The Inorganic Insecticides

R. H. Carter.

Inorganic insecticides are of mineral origin, mainly compounds of antimony, arsenic, barium, boron, copper, fluorine, mercury, selenium, sulfur, thallium, and zinc, and elemental phosphorus and sulfur.

Antimonyl potassium tartrate, tartar emetic, K(SbO)C₄H₄O₆.1/₂H₂O, is a white powder soluble in water. It is sometimes used as the toxic agent in ant poisons and for the control of thrips.

Arsenical compounds are the most widely used inorganic insecticides. Recommendations for their use date from 1681. They were probably used before that. The poisonous properties of arsenic trioxide were well known during the Middle Ages and it was a favorite instrument of murder as practiced by the Borgias. This knowledge of the poisonous properties of arsenic compounds probably led to their use as insecticides.

Arsenic trioxide, As₂O₃, also called arsenious oxide, is a white crystalline material sometimes referred to as white or gray arsenic. It is the starting material in the manufacture of arsenical compounds used as plant insecticides and it is sometimes used in weed killers. It is obtained from the flue dust from copper smelters. Our supply comes from domestic and foreign sources. It is sometimes used as the toxic agent in baits to control grasshoppers, cutworms, and other insects.

The calcium arsenate that is sold commercially as an insecticide is not a single chemical compound but a complex mixture of several calcium arsenates and an excess of calcium hydroxide. The material is made from arsenic trioxide by first oxidizing it to arsenic pentoxide with nitric acid and then reacting the solution of arsenic pentoxide or arsenic acid with a slurry of calcium hydroxide. The conditions of temperature, concentration, and duration of reaction are important because of their influence on the physical nature of the product. Commercial calcium arsenate generally is colored pink and is alkaline in reaction. It is a finely divided powder. It has been used extensively against certain insects affecting field crops, especially cotton. It cannot be used safely on apples, peaches, beans, and some other crops because of its burning effect on the foliage and fruit.

Calcium arsenate-calcium arsenite mixture is sold under the name London purple. It has some use for poisoning insects on cotton.

Among the insecticidal materials containing copper and arsenic, copperaceto-arsenite (or Paris green), 3 Cu(As0₂)₂.Cu(C₂H₃O₂)₂, is by far the most important. It has been used as an insecticide since about 1870. For many years it was the most widely used insecticide in the United States for control of Colorado potato beetles. It has largely been supplanted by some of the newer materials, but approximately 4 million pounds are used annually by farmers and gardeners.

Copper arsenite, Cu₃(AsO₃) ₂.xH₂O; copper meta arsenite, Cu(AsO₃)₂- H₂O; and basic copper arsenate, Cu₃(AsO₄,) ₂-CU (OH) ₂, have all been proposed as insecticides but have not been used to any extent. Compounds similar to Paris green made from organic acids other than acetic have also been tested but have not developed into commercial use.

Several chemically different compounds are known as lead arsenate. Two of them are commonly used as insecticides. Acid lead arsenate (dileadortho arsenate), PbHAsO,, is formed by the action of arsenic acid on litharge salt. It is a white powder, insoluble in water. Basic lead arsenate (lead hydroxy arsenate), Pb₄(PbOH) (AsO₄)₃, also is a white insoluble powder.

Both forms should contain very little water-soluble arsenic pentoxide in order to minimize plant damage. Generally they are much less apt to burn plant foliage than is calcium arsenate or Paris green. The basic compound is safer to use on growing plants in some localities (for example, the foggy regions of California) than is the acid compound, but in general it is not so toxic to insects.

Acid lead arsenate is used extensively to control chewing insects on fruits, such as apple and pear, on flowers, trees, and shrubs, and on vegetables, such as potato and tomato. It also has extensive use in treating soil to control Japanese beetle and Asiatic garden beetle larvae and related soil-infesting forms.

A number of United States patents cover processes for the manufacture of magnesium arsenates for use as insecticides. The magnesium arsenates tested as insecticides consisted generally of the dimagnesium arsenate, MgHAsO₄, the trimagnesium arsenate, Mg₃(AsO₄)₂, or the pyroarsenate, Mg₂AS₂O₇, with varying amounts of water of crystallization and excess magnesium oxide or hydroxide.

Magnesium arsenate has been tested against a large number of insects affecting fruits and vegetables and at one time was recommended for the control of the Mexican bean beetle, but its use has declined.

A crude manganese arsenate once was proposed as an insecticide for combating caterpillars on tobacco because its brown color made it less conspicuous on cured tobacco leaves than the white lead arsenate.

Sodium arsenite is formed by dissolving arsenic trioxide in sodium hydroxide solution. Depending on the ratios of the reacting materials, the products range from the monosodium compound, NaAsO₂, to the trisodium arsenite, Na₃AsO₃. A standard formula for making so-called liquid sodium arsenite requires 4 pounds of white arsenic and 1 pound of sodium hydroxide per gallon of solution.

Sodium arsenite is not used as an insecticide on field crops because of its corrosive action. It is used as an ingredient in poison baits for grasshoppers, crickets, roaches, ants, and other insects, and in stock dips. It has been used extensively as a weed killer.

Zinc meta arsenite, Zn(AsO₂)₂, is formed when a soluble zinc salt is reacted with arsenious acid or white arsenic under carefully controlled conditions, as it is soluble in either acid or alkaline solutions.

Zinc arsenite is used in wood preservation but is not used in household insecticides or as a constituent of formulations to be used on field crops.

Zinc arsenate, Zn₃ (AsO₄) ₂, has been proposed in place of lead arsenate in codling moth control, principally because it avoids lead residues.

Arsenates and arsenites of many of the other elements have been investigated for insecticidal use but none has been developed into satisfactory materials. Organic arsenicals have likewise failed to find a place as insecticides.

Barium carbonate, BaCO₃, is a white, finely divided powder which is sometimes used as the toxic agent in rat poisons.

Borax, Na₂B₄O₇, and boric acid, H₃BO₃, have been used in roach powders, but more effective compounds, such as sodium fluoride, DDT, and chlordane, are available now.

Bordeaux, or bordeaux mixture, is the name applied to the compounds formed by reacting dilute solutions of copper sulfate with calcium hydroxide suspensions. If equivalent amounts of the two materials are used, an intimate mixture of the copper hydroxide, Cu(OH)₂, and calcium sulfate, CaSO₄, is formed. This suspension has a blue color and leaves a bluish-white deposit on sprayed surfaces.

Bordeaux mixture is primarily a fungicide but is often used in connection with insecticides such as nicotine, lead arsenate, and calcium arsenate. It is sometimes used to control the potato leafhopper and as a repellent for flea beetles on various vegetables and flowering plants. It is sometimes used also as an emulsifier for lubricating-oil sprays applied to fruit trees, such as apple, pear, quince, and peach, when they are dormant.

Several other copper compounds, including the oxide, oxychloride, phosphate, quinolinolate, silicate, basic sulfate, and cyanide are used as spray materials. They have little insecticidal value but are potent fungicides.

Hydrated lime, or calcium hydroxide Ca(OH)₂, is used in the manufacture of lime-sulfur, calcium arsenate, and bordeaux mixture. When limestone, CaCO₃, is heated, the carbon dioxide is driven off, leaving the product known as quicklime, CaO. When quicklime reacts with water, heat is evolved and the resulting product is hydrated lime, Ca(OH) ₂. Hydrated lime is not primarily an insecticide but is used as a safener with some of the arsenical sprays.

Calcium cynanide, Ca (CN) ₂, reacts slowly with moisture in the air to liberate hydrocyanic acid gas, a highly toxic organic compound used as an insecticidal fumigant.

Compounds that contain fluorine have been in use as insecticides since about 1890. Barium fluosilicate, BaSiF₆, a white, finely divided powder, has been tested extensively as a substitute for arsenicals in the control of fruit and vegetable crop insects. It has some value in the control of flea beetles, blister beetles, Mexican bean beetle, and others.

Cryolite, or sodium fluoaluminate, Na₃AIF₆, is a white crystalline material. Natural cryolite (ice-stone) is mined in Greenland and imported into this country. Synthetic cryolite, of similar composition, has been manufactured and sold for insecticidal use. For most uses there is little difference in their effectiveness. Large quantities have been used on codling moth in the Pacific Northwest and on the tomato pinworm, tomato fruitworm, lima-bean pod borer, corn earworm, Mexican bean beetle, walnut husk fly, pepper weevil, cabbage caterpillars, blisterbeetles, and flea beetles. It is generally used as a spray but may be diluted with talc, pyrophyllite, or other diluents to form a dust.