Particle Size in Relation to Insecticide Efficiencv CHARLES M. SMITH AND LYLE D. GOODHUE Bureau of Entomology and Plant Quarantine, U. S. Department of Agriculture, Washington, D. C.
T
HE insecticidal action of a chemical compound obvi-
mines how well they suspend when made into sprays, how well they distribute when applied as dusts, and how well they adhere to the plant or animal to which they are applied. I n the past few years the Bureau of Entomology and Plant Quarantine has paid considerable attention to this subject, and this paper is an attempt to evaluate our present knowledge of the relation between particle size and the over-all efficiency of insecticides. The problem of investigating the relationship just mentioned
ously is influenced to some extent by the size of its particles. I n the case of gaseous fumigants, which are molecularly dispersed, the rate of diffusion through air is a function of the size of the molecules and must affect the penetration of the fumigant into crevices. The value of liquid insecticides, such as petroleum oil, both as direct sprays and as emulsions, is dependent upon the production of droplets of suitable size. With solid insecticides the particle size deter-
A number of solid insecticides have been separated by various workers into fractions of different particle size, and toxicity tests have shown the smaller particles to be, in general, the more effective. Greater subdivision of the insecticides now available only in comparatively coarse form, such as Paris green, and particularly of plant materials such as derris and pyrethrum and organics such as phenothiazine, would undoubtedly improve their action.
U. S. D. A.
In the case of oil emulsions droplet size is important, but no clear generalizations can be drawn. With oil sprays the quantity of oil appears to be more important than the size of the droplets. Solids and liquids can both be applied in aerosol form, and the degree of fineness and stability of such aerosols can be improved by means of protective smokes and surfaceactive agents to the point where many new fumigants may be developed.
photograph hy Forsythe j
Cage of Cockroaches, Reared in the Laboratory, Is Used f o r Testing the Insecticide Power of Aerosols of Rotenone and Pyrethrum
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is not so simple of approach as might be supposed. For instance, powdered insecticides always contain a variety of sizes, with the middle 80 per cent frequently covering a size range represented by a diameter ratio of 10 t o 1. It is practically impossible to choose two commercially manufactured materials whose size ranges do not overlap considerably, for one material would have to be so coarse as t o be unsuited for use as a dust. Furthermore, in the laboratory it is far from simple to segregate from commercial samples fractions of such narrow size limits as to ensure no overlapping. Sifting is of little use, since a dusting powder must consist largely of particles smaller than the holes in the smallest screens available (45 microns for the 325-mesh screen). Some form of sedimentation or elutriation must be adopted, and the difficulty of obtaining large quantities of materials by such means increases with the narrowing of the size limits between which it is desired to have the samples fall. Despite these difficulties, certain data have become available from which it is possible to get some idea of the correla-
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tion between particle size and insecticidal efficacy. data will be outlined briefly.
These
Solid Insecticides Smith (14) investigated the relation between the fineness of pyrethrum powder and its toxicity to insects. The samples were prepared by grinding commercial pyrethrum powder for different periods. When tested in water suspension against fourth instars of mosquitoes (Culex quinquefasciatus Say), the finest powder required 10.5 minutes to paralyze 50 per cent of the larvae in one series of tests, whereas the coarser powder produced this effect only after 277 minutes. When tested in aqueous suspension against A p h i s rurnicis L., the coarsest powder killed 56 and the finest 73 per cent; when applied in dust form against the same insect, the coarsest killed 66 compared with 79 per cent for the finest. Smith, Scales, and Gaines (16) studied the toxicity of twenty samples of commercial calcium arsenate to the boll weevil (Anthonornous grandis Boh.). As a criterion of the relative particle size of these samples, they considered that weight percentage which consisted of particles below 10 microns in effective diameter, as determined by sedimentation analysis. A summary of their results follows, in which the samples are combined into four groups of five each: Avera e % ’ by Wt. S p a f e r than IO Microns Diameter
40 46
GO SO
Average Net Mortality, % 72
70
66 50
A definite relation is shown, as indicated by the coefficient of correlation of -0.73 calculated by the authors. The coarser samples proved to be the most toxic, contrary to expectation and to the findings of other investigators with other materials. The explanation undoubtedly lies in the nature of commercial calcium arsenate, a complex mixture sometimes containing relatively soluble acid arsenates which crystallize readily and are thus both coarse and toxic, and sometimes containing basic arsenates which, being very insoluble and hardly crystallizing at all, are both finely divided and relatively inert, For this reason calcium arsenate is unsuitable for critical testing of insecticidal efficiency in relation to particle size. Siegler and Goodhue (IS) prepared coarse, medium, and fine samples of Paris green, cryolite, and phenothiazine by putting commercial samples through an air classifier and tested them against larvae of the codling moth ( C u ~ p o c a p ~ a pornonella La). Commercial lead arsenate was too fine to be fractionated by this method, but they prepared a coarser sample by recrystallizing the fine material from nitric acid. Table I shows the percentages of nonwormy apple plugs in relation to the degree of fineness of the samples as expressed by the middiameter-that is, the particle diameter which divided the samples into two equal parts by weight. The relationships shown are rather confused. I n the case of phenothiazine the coarsest sample is much the poorest, statistically speaking, but with lead arsenate the coarser sample is the better and the intermediate samples of Paris green and cryolite are best. Later Bertholf and Pilson (S), in studying the toxicity of certain insects to the honeybee ( A p i s mellifera L.), included among their samples the same lead arsenates, cryolites, and phenothiazines that had been tested by Siegler and Goodhue. By controlled feeding of the honeybees, Bertholf and Pilson determined the dosage mortality curves for the f i s t two materials but found phenothiazine to be practically nontoxic. The quantities of insecticide required to cause 60
U. S. D. A. p h o t o g r a p h by Forsythe Pouring Out a Rotenone Solution t o Be Tested as a Fumigant f o r Household Insect Pests
per cent mortality in 3 days, in comparison with fineness, follow : Relative Fineness of Sample Coarse Intermediate Fine
Lead Arsenate Median lethal dose, micro: grama of As per bee 185
Middiemeter, miorons 18
..2
..5
Cryolite Median lethal dose, micrograms of F per bee 13.0 5.5 4.2
Middiameter, microns 28 8 2
The greater toxic effect of the fine lead arsenate was marked in these experiments, and the finest cryolite was definitely more toxic than the other samples. McGovran, Cassil, and Mayer (9),not satisfied with the indefiniteness in the particle size of the samples used by other investigators, prepared narrow-limit fractions of Paris green and determined their effects on the Mexican bean beetle (Epilachna varivestis Muls.). By a combination of sifting, air classification, and sedimentation in alcohol, they obtained three fractions which did not overlap. Both spraying and dusting tests were made, care being taken that within each series of tests the deposits offered to the beetles were practically identical. The data on these three fractions follow: Average Particle Diameter, Microns 22 12 1.1
Spraying Tests Insecticide ingested micrograds/ Mortality gram of in 48 insect hours, % 20s 29 121 40 35 63
Dusting Tests Insecticide ingested microgrards/ Mprtality gram of in 48 insect hours, % 449 43 238 61 34 88
The results establish almost beyond doubt that, in general, fine subdivision enhances the effect of an insecticide. The greater mortality with the finer samples, in spite of the
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The experimental results cited indicate that h e subdivision of a solid insecticide is definitely advantageous. Some of our common insecticides, such as lead arsenate and many calcium arsenates, are already produced in such fine form that not much improvement can be expected. However, reduction in the particle size of Paris green might greatly increase its effectiveness; in fact, recent years have seen the appearance of so-called air-floating Paris green. Pyrethrum and derris powders are both relatively coarse a t present and probably would be more effective if they could be more finely ground, New organic insecticides intended for use as dusts should always be prepared in as fine a form as possible, even if some conditioning material must be added.
U. S . D. A. photograph by Foraythe Sprayed j r o m the Bottle onto a Hot Plate, the Rotenone Solution Immediately Vaporizes as a Thick Smoke or Fog
relatively small amount of feeding, indicates that the fine particles are more readily soluble within the insect. Chiu (6) extended the study of particle size in relation to toxicity to include a so-called inert material. He tested several fractions of crystalline silica against the bean weevil, Acanthoscelides obtectus (Say), and observed the percentage mortality as a function of time after the weevils had been heavily dusted with the powders. An abridged summary of his results follows: Diam. of Particles, Microns Average 2.9 18.8 37.5 111.0
Range 1-147 1-149 10-74 74-149
Days Required t o Kill: 50 %
1.8 4.2 10.5 14.5
100%5.0 10.0 18.0 20.0
Liquid Insecticides The correlation between toxicity of liquid spray materials and their globule size seems to have received less attention than the same problem in connection with powdered solid insecticides. The greater difficulty in studying them, due to variations introduced by method of preparation and less stability in the finished product, may account for this. However, a few investigations can be mentioned. De Ong and others (10, 11) came to the conclusion about 1925 that quick-breaking oil emulsions were more suitable for insecticidal purposes than the more stable kind, and the quickbreaking emulsions were recognized to contain larger drops. Griffin, Richardson, and Burdette (8), having noticed that miscible oils were less effective than petroleum emulsions, tested several kinds of emulsions containing different-sized oil globules against Aphis rumicis. With lubricating-oil emulsion, an oil concentration of 0.5 per cent killed 90 to 100 per cent of the aphids when the globules were between 8 and 12 microns in diameter, and only 40 to 80 per cent when the droplet size was 2 microns or less; this relationship held despite variations in viscosity and unsulfonatable residue. An emulsion made from gas oil having globules 7 to 8 microns in diameter was approximately twice as toxic as a similar oil of 2-micron droplet diameter. With emulsions of liquid petrolatum, when the oil droplets were 2 microns or less in diameter, three times as much was required to kill 95 per cent of the aphids as when the droplet diameter was 8 to 10 microns. Determinations of the oil retained by sprayed plants showed that more oil was deposited from large-droplet than from small-droplet emulsions, and this was considered the reason for the greater toxicity. English (6), in a rather comprehensive consideration of the physics of emulsions, states that increased effectiveness may or may not be accompanied by an increase in size of globules, and that increased globule size is the result of desirable qualities in the emulsion rather than the cause of greater effectiveness. I n contrast to the work just mentioned, the researches of Beran on carbolineum emulsions may be mentioned. He states ( 1 ) that opinions differ as to whether a stable or a quickbreaking type of emulsion is preferable, and gives as his own opinion that a laboratory stability test is no measure of film
Here again the correlation is unmistakable. Chiu states that with particles over 100 microns in diameter there is practically no toxic effect, and that probably only those particles under 15 microns are of any value. He believes that the effect is due to better adhesion of the fine particles. I n the last two seasons there has been under SIZEOF SEVERAL TABLE I. RELATIONBETWEES PARTICLE test at certain laboratories of the Bureau of INSECTICIDES AND PERCEKTAGE OF APPLEPLUGS NOT Entomology and Plant Quarantine a “micronENTERED BY CODLING MOTHLARVAE (la) ized” phenothiazine in comparison with phenothiazine regularly produced by manufacturers. Fineness Midwormy M The fine material has a surface mean diameter of diameter, plugs, diameter, of 2 microns compared with about 11 for the Sample mmrons yo microns commercial product, Final results are not yet Coarse 1s 82 29 88 28 89 45 Interavailable, but preliminary tests seem to show 96 8 95 10 .. mediate . . 2 l54 Fine 2 72 3 89 88 that the micronized material is decidedly more effective against the codling moth. L*Tl*iuI1T
3s 91
90
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stability on a tree. ’He describes tests (3) of five emulsions with droplet sizes ranging in diameter from 5 microns to submicroscopic. I n field tests against Lecunium cor& Bouch6 on plum and locust and Epidiaspis leperii Sign. on apples, he found increased efficiency as the particle size was diminished and concluded that for maximum effect emulsions should be of the highest possible degree of dispersion. I n connection with the application of oils alone, Burdette (4) studied the production of what he oalled “air-floated oil particles” and their relation to the insecticidal value of contact sprays. By the methods of atomization used, light petroleum oil formed a fog consisting of droplets about half of which were larger than 1 micron in diameter and the other half were ultramicroscopic. When tested against honeybees, the toxicity was mostly in that portion consisting of droplets from 1 to 10 microns in diameter. He concluded that for maximum kill sufficient material of this size range must be atomized to produce a concentration of at least 0.03 cc. of oil per cubic foot of air. Searls and Snyder ( l a ) ,also working on the application of oils by atomization, concluded that the value of an oil as a cattle spray could be judged by the function SNIT, in which X is the mean cross-sectional area of the drops in sq. mm., N is the mean number of drops per sq. mm., and T is the time in seconds required to spray 25 cc. of the oil. The function must have a value of a t least 0.03.
Aerosols
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sistent aerosol, such as the smoke from burning plant material, the formation of large crystals is retarded and the effective period is greatly prolonged. The inert aerosol provides a large surface for adsorption of vapor, and the microcrystals which form on the abundant nuclei settle very slowly. Naphthalene in the presence of smoke is from seven to ten times more effective than when vaporized alone. The use of surface-active agents has been found to increase the effectiveness of most insecticides applied in aerosol form (16). The energy required for disintegration is reduced, and the probable reduction in particle size may be a factor. Lauric and oleic acids, as well as their glycol esters, are very effective with o-dichlorobenaene, Since this discovery a small amount of oleic acid has been used with everv new insectitested in aerosol form.
Literature Cited Beran, F., Anz. Schttdlingskunde, 12,. 17 (1936). . . Ibid., 13, l ( 1 9 3 7 ) .
Bertholf, L. M., andPilson, J. E., J . Econ. Entomol.,34,24 (1941). Burdette, R. C., N. J. Agr. Expt. Sta., Bull. 632 (1938). Chiu, 8. F., J. Econ. Enlomol., 32, 240 (1939). English, L. L., Illinois Nat. Hist. Survey, Bull. 17,236 (1927-28). Goodhue, L. D., Sullivan, W. N., and Fales, J. H., J . Econ. EntomoZ., 34, 650 (1941). Griffin, E. L., Richardson, C. H., and Burdette, R. C., J . Agr. Research, 34, 727 (1927). McGovran, E. R., Cassil, C. C., and Mayer, E. L., J . Econ. Entomol., 33, 626 (1940). Ong, E. R. de, Am. PetroZeumInst. BUZZ., 7, 191 (1926). Ong, E. R. de, Knight, H., and Chamberlin, J. C., HiZgardia, 2 ,
Fumigation is usually carried out with liquefied gases such 361 (1927). as ethylene oxide, readily volatile liquids such as carbon diSearls, E . M., and Snyder, F. M., J . Econ. Entomol., 29, 1167 suliide, or solids capable of furnishing a reasonable amount of (1936). vapor such as paradichlorobenzene. I n these cases the acSiegler, E. H., and Goodhue, L. D., Ibid., 32, 199 (1939). tive compound is molecularly dispersed, and whatever adSmith, C. L., J . New York Entomol. SOC.,44, 317 (1936). Smith, G. L., Scales, A. L., and Gaines, R. C., J . Econ. Entomol., vantage accrues from fine subdivision is present to its maxi31, 677 (1938). mum extent. Sullivan, Goodhue, and Fales (17) recently Sullivan, W. N., Goodhue, L. D., and Fales, J. H., Science, 94, showed that it is possible to produce such finely divided sus444 (1941). ,pensions of less volatile materials that they also can be Sullivan, W. N., Goodhue, L. D., and Fales, J. H., Soap, 16, 121 (1940). admted to fumigation. These aerosols can be prepared by spraying a solu&on of the insecticide on a surface heated to about 375’ C . ; toxicological experiments have shown that the extent of decomposition is very slight, even with easily decomposed compounds. Rotenone and pyrethrum can be effectively applied in this way, and o-dichlorobenzene, which is only slightly toxic when sprayed, shows promise for the control of certain household insects when made into an aerosol. Tests with more than one hundred and fifty organic compounds have showed the following to be good fumigants when applied in this way: 3-chloroacenaphthene, 2chlorofluorene, 3-chlorodibenzofuran, pentachlorophenol, phthalonitrile, and xanthone. Methods of stabilizing aerosols are also being studied in the Bureau of Entomology and Plant Quarantine. One of these experiments (7) shows that, if naphthalene is vaporized by heat for the purpose of fumigation, it condenses rapidly in the form of large crystals which settle quickly. If, however, Courtesy, U. 9. D. A., Bureau of Entomology and Plant Quarantine the vaporization is carried out Deposits of Grass along t h e Shore of S t . Joseph’s Bay, Fla., Being Sprayed w i t h Creosote-Fuel Oil Mixture for Control of Dog Flies in the presence of some per-
