INSECTICIDES*
Intensive agriculture, like so many advances of civilization, brought with it its attendant evils. As land was permanently cultivated, the food for insects increased and their natural enemies became proportionately fewer; mn.e",,~ntl.r
msn
fn,,n,l
it
to the senses, with little regard to other factors now recognized as important. Natural conditions, such as light, moisture, temperature, the atmospheric gases, and compounds secreted by the plant are all worthy of consideration. Of p r i m a r y importance is a knowledge of the living habits of the insect. Leaf-chewine insects are com. . Ixited I)! poiions applicd to their food, while those insects which piercc the Ieavrs and hark and extract juiccs below t l a s ~ ~ r f a rmust r be fought with contact poisons, i. r . , iuhstances which clcitroy thr insects by snflocating them or by having a corrosive efCHARLOTTE NISSLEY fect upon their bodies. Against all insects the most effective procedure is the destruction of the eggs, accomplished by a corrosive substance applied to the dormant plants. Frequently, the application of a detergent wash in winter is a useful adjunct to insecticides, for it facilitates the reaching of the eggs by the liquid and reduces the number of crevices in which insects may deposit eggs. The effectupon the plant itself must necessarily be considered, for experimentation proves that many insecticides most effective against insects are likewise injurious to the plant itself. Obviously, the use of such an insecticide
* Prize-winning college essay,
192S29.
1244
JOURNAL on CHEMICAL EDUCATION
JULY-Auc., 1929
would defeat its own purpose. The essentials of a good insecticide, then, are cheapness, effectiveness, non-injury to the plant, and adherence. I n the past, most insecticides were found by accident. Recently, chemistry has entered more into the field, and the battle is now one of chemical warfare. The manufacture of insecticides is a distinct industry, and one of increasing commercial importance. One of the oldest, and a t the same time most common, insecticides consists of sulfur or sulfur compounds. Sulfur is one of the few insecticides applied as a solid. It is used on hops for the destruction of mildew and red spider. Its action is rather uncertain, but may be due to its slight volatility, for i t is most effective in the presence of moisture. Sometimes i t is mixed with water and applied as a wash, the action of which appears partly mechanical, for the sulfur deposited on trees glues the San Jose scale-against which it is used-to the bark and prevents egress of the young. A mixture of lime and sulfur with water is used for aphis, mites, and red spider in preference t o sulfur itself. This mixture possesses the advantage of being also a fungicide. Calcium and potassium sulfides are also used, the latter being preferable because it may be mixed with soft soap without decomposition. Arsenic is the chief ingredient of many of the most powerful insecticides. These insecticides are called arsenicals. Paris green, a compound of arsenic and copper acetate, is much used, although i t may cause injury to the foliage by scorching the leaves. Its use against the potato beetle was really the start of the insecticide industry in America. The canker worm and codling moth are also combated by it with different degrees of success. Arsenate of lead is used against the Gypsy moth, elm leaf beetle, grape worm, and codling moth. It is sold in the form of a paste, sometimes mixed with other materials, and is preferable to the home-made product, owing to the small proportion of acetic acid which it contains. Calcium arsenate was used against the boll weevil, which in 1894 almost covered the cotton areas. Nicotine is perhaps the most important of the plant insecticides. The use of nicotine dust is of recent origin. The dust which volatilizes the largest percentage of nicotine within a given time is most effective. There is an increasing tendency to use flowers as a source of dusting powders. I n the case of Pyrethrum, the dusting powder is prepared by extracting powdered flowers with mineral oil. A dust of 20% Derris root is found t o control certain lice, and other applications containing Derris are toxic to caterpillars and other pests. Hellebore, obtained from rootstock and roots of the white hellebore plant, is found effective against caterpillars on gooseberries, etc. Its effect is probably due both to its poisoning the food of the caterpillar and t o its contact with their bodies. As i t is less poisonous than arsenicals it may be safely used shortly before the ripening of the fruit.
VOL.6, NOS.7 AND 8
COLLEOS I%s.%Ys
1245
Another method of combating insects is by the use of fumigants, which, although especially adapted for indoor use, may be used out of doors as well. Of these, one of the foremost is hydrocyanic acid gas. As i t is one of the most deadly toxic gases known, care must be taken in its use. Carbon disulfide has been extensively used, hut its high explosive properties render it dangerous in other than skilled hands. An example of the type of experimentation with new compounds now being carried on by chemists is seen in the field of fluosilicates, which has offered indications of success in the past four years. Calcium fluosilicate, which has been fonnd to be among the most successful, is a by-product of the volatilization method of treating phosphate rock in the manufacture of phosphoric acid. As it is insoluble, it may safely be used without injury to foliage. By experimentation, i t has been found effective against the flea beetles, beetles attacking soy beans, and even the striped cucumber beetle, upon which arsenicals have no effect. The success of flnosilicates is in this instance probably due to the beetle's habit of cleaning its feet in its mouth. In numerous other cases, flnosilicates have been tried successfully, and their advantages over arsenicals seem to consist in controlling numerous insects resistant to arsenicals, and in being far less poisonous to man, yet fully as toxic to most insects. Improvements in the application of insecticides have been accomplished. The airplane has proved its use in the dusting of extensive infested areas, and the cooperation of the government is seen in the authorization of a large sum for experimentation in airplane dusting as a control for the sugar cane moth borer. The airplane is particularly indispensable in dusting large forest tracts, etc., which it would be almost impossible to reach by ordinary methods of insecticide application. Another method, resembling the "smoke candles" of war days, and developed by war technicians, consists in setting in the ground or carrying on poles through an infected area arsenical smokes, which fill the air with a white fog. After this has settled, the foliage is fonnd to be coated with arsenic. This method would he cheap and rapid, but as yet reports of its effectiveness are not complete. Rather than becoming less, the interest in insecticides is increasing, as people are impressed by reports of the tremendous economic loss in our own country due to insect invasions. The loss extends to growing cereals, hay and forage crops, tobacco, truck crops, cotton, fruits, forest and shade trees, live stock and stored products. In many cases the astounding proportion of one-tenth of the total crop produced is ravaged by destructive insects. Although modem agricultural methods and mechanical means may play their part, the ultimate eradication of many pests will rest upon the wise use of insecticides. The fight started in 1924 against the Mexican boll-weevil may serve as
evidence of the extensive work necessary t o combat even one variety of insect. During the experiment, 1000 poisons and 2500 of their compounds were tested for relative toxicity against the weevil. The most promising of the substances were subjected to a further test as to their toxicity on the cotton plant itself. After a long series of tests, covering a period of three years, two substances stood out as possibilities in the control of the weevil. One was a special calcium arsenate, prepared by heating calcium carbonate and white arsenic in the presence of excess air a t 650°C.; the other was sodium fluosilicate, a by-product in the manufacture of acid phosphate. Both are relatively cheap, easy to make from raw materials which are abundant, and show a toxicity equal t o commercial calcium arsenate, which is more expensive. There are minor details to be improved, such as greater adhesion in the case of the fluosilicate, which indicate that the work of the chemist is by no means complete. As in all cases, the task of choosing a suitable insecticide is made more difficult by the conditions unique t o the particular insect. I n the case of the boU-weevil, many strong poisons were found useless because of their insolubility in the system of the weevil. Also, while adhesion is necessary, it must not be too great, for the destructive effect on the weevil is due t o its habit of picking up the insecticide on its moist snout. The question of attractants and repellents likewise opens an interesting field of speculation. The weevil was found to be affected more readily by mixtures in honey than in other sweets, possibly due to a preference for the former taste. An investigation is also being carried on t o determine if there are not some substances that might be successful as repellents or irritants that would induce the weevil t o fly. The whole field of insecticides, like so many phases of chemistry, is not stationary, for a t any time substances now in common use may be replaced by others more effective for all purposes. Many substances are now claiming important places for the first time. Carbon tetrachloride is used against the hook-worm, paradichlorobenzene against the peach borer, creosote oil to preserve timbers and formaldehyde in seed treatment. The chemist has as yet to find a more completely desirable insecticideone as "harmless to the foliage as arsenate of lead, and as deadly to the insect as Paris Green and nicotine, as cheap as lime sulfur, and no more harmful to the soil than lime sulfur or nicotine." This represents a large order, but even when it is filled, the work will not cease, for indications are already seen of preventive measures in the field. I n Germany a laboratory has been established to combat the European corn borer, although corn is not now raised extensively in that country. These far-seeing people propose t o start its production on a large scale, and intend t o get a "jump ahead" of the enemy! Certainly others will follow this example, and perhaps in the distant future the use of insecticides may be largely a preventive measure rather than a cure.
Though the casual observer may readily realize the tremendous importance of the subject of insecticides to the farmer, nurseryman, fruit producer or any one whose work deals primarily with the soil and the crops produced from it, he may not a t once see that, less directly, it actually concerns each of us. Yet such is the case, for the consumer must ultimately bear the loss suffered by the agriculturist in planting, cultivating, and harvesting crops of which a large proportion is destroyed hy insects a t some time before i t reaches the consumer. Hence, whether we live in town or country, whether we are versed in agriculture or must confess to a deplorable ignorance of it, whether we pluck our dinner fresh from the garden or hastily remove it from a tin can, we cannot escape the results of these ever vigilant enemies of man. But the outlook is not wholly gloomy, for, thanks to the chemist in his laboratory, we need not stand in such great dread of the invasion of these enemies, but can look forward to the time when man may have a decided advantage in this centuries-old battle. Bibliography Chamberlain. Jos. S., Editor, "Chemistry in Agriculture," The Chemical Foundation, Inc., New Pork City, pp. 21G28. Encyclopedia Americana, Entomology, Economic, Vol. lo, pp. 401-2. Encyclopedia Brifannica, Entomology, Economic, New Vol. I, 13th edition, pp. 1011-2. Industrial and Eneineerine Chemistm. .. 16.. 1007-11 (19241, Ibid., 18, 9314, 5 7 2 3 (1926). Ibid.. 19. 493 note. 953 note, 703-11, 190 (1927). bid.; 20; 1079-80 (1928). Sanderson, E. Dwight. "Insect Pests of Farm, Garden. and Orchard," pp. 1-7, 39-58. Sciace Semite, Jan. 7, 1928, p. 13, "Europe's W a r on Insects." Thorpe, "Dictionary of Chemistry," Vol. IV, pp. 296301.
