Insecticides Applied on Livestock F. C. Bishopp and E. F. I-nipling Bbreau of Entomology and Plant Quarantine, United States Department of Agriculture, Washington, D . C . Available information on the effectiveness of insecticides against some of the more important ifisects and ticks affecting farm animals is summarized. A number of compounds show distinct promise as insecticides, but further toxicological studies and additional research on performance, both under controlled .conditions and in practical field use, are needed.
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H E several hundred insects and related parasites attacking livestock show great diversity in their life histories and habits (14) and wide degrees of tolerance to insecticides. For these reasons many problems arise in connection with their control and a variety of insecticides or repellents is necessary to provide satisfactory control by chemical methods. The control of external animal parasites usually involves direct application of chemicals; therefore, the question of the toxicity of such materials to the host is one of the most important considerations in the livestock insecticide field. The lack of toxicological information on the newer insecticides is the principal reason for delay in recommending some of them for practical appliaction. Further toxicological studies and additional research on the performance of certain compounds, both under controlled conditions and in practical field use, are needed before such recommendations can be made. Great progress has been made during recent years in conDDT ' trolling various animal pests by the use of insecticides. has been used successfully against a number of insect pests of livestock (3, 6, 12) and several other new insecticides show promise in this field. This discussion gives a brief r6sum6 of available information on the effectiveness of insecticides against some of the more important insects and ticks affecting farm animals. A number of compounds show distinct promise as insecticides but some of these have been tested in a preliminary way only and information is insufficient to appraise their value with accuracy. This review is largely limited to information obtained by the Bureau of Entomology and Plant Quarantine laboratories at Kerrville, Tex.; Savannah, Ga.; and Corvallis, Ore.
having a uniformly higher rotenone content, and the finding of ways to reduce the cost of rotenone insecticides will do much toward keeping this material in a competitive position with the newer insecticides. PYRETHRUM
Formerly pyrethrum was generally employed as the toxicant in oil-based sprays for the control of flies on cattle and as a space spray in barns. However, because of a shortage during the war, i t was largely replaced by the thiocyanates for this purpose. Oil-based livestock sprays are applied in small amounts as a fine mist to give temporary control of flies and other insects. Although not so widely used as formerly because of the availability of residual type sprays, there is still need for this type of livestock spray. The'very low toxicity of pyrethrum to higher animals and the absence of toxic residues after its use on livestock and in barns commend i t for certain uses. Progress in the development of synergists for pyrethrum (15) has greatly enhanced the initial killing action and lasting effects of this insecticide. Several compounds possessing the methylene dioxyphenyl grouping are promising pyrethrum synergists. Piperonyl butoxide is among the best of the materials which have been tested as pyrethrum synergists for the control of horn flies, deer flies, the lone star tick, and other pests. Whether or not pyrethrum alone or in combination with certain synergists can compete with the synthetic organic compounds remains to be seen, but further study is warranted. Pyrethrum is still one of the most rapid-acting insecticides known. This property, together with its low order of toxicity to warm-blooded animals, makes i t an insecticide much to be desired. High cost and instability are its chief limiting factors. A synthetic pyrethrum having the properties of the natural products would be welcomed. Much could be done with pyrethrum if i t were economically feasible to employ i t in insect control a t concentrations up to 5%, as is possible with some of the chlorinated insecticides. The development of cheaper and more effective synergists and methods of stabilizing pyrethrins warrant special attention of insecticide chemists. DDT
ROTENONE INSECTICIDES
The use of DDT has enabled us to improve methods of controlling certain external animal parasites (13). More research has been done on this material than on any other synthetic insecticide used in the field of veterinary entomology. DDT has proved excellent against several pests which previously could not be adequately controlled, and in fact, should make it possible to eliminate several of our major livestock pests. Much is known about the use of D D T against insects attacking livestock, yet many problems need further research. HOUSEFLIES. Houseflies are of major importance to the livestock industry,. primarily because they transmit a number of diseages and parasites of man and animals and are recognized as an indication of unsanitarv conditions. In the past, the most rigid sanitary practices around dairy and other kinds of livestock installations failed to give adequate control of this insect, However, DDT used as a residual or surface treatment has given excellent results (4). For maximum effectiveness the need for sanitation is still emphasized, because control of flies by DDT might be unsatisfactory where extensive and profuse fly breeding is permitted. DDT for housefly control can be used effectively as oil solu-
Until recent years rotenone-bearing materials, such as derris and cube, were perhaps the most widely employed against insects attacking livestock. Even today rotenone is the only satisfactory insecticide known for the control of cattle grubs, which are among our most destructive livestock pests (1). There has been increased interest on the part of livestock owners i n the control of this parasite. I n fact, the number of cattle treated for cattle grubs during the winter of 194E-47 exceeded 5,000,000. Rotenone has been employed successfully against several kinds of lice on cattle and other livestock, sheep ticks, fleas, and certain ticks, but for several of these uses i t is being replaced rapidly by the new chlorinated synthetics. The lack of residual insecticidal action is the major disadvantage of rotenone in comparison with the chlorinated materials, but this shortcoming is an advantage from the standpoint of toxicity to higher animals. Methods of stabilizing rotenone so that i t will continue its insecticidal action for longer periods after application would represent a major contribution. The further standardization of rotenone-bearing materials, the production of strains of plants
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tions, aqueous emulsions, or wettable powders. The amount to be applied depends on the type of formulation, nature of the surface, type of equipment used, length of the fly season, and many unknown factors. The Bureau of Entomology and Plant Quarantine suggests application of 200 mg. per square foot of surface area. One application a t the beginning of the fly season generally provides satisfactory control for the season, although sometimes a second application in midsummer is desirable. HORNFLIES. Several million head of cattle were sprayed with DDT during the summer of 1947. The insecticide has provided a much needed control measure for this insect, especially on range cattle where the cost of handling animals is a major consideration in determining the practicability of a control measure. Both DDT wettable powders and emulsions are employed to control horn fly (8). The wettable powders, because of their safety and effectiveness, are preferred for general use. Emulsions of D D T are made from a variety of ingredients, and some solvents might produce harmful effects on the host. However, when properly mixed and used, certain emulsions can be employed without harmful effects, and generally are readily applied with hand equipment. There is considerable difference of opinion as to the amount of D D T spray to use for horn fly control. Concentrations of D D T in the spray vary from 0.25 to as much as 2.5%. Usually, however, too little emphasis is placed on the quantity of spray. Recommendations vary from slightly more than 1 pint to 1 gallon for a grown cow. The Bureau of Entomology and Plant Quarantine recommends about 8 grams of DDT for a mature animal of average size. The quantity of spray for adequate coverage will depend somewhat on the size of the animals, the amount of hair, and the type of equipment used. When a large power spray unit is employed about 2 quarts per animal of a 0.5y0 solution is recommended. With a small power unit as little as 1 to 1.5 pints of a 1.5% D D T solution has been found adequate. If these amounts of DDT are applied, adequate control of horn flies can be expected for about 4 weeks. Research on the duration of effectiveness of various rates of application has shown that the lasting effect is not in direct relation to the amount of DDT applied. A dosage of 8 grams of DDT per animal, for example, will not last twice as long as 4 grams. An average of about 3 weeks' protection can be expected in the Southeast and Middle West when 3 to 4 grams are applied, as compared with 4 weeks for double this amount. For range animals, where cost of handling is a major factor, the higher rate is suggested. For dairy animals or beef animals in feed lots, which are available for treatment at all times, it might be more economical to use smaller amounts of D D T and make more frequent applications. Fairly good control of horn flies will result from making spot tri?atments on cattle. Combination treatments, designed to control lice, ticks, or other parasites along with horn flies, strongly influence the mode of application and the quantity of insecticide applied. CATTLELICE. DDT is sufficiently effective and stable to eradicate lice from herds of cattle with one application (6) if coverage of the animal is complete and thorough. I n actual practice that is seldom accomplished. Dipping animals is more likely to result in complete elimination of lice from herds than is spraying, but even dipping may fail because a few animals escape complete coverage. For both spraying and dipping, a concentration of 0.5% D D T is commonly employed, either as a wettable powder or as a good emulsion formulated with solvents having a low order of toxicity. A single treatment will accomplish excellent practical control of lice. A second treatment 3 weeks after the first is suggested when elimination of lice from the herd is the goal. Usually from 2 to 4 quarts of spray are recommended for the average mature animal. Routine spraying of cattle for horn flies during the summer gives good control of cattle lice.
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D D T dusts can also be used effectively as a louse treatment. A 10% D D T dust applied a t the rate of 6 to 8 ounces per animal provides good control, but two treatments are required for complete elimination. I t is usually more difficult to obtain thorough coverage of the animal with dusts than with sprays. LICE o s OTHERFARM ANIMALS. Lice on sheep, goats, and swince are readily controlled with DDT. For sheep and goats a dip is preferred, and a concentration of 0.2%D D T , as emulsions or wettable powders, is suggested. If all the animals are dipped, a single treatment will eliminate lice from the herds. Sheep ticks are also effectively eliminated with such treatment. For swine a spray or dip containing 0.5 to 0.75% of D D T is effective. Lice on poultry can be controlled with a 10% D D T dust. The pinch method is suggested. Sodium fluoride, which has been widely used for poultry lice, is probably as effective as DDT. TICKS. Ticks are among the most resistant of the livestock parasites to insecticides. Preliminary tests have shown that DDT is among the best of the available insecticides for the lone star tick, cattle fever tick, Gulf Coast tick, and winter tick (9, 10, 11). Additional research is needed before precise recommendations can be made for the control of various species attacking livestock. D D T is relatively ineffective against engorged forms, but possesses a high degree of toxicity to larvae and nymphs and to the unengorged adults. Its persistence on the animal tends to prevent reinfestation for a short time. STABLEFLIES. DDT treatment of animals alone will not give good control of stable flies, because thay 'are intermittent feeders and remain on the host for relatively short periods. The spraying of barns and other places where adult stable flies congregate and rest will greatly relieve annoyance from this insect (8),but will not provide satisfactory control where extensive breeding areas such as straw stacks exist away from the livestock installations. A more satisfactory chemical is needed for protecting animals from attack by stable flies. MOSQUITOES.Mosquitoes are considered one of the major pests of livestock. Much research has been carried out on methods of controlling these insects but emphasis has been placed on their control from the standpoint of protecting man. Mosquito control procedures, employing airplanes, fog machines, etc., are usually expensive and are seldom practical for the individual livestock owner. The treatment of livestock and livestock barns with DDT will, under certain conditions, give much relief from mosquitoes. Tests indicate that for about three days after treatment, mosquitoes feeding on treated animals are killed. Animal treatment together with barn spraying will usually relieve annoyance. DDT, however, is not entirely satisfactory for protecting animals, especially range stock, from mosquito attack, and a more effective insecticide is needed. HORSE FLIESAND DEERFLIES.DDT is not satisfactory for the control of horse and deer flies, which are becoming of increasing importance to the livestock industry. Many species of these flies are known, and they may vary in their degree of susceptibility t o DDT. Observations thus far indicate, however, that the application of D D T to livestock provides little relief from these pests. NEEDFOR FURTHER RESEARCH ON DDT. The widespread use of DDT, under many conditions, has given an opportunity to observe some of its limitations under adverse conditions. Reports received from various localities from time to time indicate that DDT under certain conditions loses its effectiveness mor0 rapidly than would ordinarily be expected. Such reports have been received in connection with horn fly and housefly control in Louisiana, West Texas, Arizona, and North Dakota. The type of surface on which DDT is applied influences the duration of its effectiveness. Methods of sealing porous surfaces to p r e vent the D D T from penetrating appear worthy of consideration. .Temperature is known to influence its toxicity to certain insects. Insecticide chemists could no doubt contribute toward the im-
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provement of D D T by investigating factors influencing its stability under various conditions. The type of water used in D D T sprays is believed to be one of the important factors influencing its effectiveness. Much improvement in D D T formulations is needed, such as wettable powders that will remain in suspension better than those in use at present and are more readily applied with ordinary hand equipment. It is desirable that the percentage of conditioning materials be reduced to a minimum to lessen unsightly residues and trouble in application. A good solvent that i s low in cost, nonflammable, and safe t o apply on livestock under all conditions, is urgently needed. NEWER SYNTHETIC ORGANIC INSECTICIDES
Insecticide manufacturers have made great progress in developing new and promising insecticides. One of the major problems facing entomologists today is to keep pace with the insecticide chemists in determining the place of the new materials in controlling the many insects of economic importance. This is certainly true in the field of veterinary entomology. During the war years personnel in federal and state institutions devoted little time to research on livestock pests. During the past few years most effort has been devoted to the development of DDT, although a t present more attention is being given to evaluation of the newer insecticides. The results of the work on these products by the Bureau of Entomology and Plant Quarantine are largely evaluated in direct comparison with D D T and other standard insecticides. The new or relatively new insecticides (7) of special interest a t present are benzene hexachloride (1,2,3,4,5,6-hexachlorocyclohexane), chlorinated camphene, chlordan, the methoxy analog of DDT, dimethoxydiphenyltrichloroethane,and dichlorodiphenyldichloroethane (TDE), which is sometimes referred to as DDD. BENZENEHEXACHLORIDE. The gamma isomer of benzene hexachloride possesses greater inherent toxicity to many of our livestock pests than any other new material. The principal factors limiting its use are its instability and objectionable odor. The question of its toxicological effect on livestock is also a limiting factor, although the commercial grade used a t concentrations necessary for satisfactory control of certain parasites will probably present no problem from the standpoint of acute toxicity. Because of the short duration of its residual action, benzene hexachloride does not compare favorably with D D T for horn fly control and as a residual spray in barns for controlling houseflies. However, it is extremely toxic to these insects. It is much more rapid in its action than DDT, and seems to offer promise in relieving animals from attack by deer flies and stable flies. Available information indicates, however, that its action does not persist more than a few days. For lice on cattle, sheep, and goats, benzene hexachloride is indicated to be superior to DDT. It possesses an ovicidal property (2) which D D T and other new synthetics do not have. Another property of this insecticide which might be important in practical control operations is its fumigating action. Lice on cattle have been killed for some distance from the area where the benzene hexachloride has been applied. In direct comparison with D D T technical benzene hexachloride has proved to be more toxic t o both short- and long-nosed cattle lice and to red and yellow goat lice. Concentrations of technical benzene hexachloride (containing 10% of gamma isomer) as low as 0.025% have given complete kill of long-nosed cattle lice. D D T a t the same concentration failed to kill all lice. Against the goat lice a 0.01% concentration of technical benzene hexachloride proved more effective than D D T a t the same concentration. Against the lone star tick on cattle technical benzene hexachloride at a concentration of 0.25% (wettable powder form) gave complete control of all unengorged ticks and almost complete control of engorged forms. D D T on the same basis failed
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to kill all engorged forms a t 1.5% and required a concentration of about 0.75% to kill ail the unengorged ticks, but proved superior to benzene hexachloride in preventing reinfestations. A combination of D D T and benzene hexachloride seems to offer considerable promise in tick control. Benzene hexachloride proved effective in preventing fleeceworm infestations in sheep caused by larvae of the black blow fly, Phormia regina (Meig.), and other blow flies. I n some tests it showed promise against larvae of cattle grubs, but in others it proved ineffective. In general, the action of benzene hexachloride on cattle grubs has been too erratic t o warrant its consideration for practical use. CHLORINATED CAMPHENE.Chlorinated camphene has been tested less extensively than benzene hexachloride. It is now being tested in the field in comparison with D D T and other insecticides to determine its value in the control of. horn flies. Early results of tests being carried out in cooperation with the National Livestock Loss Prevention Board and the State Agricultural Experiment Stations of Kansas and Missouri indicate that i t is probably less effective than DDT. The action of chlorinated camphene against lice on cattle (7), sheep, and goats seems t o be comparable with that of DDT, although extensive field tests have not been conducted. It may be superior to D D T in controlling the lone star tick and the winter tick. The insecticide is now under consideration for use in preventing reinfestations of susceptible animals t o screwworm and fleeceworm attack. Results of these preliminary investigations are promising. Tests against cattle grub larvae, deer flies, and houseflies indicate that chlorinated camphene is not promising. CHLORDAN.I n comparative tests chlordan is indicated as somewhat inferior to D D T against the horn fly, although i t can be classified as effective against this insect. Against houseflies chlordan has less persistence than D D T as a residual spray, but is highly effective for several weeks when tested under laboratory conditions. The insecticide is also promising for the control of lice on cattle, sheep, and goats ( 7 ) . Against the lone star tick and the winter tick i t is comparable with or perhaps superior to DDT. On the basis of lasting protection it s e e m to be equal to D D T against the winter tick but inferior against the lone star tick. Chlordan proved comparable wiLth D D T in protecting animals from attack by fleeceworms. It is relatively ineffective in protecting animals from attack by horse flies. When tested as a dust i t was ineffective against the larvae of cattle grubs. COMPOU~DS RELATEDTO DDT. Two compounds closely related to DDT, dichlorodiphenyldichloroethane TDE, (DDD) and the methoxy analog of DDT, have been given some consideration as possible substitutes for D D T in controlling livestock pests. I n preliminary tests against horn flies and houseflies the two materials showed promise but were less effective than DDT. Laboratory tests with the lone star tick indicate that these materials are less effective than DDT. Even though generally less effective than D D T for most insects attacking livestock, the methoxy analog of D D T and dichlorodiphenyldichloroethane are reported to be of a lower order of toxicity to warm-blooded animals. Preliminary tests in South Carolina suggest that the methoxy analog of D D T and combinations of this insecticide with benzene hexachloride may provide considerable relief against attack on cattle by horse flies and deer flies. Cattle treated with a spray containing 2% of the methoxy analog of D D T and 0.25% of benzene hexachloride showed a marked reduction in numbers of horse flies (Tabanus spp.) over untreated herds for about a week after treatment. Further tests employing these materials separately proved inconclusive because of low fly populations. NEED FOR FURTHER RESEARCHON NEWER INSECTICIDES. There is urgent necd for’ research on the toxicology of various
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insecticides to livestock. Because available data on the toxicology of the newer materials are limited largely to small laboratory animals, the Bureau of Entomology and Plant Quarantine has undertaken certain investigations in cooperation with the Bureau of Animal Industry, the Bureau of Dairy Industry, and certain state agricultural experiment stat,ions to determine the toxicological effects of some of the newer insecticides when applied to livestock. These effects should be studied on small laboratory animals and livestock before extensive and costly research is carried out. In order that certain aspects of the toxicological investigations with tBhenew insecticides may proceed satisfactorily, met,hods must be developed for analyzing animal tissues, secretions, and excretions for the quantitative determination of all the new insecticides. Further research on the chemistry of new insecticides is,urgentlyneeded. Much additional chemical research on the newer materials is also needed in efforts to improve formulations, such as wettable powders, eniulsions, and dusts. The trend is to employ wettable powders because they are generally considered safer. However, marked improvement of this type of product is needed, especially when i t is employed with existing equipment. The numerous effective insecticides t,hat have come to light in
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the past few years encourage the belief that many other chemical compounds possess distinct insecticidal value, and should stimulate synthesis and testing of many new compounds. LITERATURE CITED
(1) Anon., U. (2)
S. Bur. Entomol. Plant Quarantine, E-623 (1944). Barrett, W. L., Jr., and Wells, R. W'., J . Econ. Entomol., 39, 816
(1946). (3) Bishopp, F. C., Am. J . Pub. Health, 36, 601 (1946). (4) Bishopp, F. C., and Henderson, L: S., U. S. Dept. Agr., L e a f . 182, 4 (1946). (5) Bruce, W. G., and Blakeslee, E. B., J . Econ. Entornol., 39, 36774 (1946). (6) Dove, W. E., U. S. Bur. Entomol. Plant Quarantine, E-673, 2 (1945). (7) Knipling, E. F., S o a p Sanit. Chemicals, 23 (7), 127 (1947), (8) Laake, E. W., J . Econ. Entomol., 39, 65 (1946). (9) Parish, H. E., and Rude, C. S., I b i d . , 39, 92 (1946). (10 Rude, C. S..U. S.Bur. Entomol. Plant Quarantine, E-686 (1946). (11) Smith, C. N., and Gouck, H. K., J . Econ. Entomol., 38, 553 (1945). (12) Teleford, H. S., and Gnthrie, J. G., A g r . Chemicals. 1 (3), 31 (1946). (13) Twinn, C. R., C a n . J . Comp..lIed. V e t . Sci., 10,301 (1946). (14) U. S.Dept. Agr., Yearbook, 1942. .(15) Wachs, H., Science, 105, 530 (1947). RECEIVED November 22, 1947.
DDT Residues in Agricu R. H. Carter Bureau of Entomology and Plant Quarantine, Beltseille, Md. Residues of DDT that may result from insecticidal applications of this material to fruits, vegetables, and forage crops are discussed. The results of experiments on the absorption and translocation of this compound when applied as sprays and dusts to the aerial parts of vegetables are described. The absorption and storage of DDT in the tissues of farm animals and its elimination in milk are discussed.
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HE widespread use of D D T for the control of insects has
stimulated interest in the amounts of D D T residue in and on field crops and farm animals that have been treated with this insecticide. The amounts of residue are of interest from the standpoint' not only of insect control but also of their presence in products intended for human and animal consumption. Investigations have included studies on: Deterniination oi D D T residues on fruits, vegetables, and forage crops. Absorption of D D T residues by plants and its translocation into the edible portions' from applications to the aerial parts. Absorption and storage of DDT in the organs and tissues of farm animals that received small amounts ingested with the food. D D T content of milk from cows fed silage containing D D T residues. Effect of cooking mect from animals that had stored appreciable amounts of D D T in their tissues as a result of having been fed rations containing this compound. D D T content of eggs froni hens receiving this compound in the diet. METHODS OF ANALYSIS
Most of the chemical results reported in this paper were obtained by determinations of organic chlorine (8, 4 ) , or by the Schechter-Haller colorimetric procedure for D D T ( 7 , 8). In many cases determinations were made by both methods with good agreement.
A report on methods for the determination of D D T in insecticide tesidues and in animal products has been published recently (3); several methods are now being subjected to collaborative testing by federal and state chemists. DDT RESIDUES ON CROPS
The amount of residue found on agricultural crops depends on the weight of the crop per unit area, the amount of insecticide applied per unit area, the interval between treatment and collection of the sample, the gron-th since the last treatment, weathering, and various other factors. For this reason no prediction can be made as to the amount of residue that may be found on any particular sample. The author has made residue determinations of thousands of samples of fruits, vegetables, forage crops, and forest vegetation. On apples, for instance, he has found D D T residues ranging from 1 to 12.5 p.p.m., and on peaches from 6 to 23 p.p.m. On pea-vine samples he has found amounts ranging from practically nothing to 40 and 50 p.p.m. on the dry weight basis. On forage crops such as alfalfa and clover he has found residues as high as 48 p.p.m. An application of 1 pound of insecticide per acre to forage crops yielding 1 ton of hay per acre should result in a reqidue of 500 p.p.m. if all the insecticide n.ere retained on the vegetation. The residues obtained from various crops following treatment Kith D D T , according to the usual commercial practice, are given in Table I. ABSORPTION AND TRANSLOCATION O F DDT BY PLAUTS
h cooperative investigation with the Bureau of Plant Industry, Soils, and Agricultural Engineering was carried out over two seasons to determine whether vegetable plants would absorb D D T from insecticidal applications and translocate the compound into the edible portions.
