Residual Action of Organic Insecticides U
Elmer E. Fleck U . S . D e p a r t m e n t of A g r i c u l t u r e , B u r e a u of E n t o m o l o g y a n d P l a n t Q u a r a n t i n e , Beltsville, M d .
T h e residual action of nonvolatile organic insecticides is governed largely by their resistance to chemical change under field conditions. Oxidation has been shown to be a common cause of failure in pyrethrum, rotenone, phenothiazine, and DDT. Pyrethrum also fails by polymerization. These reactions are catalyzed by light and attempts to control them with antioxidation, antipolymerization agents, and ultraviolet opacifiers have been but partly successful. Other materials encountered in the application of these insecticides may act catalytically to produce decomposition and alkalies have been found to destroy insecticidal action in many cases. ~
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HE residual action of a n insecticide is determined by its vapor pressure, its sticking power, its solubility, its absorption into the surface on which it is applied, and its resistance to chemic$ change. This discussion is limited to organic insecticides of low vapor pressure and the chemical changes they undergo when exposed in thin layers t o the action of sunlight, air, and the materials they may contact during the course of their use. The action of direct sunlight is of prime importance in insecticides used on agricultural crops. The combination of temperatures up to 60" C. and bright sunlight presents unfavorable conditions for stability in an organic material that is exposed as a fine dust or as a thin film. It is safe to say that no known organic insecticide is as stable as the inorganic insecticides. Lasting stability, however, is not always desirable as it involves the problem of the removal of spray residue when edible crops are harvested. For agricultural crops, the ideal insecticide should have sufficient residual action to protect the crop from insect damage but should decompose to inert material or be easily removed between the time of harvest and the time the material is consumed. It is through a knowledge of the chemical properties of insecticides that much progress has been made toward finding the ideal. Up to this time the chief concern with the residual action of organic insecticides has been to prolong residual action.
(90). West (22) has shown that besides oxidation, polymerization occurs; this involves the pentadienyl side chain. Polymerization is inferred from the loss of knock-down power found in polymerized pyrethrum which is similar to the loss found when side chains of the pyrethrum molecules are hydrogenated. Copper and brass have been found to catalyze this polymerization. Roark ( 1 7 ) called attention to the action of soap and alkalies as materials which destroyed the effectiveness of pyrethrum. I n spite of this instability of pyrethrum the recent work of Metcalf and Wilson (16) indicates that it exerts a remarkable residual action against mosquitoes. These investigators sprayed a solution of pyrethrum extract (pyrethrins 2%) in kerosene onto plywood at the rate of 200 mg. per square foot and after 12 weeks obtained a 24-hour mortality of about 90%. Exposure to indoor light reduced the effectiveness. During this 12-week period the knock-down time changed from a n original 4 to 5 minutes to 20 to 36 minutes, with the light-exposed surfaces showing the greater deterioration. I n a n effort to prolong the insecticidal action of pyrethrum, antipolymerization and antioxidation agents h'ave been added. Bushland, Schechter, Jones, and Knipling (2) reviewed this work in connection with prolonging the usefulness of 1% pyrethrum louse powders. These studies showed that the addition of 3.3% hydroquinone or isopropyl cresol prevented detectable deterioration for 79 days, but that after exposure for a year, the dust had lost its toxicity. ROTENOKVE
Another insecticide of plant origin which has limited residual action is found in rotenone (18). Rotenone, CHiO
PYRETHRUM
The structural formulas of the pyrethrins, (?E312
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is entirely stable in the solid state when not exposed to light. Jones et al. (11) have shown that direct sunlight catalyzes its oxidation to form a mixture of dehydrorotenone and rotenonpne, which are yellow products. Oxidation was found to take place much more rapidly when the material is in solution than when it is in the solid state. Solvents such as pyridine and chloroform accelerate oxidation whereas benzene and alcohol solutions are more resistant. Under ordinary conditions of field application the insecticidal action of rotenone dust will last almost a week. Many attempts have been made t o prolong the action of rotenone by the addition of antioxidants, but none of these 4as achieved general acceptance. Alkaline materials should not ,be used with rotenone. Rotenone dusts containing lime, for instance, deteriorate during storage even in the dark (18). Prolongation of the residual action of rotenone has been partially solved by converting it into dihydrorotenone. Haller and
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recently advanced by LaForge and Soloway (IS) show the high degree of unsaturation of these compounds which readily accounts for their instability, particularly in the presence of light and air
706
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1948
Schaffer (10)were able to form this compound in quantity by hydrogenation with Raney nickel catalyst in neutral solution. Dihydrorotenone is somewhat more toxic to mosquito larvae and goldfish than is rotenone (11). During the first days of exposure to sunlight, the toxicity of the hydrogenated product remained much higher than rotenone, but a t the end of 20 days both had lost toxicity to about the same extent. PHENOTHIAZINE
Phenothiazine is an example of a synthetic organic compound whose residual action is largely limited by its instability in light and air. When this material is used against mosquito larvae in barrels and small containers it is effective for as long as a year (3). On the other hand, when phenothiazine is sprayed on foliage that is exposed to sunlight and air its effectiveness is reduced by bronzing (9). This difference is associated with the oxidation of phenothiazine. DeEds and Eddy (4) have shown that oxidation results in a complex system of phenothiazone and hydroxyphenothiazone, both of which may act in equilibrium with their leuco forms. This susceptibility of organic insecticides to oxidation in air and sunlight has led to the use of oxidation inhibitors, such as @-naphthol and hydroquinone, which materially lengthen the effective period of the insecticide (19). Control of oxidation has also been attempted by the addition of ultraviolet opacifiers, such as tetramethyldiaminobenzophenone (Michlers ketone) and cerium oxalate ( 1 ) . The theory of this control method is that the ultraviolet light is absorbed by the additive and thus prevented from acting on the insecticide. A simple, effective solution to the oxidation problem could greatly increase the use of phenothiazine as an insecticidal material. Outstanding examples of residual action in synthetic organic insecticides are found in new chlorinated products such as DDT, benzene hexachloride, chlordan, and Toxaphene. DDT is discussed in this paper because much of the chemistry of the newer members of this group has not yet been developed. DDT
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When DDT is used indoors a limiting factor on its residual action is the coverage of the deposit by dust and dirt. Lindquist et al. (16) showed that fly cages treated with DDT were still killing flies after 8.5 months. D D T is remarkably resistant to oxidation and when exposed indoors, as a fine powder, will remain unchanged almost indefinitely (6). It is not oxidized readily by refluxing in glacial acetic acid in the presence of chromic anhydride. Heating with nitric acid nitrates DDT, but does not oxidize it (24). I n contrast to this stability in air and in acid solution, D D T is readily dehydrochlorinated in alkaline solution to form a n insecticidally inert compound. c1
DDT
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Dehydrochlorinated p,p‘-DichloroDDT benzophenone
Wain and Martin (91) have shown that 0.1 N alkali will completely dehydrochlorinate p,p’-DDT a t room temperature in 30 to 60 minutes. However, this sensitivity to alkali is found only
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707
when the D D T is in solution. Because of the low solubility of DDT in water it is possible to shake suspensions of D D T in limewater for a month without a reaction occurring, but if a mutual solvent such as alcohol is added reaction takes place rapidly, even a t room temperature, to form dehydrochlorinate8 DDT. Some wetting agents may also provide the mutual solubility needed for this reaction. D D T is also susceptible to catalytic dehydrochlorination (6). Catalysts such as anhydrous ferric and aluminum chlorides dehydrochlorinate molten D D T when present in a small amount (0.01%). Other material-iron, rust, stainless steel, and chromium-exhibit the same catalytic action, but to a lesser degree. D D T on rusty fly screens will lose its effectiveness much sooner than on well painted screens or those made of copper or brass. Fortunately this catalytic action on DDT is inhibited by most of the solvents used, such as kerosene and oils in general. Fats, fatty acids, and alcohols also inhibit this action. The exceptions are the chlorinated solvents-0-dichlorobenzene, ethylene dichloride, and nitrated solvents such as nitrobenzene. Solutions of D D T in these solvents are susceptible to the action of catalysts at ordinary temperatures. The presence of water reduces this action, presumably by hydrating the anhydrous chlorides that may be present, or are formed, by chlorinated by-products of DDT. Reports vary greatly on the duration of the residual action of DDT deposits under field conditions. Thus Fleming and Chisholm (7) reported that one application of DDT was effective in controlling a Japanese beetle infestation on plum trees in New Jersey throughout a season. Gunther et al. (8) report a slow decrease in effectiveness after exposure for 3 weeks on orange and lemon trees in California. At the 86-day period from 80 to 95% of the original deposit had been lost. Lindquist ,et aZ. (14) exposed sprayed boards and glass plates to ultraviolet iight and in all cases, where solutions were used, decomposition of the D D T had occurred. They concluded that the extent of decomposition could be correlated roughly with the boiling point of the solvent. The higher-boiling solvents produced the greater decomposition of DDT. Experiments with solid D D T show that both technical and pure p,p’-DDT are rather stable against strong irradiation with ultraviolet light. The technical product becomes coated with a brown material, but the pure product shows little change (6,2325). This greater susceptibility of the technical DDT may be due to the oily by-products which tend to bring the D D T into solution under exposure to high temperatures in the summer. That D D T is less resistant to ultraviolet light when in benzene solution than it is in the solid state has been brought out by Wichmann et al. (93). They found that a t least 50% of the DDT was changed into other substances within a few hours when irradiated with ultraviolet light from a powerful source. These workers isolated the 2,4dinitrophenylhydrazone derivative of p,p’-dichlorobenzophenone from the irradiation products. Simultaneouoly in this laboratory p,p‘-dichlorobenzophenone was isolated from irradiated D D T and the compound established as a n oxidatiorl product of DDT. Both p,p’-dichlorobenzophenone and dehy.. drochlorinated DDT are shown to be much more volatile than D D T (23). This accounts for the fact that these compounds do not normally interfere with the determination of DDT in spray residues. DDT was irradiated in this laboratory in a number of solvents commonly used in DDT formulations. Of the solvents used benzene was found to facilitate decomposition to the greatest extent, and alcohol to the least. Since decomposition occurred in all cases it was concluded that D D T is more readily destroyed by sunlight when in solution than when exposed in the solid state. To determine whether reactions other than oxidation were caused by ultraviolet light, D D T was irradiated in solution and
INDUSTRIAL AND ENGINEERING CHEMISTRY
708
held in the absence of air in quartz cells. Although no p , p ' dichlorobenzophenone JTas. isolated from the reaction products, hydrochloric acid developed in each solution tried, and in some cases crystalline proddcts of higher melting point than D D T were isolated. Benzene was found to be the most active solvent under these conditions also. The results of this work indicated that ultraviolet light will either catalyze the condensation of D D T molecules or cause it to react with the solvent. D D T presented a problem in residual action that was similar to the residue problems of the inorganic insecticides in that the residue carries through to the final consumption of agricultural products. The oil-soluble nature of this insecticide presents difficulties in residue removal that are not encountered with inorganic materials. D D T is hard to remove from apples and orange peels because it tends to remain, dissolved in the wax or oil of the fruit. The usual methods of washing to remove the residue are not so effective as Kith inorganic materials. Apparently D D T is an organic insecticide with too much residual action for some agricultural uses. It is expected that through studies of the chemistry of D D T there will be developed farm practices which make use of the known weak points of D D T , as well as others yet to be discovered, t o control the length of residual action of this organic insecticide. LITER 4TUQE CITED
(1) Bousquet, E. W.,U. S. Patent 2,123,929 (July 10, 1938). (2) Bushland, R. C., Schechter, M. S., Jones, H. A., and Kntphng, E. F., Soap S a n k Chemicals, 21, 119 (1945).
Vol. 40, No. 4
Chandler, A. C., Calif. Mosquito Control Assoc., Papers of 13th Ann. Conf., pp. 86-90 (1944). DeEds, F., and Eddy, C. W., J . Am. Chem. Soe., 60, 1446 (1938). Fleck, E. E., J . Econ. Entomol., 37, 853 (1944). Fleck, E. E., and Haller, H. L., IXD. EXQ.CHEM.,37,403 (1945). Fleming, W. E., and Chisholm, R. D., J . Eeon. Entomol., 37, 1.55 (1944).
Gunther, F. A , , Lindgren, D. L., Elliot, M.I., and LaDue, J. P., I b i d . , 39,624 (1946).
Guy, H. G., Del. Agr. Expt. Sta., Bull. 206 (1937). Haller, H.L., and Schaffer, P. S., U. S. Patent 1,945,312 (January 30, 1934). Jones, H. A., Gersdorff, W. A., Gooden, E. L., Campbell, F. L., and Sullivan, W.N., J . Econ. Entomol.. 26. 451 (1933). LaForge, F. B., Haller, H. L., and Smith, L. E., Cheh. Revs., 12, 181 (1933).
LaForge, F. B., and Soloway, S.B., J . Am. Chem. Soc., 69,2932 (1947).
Lindquist,, A. W., Jones, H. A., and Madden, A. H., J . Econ. Entomol., 39,55 (1946).
Lindquist, A. W.,Madden, A. H., Wilson, H. G., and Jones, H.A., Ibid., 37,132 (1944). Metcalf, R. L., and Wilson, C. E., I b i d . , 38, 499 (1945). Roark, R. C., I b i d . , 23,460 (1930). Roark, R. C . , U. S. Bur. Entomol. Plant Quarantine E-706, pp. 11-16 (1946).
Salzberg, P. L., U. S.Patent 2,112,381 (March 29, 1938). Tattersfield, F., J. A g r . Sei., 23, 396 (1932). Wain, 11. L., and Martin, A . E., A n a l y s t , 72, 1 (1947). West, T . F., .Vatwe; 152, 660 (1943). Wichmann. H. J., Patterson, 1%'.I., Clifford, P. -4., Klein, A. K., and Claborn, H. V., J . Assoc. O f i c . A g r . Chem., 29, 218 (1946). Zeidler, O., Be?., 7, 1181 (1874). RECEIVED Kovember 2 2 , 1947.
Insecticides for Pr
rowing Crops
Bailey B. Pepper ,Yew Jersey Agricultural E x p e r i m e n t S t a t i o n , 'Vew B r u n s w i c k , il'.
Intensive research is required to evaluate the factors which are limiting the use of the new synthetic organic insecticides. The economic value of insecticides is discussed; new types of insecticides and their increased use have given the entomologist broader knowledge of the actual damage to growing crops by insects.
, P
RIOR to World War I1 the list of st'andard agricultural insecticides was relatively small. This list included: inorganic
chemicals-the arsenical and fluorine compounds; natural organic compounds-rotenone, nicotine, and pyrethrum; and a few synthetic organics-phenothiazine, xanthone, etc. Most of the synthetic insecticides made prior to the war had such minor advantages over t,he inorganics and plant derivatives that it a a s hardly worth n-hile to market. them. The int,roduction of D D T was the spark that started economic entomology on the march toward more efficient insect control. Had thjs wonder chemical never come into commercial use it would still be t.he greatest discovery in t.he history of entomology because it is responsible for the interest in research in agricultural chemicals. Practically every jnsecticide manufact,urer has launched a research program which is aimed a t an insecticide superior to DDT. Results have already been obtained in the discovery of chlordan, dichlorodiphengldichloroethane (DDD or TDE), chlorinated camphene, benzene hexachloride, and several phosphate compounds. Many promising insecticidal materials are still known only by laboratory code numbers. LIMITATIONS OF SYNTHETIC INSECTICIDES
iilthough progress has been made during the last few years in the development of synthetic organic, insect,icides, the inorganics
J.
and plant derivatives-arsenicals, rotenone, pyrethrum, and nicot,ine-cannot yet be replaced in their use against certain pests and where experience in their application has proved their value. The synthetic insecticides often give outstanding performance in insect toxicit,y: on the other hand, the following objections and/or limit,at,ionsare unknown factors connected with their use: specific t,oxicity (limited to certain species of insects) ; toxicit,y t,o man and animals; toxicity to plants; toxicity to beneficial insects and animals; and possible residual effect in the soil. SPECIFICITY.The farmer would like a single insecticide to control all species of insects at,tacking his crops whereas current trends indicate the development of an insecticide to control each insect or group of insects. D D T gives excellent control of cabbage caterpillars, leafhoppers, flea beetles, codling Moth, and Oriental fruit, moth, but the Mexican bean beetle, grasshoppers, plum curculio, and others are lit,tle affected by it. Chlordan, dichlorodiphenyldichloroethane, benzene hexachloride, methoxy analog of DDT, and hexaet,hyl tetraphosphate also she\\- high toxicity t o some species of insect's but are ineffective against others. However, each o f these chemicals is t,oxic to certain species against which the other materials are ineffective. TOXICITY TO klzx AND ANIMALS. All the new synthet'ic insecticides are considered t o represent a hazard to man and animals: t o the user during application, and t80tjhe consumer as a residue on food plants. As knowledge of minimum concenlrations, met,hods of formulation, and methods and schedules of application is gained, these risks can be great,ly reduced or eliminated. TOXICITY TO PLANTS. Each of the synthetic iusccticides, if used in sufficient concentration t o control a given pest, will injure some species of plant#. Such factors as soil, climatic conditions, and formulation of the insecticide influence phytotoxicity. It is the contention of the author that, entomologists
