NICOTINE THIOCYANATE A Contact Insecticide J. S. McHARGUE AND R. K. CALFEE Kentucky Agricultural Experiment Station, Lexington, Ky.
equipment. Rotenone and pyrethrum are relatively expensive and are not available in sufficient quantity to replace lead and arsenic sprays. Pyrethrum is considered idferior to nicotine (1) as an aphidicide. Some of the recently developed organic thiocyanates (4) possess marked ovicidal properties, and a few have been used with great effectiveness and apparent safety. In other cases considerable injury to foliage has occurred.
Nicotine thiocyanate was prepared by decomposition of ammonium thiocyanate. The compound is very effective as an aphidicide and,in sufficient concentration and with a suitable spreader, controls red spider. I t is decomposed by the more positive ions in solution, losing much of its effectiveness, and is then capable of serious foliage injury. Practical spreaders for use with this insecticide are given.
T
Preparation
HE wide use of insecticides which leave a residue of poisons on foods has become an increasingly important factor in public health. Extensive analyses (3) of marketed vegetable foods proved that lead and arsenic are often present in excess of the amount considered safe for human consumption. Although no cases of fatal poisoning have been definitely traced to residual arsenic or lead on vegetable foods, many deaths are known to have resulted from the careless use of these and other insecticides. Fatalities to domestic animals, birds, and bees have been definitely traced to spray residues left from the applications of insecticides. The Food and Drug Administration (8) has severely criticized the reckless use of insecticides and has urged the experiment stations to use extreme caution in their spray recommendations. It is possible to remove a major portion of the residue from some classes of fruits and vegetables by washing with dilute acids (6),particularly if a carbonate was present in the spray. Such a method, however, is not generally applicable, and since arsenic and lead are in effect cumulative poisons, such a treatment does not eliminate an undesirable condition. The use of spray materials that are nontoxic to higher animals and are slowly volatile, or of those that decompose rapidly to nontoxic products, is much safer if poisonous residues from vegetable foods are to be avoided. To compare with residual poisons in effectiveness, such compounds must minimize the necessity of residual effects by giving a high degree of insect control at all stages of the life cycle. Nicotine, derris extracts, pyrethrum, and “summer oils” have been used as contact insecticides with considerable success (1) to avoid poisonous residues. Smith et al. (6, 7) showed that fluorine compounds are physiologically harmful, making the residues undesirable on vegetable foods. The summer oils are effective on a large variety of insects and in many cases have pronounced ovicidal action. The use of such sprays is limited chiefly by their adverse effects on plant tissues and exacting weather requirements. Atomized oil fogs carrying a toxicant are future possibilities, depending on the perfection of adequate spray
Nicotine, a by-product of the tobacco industry, has been available commercially as an insecticide for many years. The alkaloid is a strong poison but, being very soluble, does not remain long as a residue. The salts at present used as insecticides decompose rapidly under atmospheric conditions, forming volatile or nontoxic products. The present nicotine insecticides are noninjurious to foliage in any practical spray concentration. Nicotine and the salts used as insecticides are not effective in practical spray concentrations for the control of a large number of insect species and have no ovicidal action. Combinations of nicotine and the thiocyanate radical were investigated as a means of increasing the effectiveness of nicotine insecticides. The least complex compound of this type is the nicotine salt of thiocyanic acid. Absolute thiocyanic acid is a pungent, colorless liquid that decomposes rapidly in air at ordinary temperatures. To avoid the difficulties in preparing and handling the pure acid, nicotine thiocyanate was prepared by decomposition of the ammonium salt. Pure, colorless nicotine was prepared by subjecting the commercial 95 per cent product to a temperature of 150” C. for 30 minutes and then distilling under reduced pressure. The air in the distillation system was displaced with nitrogen gas before evacuation. The liquid was stored under nitrogen until required for use. Reagentquality ammonium thiocyanate used in the preparation was previously dried over sulfuric acid under reduced pressure. Nicotine thiocyanate was prepared by adding 100 parts of cold anhydrous nicotine to 46.5 parts of dry ammonium thiocyanate in a round-bottomed flask provided with bubbling tube and vacuum connections. The flask was immediately evacuated and kept at 16’ C. by cooling with running water. A stream of nitrogen gas, passed through the reacting mixture, kept it stirred and assisted in removing the evolved ammonia. The temperature of the reaction was slowly increased to 60’ C. in order to maintain a viscosity sufficiently low to permit free bubbling of the nitrogen. The tendency for the compound to decompose increased rapidly with elevation in temperature. The final detectable traces of ammonia and the slight excess of nicotine were removed by allowing the compound to remain for several weeks in shallow dishes over sulfuric acid in an evacuated desiccator. Thin films on glass slides formed light, featherlike crystals upon cooling and touching with a stirring rod. The compound has approximately the following average composition: thiocyanate, 26.33 per cent; nicotine, 72.75 per cent. It is very hygroscopic.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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TABLEI. EFFECTOF SPREADERS WITH NICOTINE THIOCYANATE ON RED SPIDER Spreader Sodium oleate
Diglycol oleate
Dilution 1 t o 800
Dilution of Nicotine Thiocyanate 1 to 1000
No. Tests 3
No. Insects 99
1 t o 800 1 t o 400
1 t o 500 1 to 1000
3 3
1 t o 800 1 t o 800
1 to 1000 1 t o 500
3 2
85 73
No. Kdled 50 46 44
Per Cent Killed 50.5 54.2 60.3
84 40
73 38
86.9 95.0
Injury SeveTe marginal chloroeis Severe defoliation Severe drip &reas None Some; marginal
1 to 800
1 to 500
3
62
62
100.0
1 t o 800 1 to 800
1 t o 1000
1 t o 2000
3 3
43 62
40 55
93.0 88.7
Some; marginal and drip areas Very slight; drip areas None
Rosino
1 to 1000 1 to 1000 1 t o 1600
1 to 1000 1 to 2000 1 t o 1600
2 2 2
64 50 50
63 48 49
98.5 96.0 98.0
None None None
Sulfonated alcohol
1 t o 800 1 to 1600 1 t o 2000
1 t o 1000 1 t o 1000 1 to 2000
3 3 2
83 76 47
83 76 39
100.0 100.0 83.0
None None None
Naphthalene sodium sulfonate
1 t o 800 1 t o 2000
1 t o 1000 1 t o 1000 1 t o 1000
2 2 2
49 60 71
48 54 68
98.0 90.1 95.8
Triethanolamine oleate
1 t o 1600 Emulsified with triethanolamine oleate, 1 t o 800.
Larger quantities of nicotine thiocyanate for insecticidal purposes were repared by heating technical ammonium thiocyanate with the cayculated quantity of distilled nicotine under reduced pressure until the odor of ammonia was not perceptible. This product was a very viscous liquid and was light brown in color. The salt was stable in the absence of moist air. Aqueous solutions decom osed in about 2 weeks; hydrogen sulfide, hydrogen cyanide, carion dioxide, and ammonia were present in old solutions.
Effect on Aphids The aphidicidal properties of nicotine thiocyanate were tested on the nasturtium aphid (Aphisrumicus 1). The solutions were applied with a continuous sprayer as a fine mist to insects on the underside of the leaves. A spreader was not used. After spraying, the leaf stems were placed in water through perforations in cardboard disks. All dilutions were made on a basis of the actual nicotine content. Spray tests were carried out in the shade at 80" F. The following counts were made 24 hours after spraying: No. Dilution Tests Nicotine thiocyanate: 1 to 615 2 1 to 1230 2 1 to 2468 2 1 to 6150 2 40 per cent nicotine sulfate: 1 to 1200 2
No. Insects
No. Killed
Per Cent Killed
44 55 48 45
44 52 48 43 (2 paralyzed)
100 100 100 95.5
60
58
96.7
The dilution of 40 per cent nicotine sulfate (1 to 1200) found necessary to control this species of aphid, under the conditions of the tests, corresponds in effectiveness to a 1 to 3000 dilution of nicotine thiocyanate. With an efficient soap spreader present, some species of aphid can be controlled by nicotine sulfate solutions as dilute as 1 to 1500, equivalent to a 1 to 3750 dilution of nicotine thiocyanate. Nicotine thiocyanate solution in practical spray concentrations had no effect on bean beetle, their larva, or red spider, although temporary paralysis resulted from high spray concentrations. Soap spreaders used in conjunction with nicotine thiocyanate induced a more complete contact but resulted in severe foliage injury to many plants. The toxicity of nicotine thiocyanate with soap spreaders compared with nicotine sulfate was much less than expected. The use of water containing considerable amounts of dissolved salts also resulted in an apparent lowering of toxicity, accompanied by intense foliage injury. To ascertain the effects of reaction between the ionizable thiocyanate and alkali ions in solu-
Some general chlorosis None None
tion, diglycol oleate and triethanolamine-oleic acid emulsions were tried as spreaders. A high degree of toxicity was retained by the nicotine thiocyanate, and foliage injury was negligible under experimental spraying conditions. The organic emulsions of oleic acid were not quite as efficient spreaders as the sodium salts. The addition of a small quantity of rosin greatly increased the wetting properties of the organic emulsions. The sulfonated higher alcohols and the sodium salts of naphthalene sulfonic acids were also found suitable as spreaders for nicotine thiocyanate, since the concentration of sodium ions is small, owing to very efficient coverage. Other factors conducive to plant injury by nicotine thiocyanate sprays are application at high temperature or in bright sunlight, and excessive spraying. Injury is less apt to result in the open than in the greenhouse.
Effect on Red Spider Spray tests on red spiders (Tetranychus telarious) were made in the greenhouse on infested bean and cucumber plants on cloudy days a t temperatures between 75" and 80" F. with a continuous sprayer. Since the spreaders do not wet the web, plants were selected that had not been infested to the extent of web formation. Counts were made microscopically 24 hours after spraying. Results are given in Table I. The dilutions given represent the actual nicotine content of the spray. Nicotine thiocyanate is toxic to red spider, provided the solution is sufficiently concentrated and a suitable wetting agent is present. The loss of effectiveness by combination of the thiocyanate ion with an alkali ion in the spray solution is clearly demonstrated by comparison of the per cent kill obtained with sodium oleate and with the organic oleatea as spreaders.
Literature Cited Alvord and Diets, IND.ENO.CHEM.,25, 629-33 (1933). Craig and Richardson, Iowa State College J . Sci., 7,477-85(1933). Meyers, Throne, Gustafson, and Kingsbury, IND.ENQ.CHEM., 25, 624-8 (1933). (4) Murphy and Peet, Ibid., 25, 638-9 (1933). ( 5 ) Robinson, R. H., Ibid., 25, 616-20 (1933). (6) Smith and Lantz, J . Biol. Chem., 101,677-83 (1933). ENO. CHEM., 26,791-7 (1934). (7) Smith and Leverton, IND. (8) White, W. E., Ibid., 25, 621-3 (1933). RECEIVED June 29, 1937. The investigation reported here was carried out in connection with a project of the Kentucky Agricultural Experiment Station and is published by the permission of the Director.
