufacturing a red-squill powder of high and uniform toxicity. Commercial manufacturers of rat poisons have been quick to place red-squill products on sale in practically every State in the Union, and these are being used in large quantities, with results fully up to first expectations.
Experiments with red squill indicate that it is efficient, economical, and safer than any other rat poison. Its chief appeal and value lie in the fact that it approaches the unique position of being a poison specific for rats. In laboratory experiments red squill was fed to cats, dogs, poultry, pigeons, a pig, and to several native rodents with no apparent ill effect, although similar doses were uniformly fatal to rats. Field experiments on baby chicks from 5 to 18 days old indicated that they are not likely to eat in two consecutive days enough feed containing 10 per cent of red-squill powder to cause death, even when it is given to the exclusion of other feeds. Two hens that were given feed containing 10 per cent of red squill during a period of two weeks apparently thrived as well as others fed on untreated mash. These reactions to the poison may be explained by the highly acrid taste of squill, which is objectionable to most mammals other than rats, and by its strong emetic action, which causes most mammals promptly to vomit it. Rats can not vomit and are, therefore, unable to escape the effects of the squill. Chickens and other birds seem highly resistant to it. For these reasons red-squill baits are recommended in preference to other stronger poisons commonly used, particularly on farms and in places where poultry, livestock, and other domestic animals would be endangered, or where food supplies are stored.
The toxicity of red-squill powders now on the American market ranges between 250 and 500 milligrams of dry squill for each kilogram the rat weighs (about 1.5 to 3 grains for each adult rat). This toxicity to rats is approximately twice as great asin barium carbonate. Squill is used in baits in concentrations ranging between 5 and 10 per cent. It is advisable to expose first a variety of unpoisoned baits, which will serve both to overcome the natural suspicion of the rats and to indicate the foods for which the rats show a preference. Such prebaiting, followed by a liberal application of squill baits, will usually result in the destruction of all the rats on any premises.
Thallium sulphate, which first came into use as a rat poison in Germany about 1920, has properties indicating that it might solve some of the difficult rodent-control problems in this country. Experiments conducted with it since 1924 have shown its value and resulted in its adoption to a limited extent. Thallium sulphate is a heavy mineral poison belonging to the lead group. It is more toxic than arsenic, is both tasteless and odorless, and is slow and cumulative in action. These properties make it exceptionally effective as a rat poison, but at the same time unfortunately render its use much more dangerous than many others. Most of the common poisons have properties that quickly identify them, such as the intense bitterness of strychnine or the smoke, odor, or luminousness of phosphorus. If these are lacking, there may be an effective antidote, as with barium carbonate, but thallium sulphate has no warning property and, so far as known, no antidote. There is also danger of absorption through the skin, attended by impairment of the sexual glands, loss of hair, and the possibility of death. Furthermore, the bodies of rodents killed with thallium are a menace to the lives of other animals that may feed upon them. These factors render its use by the general public inadvisable, but in the hands of specialists it has been thoroughly demonstrated to be an effective poison against rodent pests. Thallium has been used most extensively in combating the California ground squirrel and species of prairie dogs that are especially difficult to control.
The minimum lethal does of thallium sulphate has been found to be approximately 25 milligrams to 1 kilogram of white rat (or 0.13 grain to one adult rat). An effective proportion of thallium sulphate is about 1 ounce to 12½ pounds of suitable rat bait.
Experiments with calcium cyanide in fumigating rodent burrows were probably first conducted in 1921. To-day vast quantities of it are being used throughout the United States for that purpose. It is a highly volatile chemical that rapidly gives off hydrocyanic-acid or prussic acid gas when acted upon by atmospheric moisture. That this gas is quick acting and deadly when present in lethal concentration renders it an effective means of destroying any of the deep- burrowing rodents. Because of its danger to human beings, it should be used carefully and the hands washed afterward. None should be taken into the mouth, and operators should stand to windward so as not to inhale the fumes.
Fumigation with calcium cyanide presents the most practicable means of destroying rats in their burrows, which are usually shallow, extensive, and with numerous openings that allow the rats to escape or retreat if any slow-acting gas is used. When calcium cyanide is forced through their runways, however, rats are usually overcome before they can make any successful move toward self preservation. This method may be used successfully in fields, along ditch banks, in refuse and garbage dumps, under tight floors, and, when the air is quiet, even in piles of lumber or trash and in corncribs. (Fig. 196.)
There is little danger attending the use of calcium cyanide in corn or other edible products if these are not fed for several days after the fumigation, for after the gas has entirely been given off the residue is nonpoisonous.