Insecticides are substances used to kill insects, as poisons, washes and gases. Insects are subject to many natural checks, such as wind, rains, sudden changes of temperature, the attacks of parasites and predaceous enemies, and are often destroyed in great numbers by bacterial and fungous diseases. In spite of these natural checks it is, however, usually necessary to resort to a spray or some other artificial insecticide for the protection of our crops.

The essential requirements for a satisfactory insecticide are: efficient killing power, safety to the foliage, cheapness and ease of application. The choice of an insecticide for any particular case will depend upon a number of factors: upon the structure, habits, and life-history of the insect to be killed; and upon the susceptibility of the host plant to injury, its mode of growth and the conditions under which it is cultivated. Some insects, as the plant-lice, are soft-bodied and provided with a thin and delicate integument; others, like the beetles and wireworms, have hard, horny shells impervious to ordinary spray liquids; some insects bite off and swallow portions of the plant, while others merely suck out the sap by means of a slender tube; some are injurious in the larval stage, others as adults; some attack the roots, some the foliage and fruit, while others burrow in the trunk and branches. Plants vary greatly in their susceptibility to injury from the use of insecticides; the peach and Japan plum have especially tender foliage, while the apple is not so easily injured. All these points and many more must be considered in selecting an insecticide which will be adapted to the control of any injurious insect.

Our methods of fighting insects are constantly changing as new facts are discovered, new methods devised and new insecticides invented. Our present methods are the results of a more or less unconscious cooperation extending over many years between the practical grower, the student of insect fife and the progressive manufacturers of spraying materials and spray machinery.

Insecticides may be classed into those which are eaten with the food and kill by poisoning; those that kill by contact with the insect's body; and fumes of gases used for fumigation. The poisons are effective against the biting or chewing and lapping (fruit flies) insects; the contact insecticides are used as a rule against sucking insects; and fumes and gases are employed principally in greenhouses and for the fumigation of nursery stock, stored seeds, and citrus trees.

Poisoning Insecticides

The moat widely used substance for the poisoning of insects is arsenic in its various compounds. For this purpose only compounds insoluble in water can be used, as soluble arsenic is very injurious to foliage.

White Arsenic

This is the cheapest form in which arsenic can be obtained. It is a white powder, soluble in water and very injurious to foliage. A cheap and efficient insecticide may be prepared from it as follows:

For use with bordeaux mixture only. Sal-soda, two pounds; water, one gallon; arsenic, one pound. Mix the white arsenic into a paste and then add the sal-soda and water, and boil until dissolved. Add water to replace any that has boiled away, so that one gallon of stock solution is the result. Use one quart of this stock solution to fifty gallons of bordeaux mixture for fruit trees. Make sure that there is enough lime in the mixture to prevent the caustic action of the arsenic.

For use without bordeaux mixture. Sal-soda, one pound; water, one gallon; white arsenic, one pound; quicklime, two pounds. Dissolve the white arsenic with the water and sal-soda as above, and use this solution while hot to slake the two pounds of lime. Add enough water to make two gallons. Use two quarts of this stock solution in fifty gallons of water.

As there is always some danger of foliage injury from the use of these home-made arsenic compounds, and as they cannot be safely combined with the dilute lime-sulfur when used as a summer spray, they are now rarely employed in commercial orchard spraying.

Paris Green

Paris green is composed of copper oxid, acetic acid and arsenious oxid chemically combined as copper-aceto-arsenite. By the National Insecticide Law of 1910, paris green must contain at least 50 per cent arsenious oxid and must not contain arsenic in water-soluble form equivalent to more than 3 1/2 per cent arsenious oxid. For many years paris green has been the standard insecticide for orchard use, but is now largely replaced by the safer and more adhesive arsenate of lead. In spraying apples, paris green is used at the rate of one-half pound to one hundred gallons of water or bordeaux mixture. When used with water, lime twice the bulk of the paris green should be added to lessen the danger of foliage injury. Paris green cannot safely be used with either the dilute lime-sulfur as used for summer spraying or with the self-boiled lime-sulfur.

London Purple

London purple is an arsenite of lime and is a by-product in the manufacture of aniline dyes. Its composition is variable, the arsenic content varying from 30 to 50 per cent. Owing to the presence of much soluble arsenic it is likely to cause foliage injury, and it is now little used in commercial spraying.

Arsenate Of Lead

Arsenate of lead was first used as an insecticide in 1893, in Massachusetts. It has now almost entirely replaced paris green for orchard work throughout the country. It adheres better to the leaves, may be used at considerably greater strength without injuring the foliage and may be combined with a dilute lime-sulfur solution or with the self-boiled lime-sulfur. Chemically, arsenate of lead may be either triplumbic arsenate or plumbic-hydrogen arsenate. The commercial product usually consists of a mixture of these two forms, the proportion depending on the method of manufacture employed. It is usually sold in the form of a thick paste, but for some purposes the powdered form is preferred. Under the National Insecticide Law of 1910, arsenate of lead paste must not contain more than 50 per cent water and must contain the arsenic equivalent of at least 121/2 per cent arsenious oxid. The water-soluble arsenic must not exceed an equivalent of three-fourths of 1 per cent of arsenic oxid. In the best grades of arsenate of lead paste the chemical is in a finely divided condition, and thus when diluted for use remains in suspension for a considerable time. Arsenate of lead is used at various strengths, depending upon the insect to be killed and on the susceptibility of the foliage to injury.

Four pounds in one hundred gallons can be used on the peach if combined with the self-boiled lime-sulfur; on apple, four or five pounds in one hundred gallons is usually sufficient; on grapes for killing the grape root-worm beetles and the rose-chafer, eight to ten pounds in one hundred gallons have been found necessary. The poison is more readily eaten by these beetles if sweetened by two gallons of molasses in one hundred gallons, but, unfortunately, the addition of molasses greatly decreases the adhesiveness of the poison. Some species of fruit flies may be controlled by the use of sweetened arsenate of lead sprayed on the foliage of the plants at the first appearance of the flies. They lap up the poison with their fleshy tongue-like mouth-parts and succumb before ovipositing.

Arsenite Of Zinc

Arsenite of zinc is a light fluffy powder and contains the equivalent of about 40 per cent arsenious oxid. It has been used extensively on the Pacific slope as a substitute for arsenate of lead. It kills somewhat more quickly and is fairly safe on apple foliage when used with bordeaux mixture or with lime. When sweetened with molasses, it is injurious to foliage. One pound of zinc arsenate is equivalent to about three pounds of arsenate of lead. In orchard experiments, as a rule, it has not shown that it is superior to the latter.

Hellebore

Hellebore is a light brown powder made from the roots of the white hellebore plant (Veratrum album), one of the lily family. It is applied both dry and in water. In the dry state, it is usually applied without dilution, although the addition of a little flour will render it more adhesive. In water, four ounces of the poison is mixed with two or three gallons, and an ounce of glue, or thin flour paste, is sometimes added to make it adhere. A decoction is made by using boiling water in the same proportions. Hellebore soon loses its strength, and a fresh article should always be demanded. It is much less poisonous than the arsenicals, and should be used in place of them upon ripening fruit. It is used for various leaf-eating insects, particularly for the currant-worm and rose-slug.