VIII.
PROBLEMS IN THE PRODUCTION AND USE OF INSECTICIDES IN THE SOUTH
R. C. RoARK,I~ssm~cmg DIVISION, BWAU MENT OB
OB CHEMISTRY AND Sons, U.S. DEPARTAGRICULTORE, WASHINGTON, D. C.
The southern states consume the greater part of the calcium arsenate produced in this country and use large quantities of other insecticides but at present do not rank high in the production of these materials. New insecticides are now being developed, however, and the South is the logical place for the manufacture of many of these on account of the availability of raw material and the nearness of the market for the finished product. Arsenicals Certain compounds of arsenic continue to constitute the major portion of the tonnage of manufactured insecticides. Nearly 30,000,000 pounds each of lead arsenate and calcium arsenate are produced yearly in the United States. At least 6,000,000 pounds of Paris green are manufactured annually, together with much smaller quantities of magnesium arsenate, manganese arsenate, and zinc arsenite. Calcium arsenate is made from arsenic acid and lime. Arsenic acid is made by oxidizing white arsenic, which is a by-product resulting from the smelting of arseniferous copper ores. White arsenic is not produced in the South but could be brought into the Gulf ports from Mexico, Japan, and Germany at as low a cost as it is now brought into northern ports. About 21,000,000 pounds of white arsenic were imported into the United States in 1928. Lime of high quality is found at many places in the southern states. There seems therefore to be no good reason why calcium arsenate, which is largely consumed in the South, could not be ecouomically produced there. Calcium arsenate, on account of its cheapness, will doubtless continue to be used for dusting cotton, but uon-arsenicals are needed for treating fruits and vegetables. The British government prohibits the importation of apples on which remains an arsenical spray residue equivalent to more than 0.01 grain arsenic (Asz03)per pound of fruit. The Department of Agriculture in this country has a higher tolerance (0.017 grain Asz03 per pound) but this may be lowered in the future. These restrictions are aimed primarily a t lead arsenate which is used for spraying apples, peaches, grapes, and many other products. Lead is a dangerous cumulative poison, and the combination of lead and arsenic in lead arsenate constitutes a health hazard to man or animals eating sprayed fruits and vegetables. 2301
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The recent outbreak of the Mediterranean fruit fly in Florida has emphasized another shortcoming in lead arsenate. When lead arsenate is sprayed upon an orange tree the metabolism of the tree is profoundly affected. The fruit formed is deficient in sugar and in acid to such a degree as to become insipid and such fruit has a lower market value than normal fruit. Tobacco grown in certain regions in the South has been shown to sometimes retain a portion of the arsenical insecticide used to poison the caterpillars that attack the plant. This arsenic may be carried through the manufacturing processes and appear in the finished cigar or cigarette. The injuriousness of a minute quantity of arsenic in tobacco to the smoker has not been proved, but the desirability of finding an effective insecticide which is free from any possible toxic action upon man is apparent. The position of Paris green in the insecticide field is peculiar. Paris green was the first arsenical successfully used for poisoning insects on growing plants. It came into widespread use against the Colorado potato beetle, and before the insecticidal properties of lead arsenate were discovered, Paris green was used to control such insects as the codling moth in apples. It might have been predicted that lead arsenate and calcium arsenate would supersede Paris green but there is more Paris green used today than ever before. The discovery that Paris green when scattered over the surface of swamps is effective in killing mosquito larvae in the water has greatly stimulated its use and large quantities are used in the malarial regions of Louisiana and other southern states. The green color of Paris green, simulating that of algae, apparently induces the mosquito larvae to swallow the fine particles. For the purpose of mosquito control Paris green does not suffer from competition with other arsenicals, and its larvicidal properties will doubtless increase the demand for it. Arsenical cattle dip is one of the few insecticidal materials which are used exclusively in the South. It consists of a concentrated solution of sodium arsenite, sodium cresylate and soap, and when properly diluted in water is efficaciousin destroying ticks on cattle without injuring the animals. Owing to the efforts of the Unitedstates Department of Agriculture the area infested with the cattle or Texas fever tick has been greatly reduced within recent years. Arsenical cattle dip is one of the insecticides for which a diminishing rather than an increasing market may be predicted. Fluorine Compounds Non-arsenical insecticides which will be effective in combating chewing insects are urgently needed. A great deal of research is now under way to develop new insecticides of this character. Among materials of mineral origin certain compounds of fluorine appear most promising as substitutes for lead arsenate. Fluorine occurs in the phosphate deposits of Florida and is a by-product in the manufacture of phosphatic fertilizers. Phosphate
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rock from the southern states contains in addition to calcium phosphate, fluorspar (calcium fluoride), and sand (silicon dioxide). In the manufacture of phosphate fertilizer, this rock is heated with sulfuric acid, which reacts with the fluoride present to form hydrofluoric acid, which in turn reacts with the sand to form gaseous silicon tetrafluoride. As this gas is very destructive to plant and animal life, its escape into the atmosphere is prevented by passing it through water or solutions of chemicals, in which it is absorbed. I t has been estimated that phosphate rock contains on an average approximately 60 pounds of fluorine per ton, and as more than 3,000,000 tons of phosphate rock are consumed annually in the United States, there is an ample supply of fluorine for insecticidal purposes. In the form of double fluorides, fluorine has given promising results in dusting tests against the cotton boll weevil, the Mexican bean beetle, the sugar cane borer, and other insect pests. Many investigators believe that a satisfactory substitute for arsenic as a stomach poison to insects may be found among the fluorine compounds. The South is the logical place for the manufacture of fluorine insecticides. Nicotine Nicotine, the principal alkaloid in tobacco, is one of the most valuable insecticides that kill by contact. I t is extracted from the stems and from the refuse formed in the manufacture of cigars. I t is doubtful if tobacco can be grown a t a profit for the extraction of nicotine only. Nicotine will probably remain a by-product of the tobacco industry. No complete figures on the production of nicotine are available, but the exports of nicotine sulfate in 1928 amounted to about $90,000. The production of nicotine may be expected to increase with the production of tobacco, which will doubtless remain one of the principal crops of the South. Pyrethrum Flowers The flowers of a species of chrysanthemum, commonly called pyrethrum flowers, contain two esters, Pyrethrin I and Pyrethrin 11,which are deadly to many insects but have little or no toxic action upon higher animals and do not injure delicate plants upon which they are sprayed. These esters are the basis of most of the fly sprays now so widely sold in the United States. I t has been estimated that the sale of pyrethrum fly sprays amounts to about $20,000,000 annually. Except for a negligible supply from California the pyrethrum flowers consumed in this country are imported from Yugo-Slavia and Japan, the greater part coming from the latter country. The imports of pyrethrum flowers in 1928 amounted to more than 11,000,000 pounds, valued a t nearly $2,500,000. Pyrethrum flowers can be grown in the southern states, having been cultivated successfully
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for a number of years a t Arlington, Virginia, but whether their commercial cultivation in this country could compete with that in Japan is doubtful. Hand labor is still extensively employed in harvesting the flowers and Japanese labor is very much less costly than American. Fish-Poisoning Plants as Insecticides Certain species of tropical plants used to poison fish contain constituents that are highly toxic to insects. For example, Derris elliptica (tuba root), an East Indian plant, and Lonchocarpus nicou ("cube"), which grows in the Amazon basin, contain a white crystalline compound called rotenone. This is thirty times as toxic to aphids as is pure nicotine and also exceeds lead arsenate in toxicity to caterpillars. Rotenone when taken by mouth is without effect upon domestic animals, indicating that it is also harmless to man. Fruits sprayed with rotenone would therefore be free from a toxic spray residue. Rotenone appears to be an ideal insecticide and there is a very large potential market for it. It may be possible to grow Derris and Lonchocarpus in southern Florida and, if successful, their cultivation should be highly profitable. Sulfur Sulfur in the form of sulfur dust or wettable sulfur or as lime-sulfur solution is a valuable insecticide. Finely powdered sulfur has been used successfully in Texas for controlling the cotton flea, and, in combination with lead arsenate, it is used against the strawberry root weevil and other insects. Lime-sulfur solution for years has been the standard dormant spray material for use against San JosC and other scale insects infesting orchards. Properly diluted it is a valuable fungicide. The South produces nearly all the sulfur in this country and should manufacture a larger share of the insecticides and fungicides containing sulfur.
Oil Emulsions Emulsions of petroleum oils are now extensively used in place of limesulfur solution for combating scale insects. In California oil emulsions are used instead of hydrocyanic acid for treating citrus trees. In Florida and the Gulf States oil emulsions are used to control plant lice and other insects on citrus trees and in Arkansas and Virginia they are employed against apple pests. An emulsion of pine oil has been found to be effective against overwintering larvae of the codling moth. The addition of para-dichlorobenzene to this pine-oil emulsion makes a mixture effective against peach tree
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borer. The possibilities of pine oil as an insecticide are well worth further study. New Fumigants Natural gas in West Virginia is the raw material from which two new fumigants are made. These are ethylene dichloride and ethylene oxide. Ethylene dichloride is replacing carbon disulfide for the fumigation of carpets and upholstered furniture in warehouses, and ethylene oxide appears to be the ideal fumigant for dried fruits and other foodstuffs because it leaves no toxic residue. It may be possible t o produce many other insecticides from natural gas, a development in which the South should play a prominent part. Repellents The screw worn fly and other blowAies in Texas and the Southwest may be deterred from laying their eggs in wounds on domestic animals by an application of pine tar oil. This oil is a black viscous liquid obtained by the destructive distillation of pine stumps, roots, or wood. Thevalue of this insecticide may be gaged from the fact that this treatment greatly diminishes the loss to the livestock industry caused by blowflies-a loss estimated conservatively at $4,000,000 annually. The production of pinetar oil and other pine products is confined to the South. Future Possibilities The most effective insecticides of the future will undoubtedly be synthetic organic compounds. The starting point for the manufacture of these compounds may be coal tar, petroleum, pine oil, or natural gas. The southern states are rich in all these raw materials and possess a monopoly of pine oil. Valuable insecticides and fumigants are now commercially prepared from petroleum and from natural gas, and the possibilities of pine oil as a raw material for the syntheses of organic insecticides may be as good as those of the other materials. Conclusions Paris green will probably be increasingly used for mosquito control, and calcium arsenate seems destined to hold its own as a cheap cotton-dusting material. A substitute for lead arsenate must be found, however, and new stomach insecticides are needed for the control of the Mediterranean fruit fly, the Mexican bean beetle, the sugar cane borer, the codling moth, the Oriental fruit worn, and many other pests. Fluorine appears the most promising arsenic substitute among the mineral compounds. Rotenone, a constituent of tropical fish-poisoningplants, is both a contact and a stomach poison to insects and on account of its non-toxicity to mammals when taken by mouth promises to be an ideal insecticide.
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The South is a large consumer of calcium arsenate, Paris green, and other insecticides and is strategically located for the manufacture of insecticides and fungicides containing fluorine or sulfur, or depending upon petroleum oil, natural gas, or the products of pine distillation as raw materials. There appears no reason why the South should not manufacture the arsenicals consumed there and also produce many of the newer insecticides.
