Development and Use of Synthetic Organic Insecticides CHARLES E. PALM
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Cornell University, Ithaca, Ν. Y.
The synthetic organic insecticides offer great advances in insect control. Their high degree of toxicity and specificity to certain species are noticeable features. Hazards to health in manufac ture, formulation, and application are primary considerations. New methods of application are being developed. Official guidance on residue hazards will aid entomologists and farmers in developing application programs. Resistance to insecticides is of growing importance. The effect of pesticides on naturally occurring beneficial insects is causing heretofore unimportant forms to become pests of major concern. The search for the solution to the insecticide problem is a specialized and coopera tive venture among a team of scientists trained in many fields.
Frequently it helps us to understand a current problem if we review what has taken place i n previous years. This is particularly true with the present situation in the field of synthetic organic insecticides. The use of plant extracts for insect control dates into antiquity; the use of Paris green as an insecticide for control of the Colorado potato beetle in 1867 probably marks the beginning of the modern era of chemical control of injurious insects. The develop ment of lead arsenate followed later i n the nineteenth century for gypsy moth control. The commercial production of nicotine insecticides, the production of calcium arsenate at the time of the first world war, and the use of fluorine, arsenical, and cyanide compounds, as well as other inorganic chemicals for insect control, were important steps i n pest con trol. These chemicals were applied largely by dilute high pressure sprays or dusts. The concern over fluorine, lead, and arsenical residues in the 1920's and 1930's seems to have tied i n with greater interest i n the development and use of the botanical insecti cides, pyre thrum, rotenone, and nicotine. E v e r y effort was made to supplement the effectiveness of these materials through formulation, combinations, or, as i n the case of lead arsenate, by adding deposit builders, spreaders, and other similar materials. Progress with application equipment continued along established lines with streamlining of equip ment, increased pressures, and similar measures. Farmers developed a background of experience in the use of these insecticides through the years and knew fairly well what to expect i n terms of hazards of use and performance. The developing problem of harm ful residues at harvest time alerted growers, entomologists, and the consuming public to the need for research in the production of safer insecticides.
Recent Developments A l l of us have witnessed developments of the past decade when a second world war engulfed most of the civilized world. Insect control was of prime importance to the pro tection of the armed forces against insect vectors of disease organisms as well as to the pro218
In AGRICULTURAL CONTROL CHEMICALS; Advances in Chemistry; American Chemical Society: Washington, DC, 1950.
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duction and protection of food and fiber. The American farmer was called upon to i n crease production beyond all previous limits. Shortages existed i n the supply of almost all insecticides and application equipment. The loss of the D u t c h East Indies to Japan early i n the war shut off the principal supply of rotenone to the Allies. I n spite of every conceivable handicap, industry and government teamed up to aid the farmer to make the excellent production record which is so well known to everyone. The advent of D D T brought about revolutionary practices in economic entomology. Its effectiveness for many pests was nothing short of miraculous. The uncertainty of use and residue hazards brought about precautions for grower application. Control of many insects became feasible for the first time because of the effectiveness of D D T as well as the small amounts of the insecticide needed. It is still amazing to watch a plane applying one gallon of insecticide solution per acre of forest, at the rate of 100 acres per minute, with results of near eradication of a pest like the gypsy moth. Technology i n the field of synthetic organic insecticides and application equipment has recently made unbelievable strides. To the uninitiated, i t seemed certain that D D T was almost the final answer to all our insect problems. Following D D T , other chlorinated hydrocarbons were introduced, including benzene hexachloride (hexachlorocyclohexane), chlordan, toxaphene, and others. More recently, there have been the organic phosphates including parathion, tetraethyl pyrophosphate., and hexaethyl tetraphosphate, several of the dinitro compounds, and many others still i n the research and development stages. The rapid development of the chemical industry i n producing new insecticides left all, including the farmer, without a background of use experience with these materials, which for the most part w ere more potent i n killing insects than anything that had ever been used. Is i t any wonder then, that the average farmer and entomologist became and are still confused? Close working relationships have of necessity developed with this rapid introduction of new insecticides. N o longer can an insecticide be recommended for use on the basis of its insect-killing properties alone. The chemists, toxicologists, pharmacologists, physiologists, manufacturers, and others are all contributing to the story. r
Problems with New Materials What are some of the problems that exist today relating to the development and use of new insecticides? Obviously there are hazards i n manufacturing, formulating, and applying these chemicals. Doubtless the first two processes are under more constant supervision than the last. M a n y farmers and farm workers are careless about reading precaution labels before handling and applying insecticides. The use of a mask or respirator, rubber gloves, and other protective clothing with some of the newer materials is not, unfortunately, general practice i n the field. Part of the negligence has been blamed on D D T , because few if any of the possible difficulties in the use of D D T insecticides ever m a terialized. I t is now difficult to make many farm workers realize that all the new chemicals are not i n the same safety category, particularly if they or their neighbors have applied one of the phosphate compounds, for example, without suffering i l l effects. The three deaths reported from use of parathion in the field i n the summer of 1949 are creating more respect for the material among growers than all the words of warning could ever do. In brief, the problem of educating the farmer on safety precautions to be observed in applying insecticides is not solved. The very nature of the over-all problem has made i t impossible for an agency of government like the Food and Drug Administration to establish official guidance in terms of the residue hazards and degree of contamination that might be permitted if a given chemical must be used for pest control i n a production program. Through its research, the F D A has been extremely helpful i n giving guidance during this critical period. New Y o r k farmers, like others throughout the nation, are anxious to know what levels of contamination will be considered safe on fruits, vegetables, and other commodities, i n order to guide their choice and use of insecticides. Certainly, the entomologist will be glad to receive this guidance i n making recommendations. The hearings by the Food and D r u g Administration to deal with residue hazards i n the use of insecticides deserve the supIn AGRICULTURAL CONTROL CHEMICALS; Advances in Chemistry; American Chemical Society: Washington, DC, 1950.
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port of all who have data bearing on the problem and an interest in i t . I t is a very t a n gible illustration of the important role of toxicologists and pharmacologists on the team.
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Variation in Insect Species Insects, like other living organisms, show variation within species. This has long been recognized i n so far as structure, size, and color are concerned. I t has been demonstrated from a physiological viewpoint where two strains, seemingly identical structurally, behave differently i n their habits and seasonal history. Insect toxicologists have consistently demonstrated the variation i n individual susceptibility of a population of an i n sect species to a uniform dosage of an insecticide. Variation among living organisms is a basic concept of biology. I t is not startling, then, that we should find the development of resistant or tolerant strains of a species after continued use of an insecticide. M u c h is being said about the development of resistance to insecticides, and i n a number of i n stances data substantiate the claim. I t is also likely that resistance will be blamed i n many cases of faulty timing, poor application, or poor materials.
Insect Resistance to Insecticides The development of resistant strains of an insect to a given insecticide is not new. Melander (7) i n 1914 pointed out that the San Jose scale in Washington had developed a resistance to lime-sulfur sprays. Recently Babers (1) of the Bureau of Entomology and Plant Quarantine brought together an excellent evaluation and summary of the literature dealing with the development of insect resistance to insecticides; he lists 111 references to work on this phenomenon. Quayle (11, 12), working i n California, noted resistance of the California red scale, black scale, and citricola scale to hydrocyanic acid gas. Hough (5) first called attention to the developing resistance of the codling moth to arsenicals. I n South Africa a blue tick, Boophilus decolor'atus, was found b y D u Toit (3) and others to be resistant to arsenic. Boyce and Persing (2) reported resistance among the citrus thrips, Scirtothnps citri, to tartar emetic sprays. K n i p l i n g (6) found that the larvae of the screw worm fly, Cochliomyia americana, could acquire resistance to phenothiazine. Mosna (9) i n Italy reported a variety of mosquito resistant to D D T . Missiroli (8) also working i n Italy reported on the failure of D D T to control houseflies i n 1945 and 1946, owing to resistance to D D T . W i l son and Gahan (13) developed a laboratory strain of houseflies resistant to D D T and several other insecticides. These examples from the literature compiled by Babers indicate that resistance to different materials is developing among different species and i n several localities. I t may become more extensive with organic insecticides because of their more widespread use as well as the greater number of chemicals that will be applied i n the field.
New Pest Species and Resistance in New York Several examples of resistance have been observed i n New Y o r k . The codling moth has long been considered the N o . 1 pest of commercial apple production. I n the Hudson River Valley, Chapman of the New Y o r k Agricultural Experiment Station staff reported one orchard i n 1930 where codling moth could not be controlled with the usual lead arsenate schedule. Since that time practically the entire orchard area of the Hudson Valley has developed a codling moth problem which lead arsenate will not handle satisfactorily. The same experience was noted i n western New Y o r k , except that it began a few years earlier. Harman (4), reporting to the Horticultural Society i n 1945, indicated that i n spite of everything growers could do i n areas of western New Y o r k , inability to control codling moth made apple growing unprofitable except for the very high prices being received for fruit at that time. The number of cover sprays increased to as many as six i n a season. Deposit builders, stickers, spreaders, oil added as an ovicide, nicotine, and other materials supplementing lead arsenate comprised the only weapons until D D T proved to be so tremendously effective i n codling moth control. Thus far there has been no sign In AGRICULTURAL CONTROL CHEMICALS; Advances in Chemistry; American Chemical Society: Washington, DC, 1950.
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of a reduction i n efficiency of D D T for control of the codling moth. Spray practices of applying the minimum effective dosages of lead arsenate for economy and residue considerations may have favored the development of resistant strains. The greenhouse red spider, more properly called the two-spotted spider mite, has developed a resistance in New Y o r k to two chemicals, an ammonium potassium selenosulfide compound, marketed as Selocide, and parathion. W i t h both Selocide and parathion excellent initial kills were obtained. The experience with parathion is still i n progress. Blauvelt of the Cornell University staff reports some cases where parathion aerosols no longer will effectively kill spider mites in greenhouses. There are strong indications that the rapid development of parathion-resistant populations i n some ranges was associated with the use of a marginal program of treatments. This allowed a considerable build-up of populations between treatments and thus may have afforded favorable conditions for the selective survival and increase of resistant individuals or strains originally present i n small numbers. On certain ranges where cooperative trial programs with parathion aerosols have been carried on for over two years, no significant increase in resistance to parathion has yet developed where applications were made often enough to keep the mite population at a very low level from the start. The short life cycle and more or less continuous breeding of the spider mites in greenhouses favor a more rapid development of resistance than is possible with a slower breeding species like the codling moth. Another interesting case is the growing feeling among farmers that a 0.75% rotenone dust is no longer effective against Mexican bean beetle—dosage of 1% seems to be required. There are also complaints that D D T wettable powders are not as effective against the potato flea beetle as i n former years. Such circumstantial evidence cannot be accepted as proving resistance to insecticides, but it bears watching and investigation. New Y o r k has experienced a rather widespread breakdown in housefly control with the use of D D T . Schwardt of the Cornell University staff first noticed this failure of D D T in 1948. I n 1949 the fly problem was very bad. Farmers, remembering the exceptional control of the past few years with D D T residual sprays, were greatly disturbed when D D T was first withdrawal from use i n dairy barns because of the danger of D D T contamination in milk. Methoxychlor under conditions in 1949 did not measure up to the performance of D D T in other years; neither did D D T . Lindane (gamma isomer of hexachlorocyclohexane) has been hailed by many dairymen as the successor to D D T , and by some farmers the question is raised—"What is the successor of lindane to be?" Along with development of resistance has come the destruction of important natural enemies of pest species, which has been attributed to field use of insecticides and certain fungicides. Pickett (10) recently presented evidence to show that the development of the oyster shell scale problem i n N o v a Scotia orchards is related to the use of sulfur fungicides. The scale was present as an occasional pest of apples for over half a century, but beginning i n 1930 it built up to dangerous levels. Sulfur was shown to destroy the parasitic wasp and the predatory mite responsible under natural conditions for keeping the pest under control. Prior to 1930 lime sulfur was used as a fungicide and it served also as an insecticide for the scale. W i t h the change to mild elemental sulfurs about that time, the oyster shell scale population built up to injurious levels, because the milder sulfurs continued to kill the parasites and predators but did not kill the scales. Pickett further recorded a high build-up of the European red mite on apples when mild sulfurs were used as a fungicide as opposed to copper fungicides. Again sulfur is charged with destruction of parasites and predators of the red mite. The same correlation between copper and sulfur fungicides and the degree of codling moth infestation is presented by Pickett over a 5-year period i n N o v a Scotia. H e thinks possibly sulfur has reduced the natural enemies of the codling moth to a greater extent than copper fungicides.
Upsetting Balance in Nature D D T has been blamed b y some for upsetting the balance i n nature. Certainly the balance i n nature was upset by many factors long before D D T , not the least offender being man. We recognize specific problems, however, where the use of D D T for the control of one pest has been associated with the rise to prominence of other pests. One example In AGRICULTURAL CONTROL CHEMICALS; Advances in Chemistry; American Chemical Society: Washington, DC, 1950.
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from experience i n New Y o r k is the red-banded leafroller. The incidence of this pest is closely correlated with the use of D D T as a codling moth insecticide. I t is possible that the natural enemies of the leafroller which were not disturbed b y lead arsenate were killed b y D D T . The red-banded leafroller is now a major pest capable of destroying an entire apple crop. Similarly, the two-spotted spider mite became an economic problem in many New Y o r k orchards following the use of D D T for codling moth control. This species was never considered as an orchard pest prior to the use of D D T . Although the reasons for this development are not completely explained, it again is probable that para sites and predators of the mite have been reduced. The European red mite on apples has increased i n importance i n New Y o r k with the use of D D T . Y e t research workers have not been able to say that the rise in populations of this species is directly attributable to the use of D D T .
Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
Babers, F. H . , U . S. Dept. Agr., Bur. Entomol. Plant Quarantine, Bull. E 7 7 6 , 1-31 (1949). Boyce, A . M . , and Persing, C. Α., Calif. Agr. Expt. Sta., News Letter, 2 1 (1942). Du Toit, R., Graf, H . , and Bekker, P. M . , S. African Vet. Med. Assoc. J., 1 2 , 50-8 (1941). Harman, S. W., Proc. Ν. Y. State Hort. Soc., 1 9 4 5 , 46-53. Hough, W. S., J. Econ. Entomol., 21, 325-9 (1928). Knipling, E . F., Ibid., 35, 63-4 (1942). Melander, A. L., Ibid., 7, 167-72 (1914). Missiroli, Α., Riv. parassitol., 8 (2/3), 141-69 (1947). Mosna, E . , Ibid., 8 (2/3), 125-6 (1947). Pickett, A . D., Can. Entomol., LXXXI, 67-76 (1949). Quayle, H . J . , Calif. Univ. J. Agr., 3 , 333, 358 (1916). Quayle, H . J . , Hilgardia, 11 (5), 183-210 (1938). Wilson, H . G., and Gahan, J. B., Science, 1 0 7 , 276-7 (1948).
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