AGRICULTURAL CONTROL CHEMICALS

Some Poisonous Residue Factors in Use of Two New Organic Insecticides G. S. HENSILL and L. R. GARDNER

Downloaded by CALIFORNIA INST OF TECHNOLOGY on September 21, 2017 | http://pubs.acs.org Publication Date: January 1, 1950 | doi: 10.1021/ba-1950-0001.ch021

California Spray-Chemical Corporation, Richmond, Calif.

Laboratory and field research work has developed methods for using two new organic insecticides with freedom from poison residue. With pure gamma isomer of hexachlorocyclohexane proper formulation and timing of application are the essence of success. On some crops applications must be made before fruits or heads form; on others applications may be made to within 2 or 3 weeks of harvest, and in others, within a few days of harvest. Commercial usage with proper timing and application has resulted in no poison residues or undesirable taste residues. With proper usage, dosage, and application the pure gamma isomer of hexachlorocyclohexane leaves no residues that would constitute a health hazard or produce off-flavors in food products. The unstable nature of tetraethyl pyrophosphate makes treatment of crops up to harvest time possible. This chemical is therefore an effective agricultural insecticide against many pests.

T h e introduction of the use of D D T (dichlorodiphenyltrichloroethane) as an insecticide during World W a r I I initiated a revolution i n the problems of insecticide residues. The resulting changes have been so far-reaching that today, only a few years later, most major insecticides are new synthetic organic chemicals. Prior to the introduction of D D T the major insecticides were inorganic chemicals, except for some few relatively expensive organics of plant origin such as nicotine, pyrethrum, and rotenone. Two of the most important synthetic organic insecticides which have been introduced following D D T are the technically pure gamma isomer of hexachlorocyclohexane (lindane) and tetraethyl pyrophosphate. The technically pure gamma isomer was developed as the result of investigational work on the insecticide, benzene hexachloride, which was first used i n England and France during World W a r I I and was used i n the United States on agricultural crops at the same time. I t was found to have insecticidal value equal to D D T i n most respects and better i n others. Considerable residue action was evident, and i t was toxic to a wider range of insects than was D D T , and had vapor action not evident with D D T . Of five or more isomers, the only one that is appreciably insecticidally active is the gamma isomer, which occurs i n various percentages, usually 12 to 1 3 % , depending on methods of manufacture. This mixture of isomers results i n a compound of strong and persistent odor, mostly due to the beta isomer, which odor is retained by some fruits, vegetables, and animal tissues after treatment for insect infestations. Research work soon showed the possibility of producing the pure or technically pure gamma isomer. This production was finally accomplished on a large commercial scale and the compound has now been introduced as a large-volume insecticide on the American market. Tetraethyl pyrophosphate was first manufactured i n Germany about 1940 as hexa102

AGRICULTURAL CONTROL CHEMICALS Advances in Chemistry; American Chemical Society: Washington, DC, 1950.


HENSILL AND GARDNER—RESIDUE FACTORS IN NEW ORGANIC INSECTICIDES

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ethyl tetraphosphate or the hexaethyl ester of tetraphosphoric acid. This compound and its manufacturing methods were discovered i n Germany by chemists with the allied occupation forces and the information was brought to the United States. Research work on this chemical finally indicated that i t contained as the active agent tetraethyl diphosphates which are generally described as tetraethyl pyrophosphate, and this is now well established i n the insecticide and chemical industry. As the value of these two new chemicals for insecticides became more evident, the need for extended experimental and test work was definitely established. I t was necessary to determine chemical formulas, work out analytical methods, obtain knowledge of various physical and chemical characteristics, and complete evaluation of insecticidal action as well as toxicity and effect of residues. Toxicity was concerned with not only insects but humans and other warm-blooded animals. Residual studies included information on persistence and type and amount of residue. This information, once accumulated, must be correlated with similar information on other insecticides.

Pure Gamma Isomer of Hexachlorocyclohexane Investigational work on the pure gamma isomer of hexachlorocyclohexane involved both laboratory and field tests. The pure gamma isomer was found to have retained practically all of the insecticidal value of the parent commercial benzene hexachloride containing 12 to 13% gamma isomer. The three-way action of contact poison, stomach poison, and vapor action poison was also evident with the pure gamma isomer. I n addition, methods of manufacture were found which retained the insecticidal action of the parent compound and yet removed the objectionable odor, thus making available a fine chemical for killing insects. According to Lehman (#), toxicity tests i n the laboratory with small animals such as rats and dogs indicated that the refined or technically pure gamma isomer has a mean lethal dose of 125 mg. per kg. of body weight. This gives i t an acute oral toxicity about twice that of D D T . Nicotine is twelve times and tetraethyl pyrophosphate sixty times as toxic as gamma isomer. On the same basis arsenic is about four times as toxic. A c cording to Lehman, pure gamma isomer shows much less tendency toward storage i n body tissues than does D D T . Pure gamma isomer is stored i n body fat at a level about equal to the amount of dietary intake; D D T is stored i n body fat at a level four to ten times that of the dietary intake. T h e pure gamma isomer of hexachlorocyclohexane shows practically no dermal toxicity on skin application. Its chronic toxicity is four times less than D D T . Laboratory studies showed the pure gamma isomer of hexachlorocyclohexane to have residual life equivalent to that of the normal mixed isomers. The material does not break down i n normal storage, as do mixed isomers. It was also established i n laboratory work that this product could be readily formulated into dusts, wettable powder, or liquid formulations. Liquid formulations were more readily made with this than with the commercial benzene hexachloride, because of the higher concentration of the gamma isomer. Field test work with technically pure gamma isomer of hexachlorocyclohexane has been extensive and involved and is being continued. I t was necessary to know such factors as insecticidal value i n field applications as compared to other insecticides, as well as residual life, residue from the poison standpoint, and residual taste or odor factors. These factors have been worked out on numerous crops and some of the results are dealt with i n this paper. Because the pure gamma isomer was found to be effective on insects in the soil as well as on insect infestations on plants, its residual life i n soil of all types and effects on tuber and root crops were also of major importance. The residual toxicity of the pure gamma isomer was found to be equivalent to that of ordinary commercial benzene hexachloride. Commercial usage has shown that the residual action is effective for a longer time with dust or wettable powder spray applications than with emulsive solvent-type formulations. T h e over-all residual life of the chemical is on the order of 4 to 8 days as compared to 14 to 21 days for D D T . T h i s is, of course, adequate residual life for good insect control i n most cases^ and the shorter AGRICULTURAL CONTROL CHEMICALS Advances in Chemistry; American Chemical Society: Washington, DC, 1950.

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life and the smaller amount used make the pure gamma isomer especially important because of freedom from residue problems. Despite the fact that i t has a low vapor pressure, 0.00001 m m . at ordinary temperature and 0.2 m m . at the temperature of boiling water (1), i t is an effective fumigant and its volatility is one of the major reasons for its disappearance from a plant crop (Figure 1). M o r e important than this is the high po tency to insects, which is approximately ten times that of D D T . This means that lower dosages are used, thus greatly reducing any residue problem.

30

100

200

TEMPERATURE, F. β

Figure 1. Vapor Pressure of Gamma Isomer of HexachJoro cyclohexane and DDT T o date, within the scope of the writers' information, there has been no residual deposit or poison residue recovered from treated fruits or vegetables, where proper formu lations and amounts of the pure gamma isomer have been used not later than 2 weeks prior to crop harvest. Likewise, there is no known record of poisoning to man or animals from applying the insecticide or eating food treated with the insecticide. Freedom from poisonous residue and undesirable taste i n the use of the pure gamma isomer of hexa chlorocyclohexane is achieved therfeore by proper formulation, timing, and application of insecticide treatments. The pure gamma isomer has been found to be an excellent soil insecticide for control of most common soil insects. Persistence of the material i n the soil is longer than on plant surfaces. T h e possibility of flavoring of root and tuber crops grown i n infested soil is also a problem and this has been worked out i n the following manner: First, light dosages such as 0.125 to 0.25 pound of gamma isomer per acre may be used i n soils to be planted to potatoes, which are probably the most sensitive crop as regards imparting of flavor, or to other tuber crops. Such treatment has not been found to impart unfavorable taste to most root crops. Second, the soil may be treated with a heavier dosage, such AGRICULTURAL CONTROL CHEMICALS Advances in Chemistry; American Chemical Society: Washington, DC, 1950.


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as 0.25 to 0.5 pound of gamma isomer per acre, and a nonaffected crop planted for that season. So far this procedure has not produced undesirable results i n most tuber or root crops grown during the season following treatment (Table I ) . Table I. Gamma Isomer, Lb./Acre


0.125 0.25 Check untreated (second year)

Pure Gamma Isomer Taste Test on Potatoes

Taster 1

Taster 2

Taster 3

None None None

None None None

None Slight None

Objectionable Taste Taster 4 Taster 5 None Slight None

Slight None None

Taster 6

Taster 7

Taster 8

None Slight None

None None None

None None None

The application of pure gamma isomer to plants for insect control has created similar problems, i n that undesirable taste might be imparted to mature fruits or vegetables (Table I I ) . Table II.

Pure Gamma Isomer Taste Test on Canned Tomatoes

Gamma Isomer, Lb./Acre

Taster 1

°-25 0,5 Check untreated

None None None

Objectionable Taste Taster 2 Taster 3 None None None

None Slight None

Taster 4 None None None

This factor is handled by applying the chemical not later than 30 days before harvest on crops that might hold some residual taste. O n other crops i t has been possible to use the chemical to within 2 weeks of harvest without retaining undesirable taste. This point is largely one of varieties, so that it becomes necessary to specify on labels which crops must be treated before fruits or heads form, which can be treated up to 2 or 3 weeks prior to harvest, and which have no particular time limit. T h e following fruits and vegetables are among those satisfactorily treated. I n the eastern United States apples of the Delicious variety treated 10 days prior to harvest with pure gamma isomer showed no trace of off-flavor at harvest time. Dusts and sprays applied to carrot seedlings produced no traces of off-flavor i n the mature vegetables. Several tasters could not differentiate between peaches sprayed within a few days of harvest and check fruit. Celery i n Florida sprayed twice, once within 6 weeks of harvest and once within 30 days of harvest, was canned and put through a severe series of tests. TIME OF HYDROLYSIS, HOURS 10

Figure 2.

20

30

Tetraethyl Pyrophosphate Persistence Test

Filter papers treated with solution and exposed to Drosophila


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Reports by the National Canners Association showed negative results on both taste tests and analytical tests for residual chlorides. Asparagus treated and canned i n a manner comparable to the celery also gave completely negative results. It has thus been possible to work out satisfactory applications of this chemical on many crops without the danger of poison or other undesirable residues on the har vested crops. The use of pure gamma isomer of hexachlorocyclohexane on livestock has also been worked out. I t has been found possible to use the wettable powder formulation dispersed i n water as a spray on livestock for control of flies, lice, and ticks. Proper dosage and application must be used, of course, but this is again indicative of the safety factor of this insecticide.

Tetraethyl Pyrophosphate E a r l y work i n the laboratory and i n the field soon established the fact that there would be no poison residue factor with this chemical, owing to its rapid decomposition. W i t h i n a relatively few hours the chemical broke down into diethyl phosphoric acid DAYS WEATHERING 1

2

3

4

5

50

150

=5

Ζ

900

Figure 3.

\

J|-

Tetraethyl Pyrophosphate Persistence Test

Fly cage sprayed with tetraethyl pyrophosphate and weathered in laboratory. Flies introduced at stated intervals

and finally ethyl alcohol and phosphoric acid, both of which are nonpoisonous i n rela tively large amounts, especially i n view of the low dosage used (Figures 2 and 3). A l though this chemical was found to be highly toxic i n its pure form to both insects and warm-blooded animals, the rapid hydrolysis on exposure to air or moisture eliminated the AGRICULTURAL CONTROL CHEMICALS Advances in Chemistry; American Chemical Society: Washington, DC, 1950.



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poison residue problem. This has been verified b y field usage experience and laboratory tests. I n the laboratory the chemical was put into water at the ordinary spray dilution of 1 to 800 and after 24 hours' standing the treated water was used as drinking water for test animals. There were no reactions, evidence of poison, or undesirable effects on any animals as a result of these tests, even with long feeding periods. I t was not possible to differentiate between test animals and check animals by any of the customary tests. Formulations of tetraethyl pyrophosphate as an emulsive concentrate proved to be relatively complex, considering the apparent ease of formulation. Because of the u n stable nature of the chemical, dust formulations were considered impossible. I t was found after extensive research work that a dust which would be stable for 10 days to 2 weeks could be made with a specially selected and processed filler. Field test work with the chemical has consisted of many tests and a large number of commercial applications i n both spray and dust forms. Insecticidal action has been satisfactory i n all cases where materials have been properly applied. N o toxic residue has been found on any treated plants or food crops, which include most varieties of crops. A sample of hops which had been treated with tetraethyl pyrophosphate showed a negative chemical analysis. The plant material was also extracted and the extract added to the drinking water of test animals and sensitive insects. The animals and insects that drank this treated water for several days showed no reaction. W i t h the sensitive insects i t would have been possible to detect even a few parts per million. I n addition, there have been extensive commercial field applications of the chemical i n dust and spray form to crops such as apples, pears, grapes, celery, broccoli, Brussels sprouts, and others up to within a few days of harvest; there has been no detectable poison residue on any of the crops. The lack of poison residue with use of tetraethyl pyrophosphate is due to the fact that it hydrolyzes within a few hours of application, breaking down into transient nonresidual and nonpoisonous chemicals. Thus i t is possible to use tetraethyl pyrophosphate well up to harvest time of food products without danger of residual poison on crops. The fact that the chemical is used i n extremely small amounts is a definite advantage i n respect to freedom from poison residue.

Literature Cited (1) Balson, E. W., Trans. Faraday Soc., 43, 54-60 (1947). (2) Lehman, A . J . , "Toxicology of the Newer Agricultural Chemicals," reprint from conference of 42nd annual convention of National Canners Association, Atlantic City, N . J., Jan. 17, 1949.


AGRICULTURAL CONTROL CHEMICALS

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