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Methylene chloride plus Freon 12 constituted a satisfactory propellent. A 50-50 mixture of the two liquids gave a vapor pressure of 41 pounds per square inch a t 70” F., and after the addition of the pyrethrum extract, DDT, and solvents, the vapor pressure of the solution was approximately 37 pounds per square inch a t 70” F. Less solvent was required for the DDT when methylene chloride TT-as used instead of Freon 11. Pentane was incorporated with the mixture of the two Freons t o lower the vapor pressure, but there appeared t o be no advantage in its use because of its poor solvent properties. Deobase was used to lower the vapor pressure of Freon 12. Approximately 70y0was necessary t o reduce the pressure below 40 pounds per square inch gage, and the resulting mixture was oily and produced a very coarse mist. Propane (15yo)plus methylene chloride (85%) made a good propellent for aerosols, but a fire hazard could be caused by the stratification of the two vapors. Carbon dioxide added to methylene chloride containing approximately 5% Freon 12 increased the pressure of the solution. The high percentage of methylene chloride eliminated the necessity for auxiliary solvents. However, if the carbon dioxide is lost by faulty mechanism before the contents are used, the performance of the remaining portion of liquid will be unsatisfactory.
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1,l-Difluoroethane added t o Freon 11 for use as a propellent required the use of auxiliary solvents. l,l,l-Difluorochloroethaneand Freon 12 formed a satisfactory propellent with auxiliary solvents to hold the D D T in solution. A number of possible propellents for lon--pressure aerosols will produce satisfactory insecticidal aerosols. I n developing new formulas it is necessary t o make a thorough study of the complete formula in regard to compatibility, pressure characteristics, and action on the valve mechanism. I n the production of an effective low-pressure aerosol the use of a release device suited t o the particular formula will be a requirement. LITERATURE CITED (1) Goodhue, L. D., Schulte, F. S., and Wilkins, P. W., Chem. Ind., 60, 602-4 (1947). (2) Peterson, H. E., “Low Pressure Aerosols,” Bull. 14, Continental Can Co., 1947. (3)
Rhodes, M7, W., and Goodhue, L. D., Soap Sanit. Chemicals, 23, NO. 10, 122-3, 151 (1947).
RECEIVED S o v e m b e r 22, 1947. .I portion of this research was conducted as p a r t of a programsupported by transfer of funds from the Office of Quartermaster General, U. 9. Army, to t h e Bureau of Entomology a n d Plant Quarantine.
Surface Active Agents in Foliage S Frank Wilcoxon and R . L. Morgan American Cyanamid Company, Stamford, Conn.
Surface-active agents are used i n . foliage sprays to render powdered solids wettable, or to promote emulsification. They increase the tendency of the spray to wet the leaves and insects. Their presence usually decreases the amount of actual deposit, because of increased runoff. Their effect on rain resistance is variable, and there is often an optimum concentration for best results. The effectiveness of contact sprays is increased by surface active agents, but that of residual type sprays may be increased, decreased, or not affected depending on materials and concentrations used. The spreading coefficient is a fairly satisfactory measure of wetting tendency; whereas the surface tension of the spray is inadequate for this purpose. The value of addition agents in promoting rain resistance is best determined by actual rain or washing tests.
F
UKGICIDES and insecticides are generally applied to foliage in the form of aqueous emulsions or suspensions, or as dusts. It is a fact, however, that leaves of many species, as well as the integument of insects are not readily wetted by water. Moreover, water, on account of its high surface tension, appears t o be unable to penetrate small capillaries such as the tracheal system of insects. For these reasons entomologists and plant pathologists have for a long time been interested in surface active agents which could be added to a spray suspension or emulsion to modify its physical properties. There are at least three ways in which such agents affect the spray. I n the preparation of the spray, they are used to form an emulsion in the case of a liquid toxicant, and to render a powder wettable in the case of a solid toxicant, When the spray is applied to leaves or fruits, surface active agents may influence the wetting and spreading power of the spray with a consequent change in effectiveness. Finally, where it is desirable to maintain a protective coating on leaves or fruits for a considerable
period of time, the presence or absence of surface active agents may affect both the amount of initial deposit per unit area, and its resistance to rain and weathering, Early work on addition agents t o sprays stressed the importance of lowering the surface tension of the spray fluid to obtain good netting, but as early as 1912, TTermoreland Dantony ( I S ) concluded that surface tension of the spray was not the sole criterion of netting poxer; they observed that two diffcrent sprays of equal surface tension might exhibit diffe1en.t wetting properties for different species of leaves, whereas two diffcrent sprays with widely different surface tension values wetted the leaves of a given species equally well. SPREADIhG COEFFICIEST
The work of Harkins and others (6) on the spreading of one liquid over another has led to the concept of the spreading coefficient as a measure of spreading tendency. This is simply the diffeience between the vork done in separating one liquid from another to produce 1 sq. em. of each, and the work done in separating the first liquid from itself to produce 2 sq. em. of surface. I n the first case the work is TL, TLz = TLIL2, and in the second case the work is ~ T L ~The . difference TL2- TL, = TLiL,expresses the spreading coefficieiit in terms of the surface tension of the two liquids and the interfaoial tension between them, all quantities accessible t o measurement. I n the case of an aqueous liquid spreading over a solid surface, presumably, a similar relation would hold, but the only quantity susceptible t o measurement is the surface tension of the aqueous liquid. However, if the aqueous liquid meets the solid a t a definite angle of contact, the spreading coefficient may be expressed in terms of the surface tension of the liquid and the cosine of the coniact angle is expressed by the formula: -S. C. = TLI(cos. 8 - 1). Many attempts have been made to determine the spreading coefficient of spray liquids on the surface of leaves and insects by measurement of contact angles ( l a , 16). Difficulties are
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INDUSTRIAL AND ENGINEERING CHEMISTRY
encountered, however, because leaves and insects do not have a uniform well defined surface, and also, there is some unhertainty as to whether the angle observed corresponds t o a condition of equilibrium. For these reasons some investigators prefer to use a standard oil as a reference substance; they assume that spreading coefficients, determined in this way, will indicate the spreading tendency toward leaves, fruits, and insects. Cupples (1, 2 ) in a series of papers determined the spreading coefficient of a number of aqueous solutions by m e of a highly refined mineral oil as reference substance. This author also studied the relation between spreading coefficient and the initial retention of spray on apples. It was founddhat the relation was a n inverse one, the larger the spreading coefficient and the better the wetting, the less spray was retained on the apple surface. This is, of course, what might be expected since the better the wetting and spreading properties of the spray, the greater the runoff from the sprayed surface. Similar results were obtained by the present authors using disks of cabbage leaf. It appears, therefore, that good wetting and spreading properties may be obtained a t the expense of a lower actual deposit of toxicant'per unit area of the sprayed surface, and the question arises as to the relative importance of these two factors.
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CONTACT AND RESIDUAL SPRAYS
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Insecticidal sprays may be classified roughly ad contact sprays or residual sprays. I n the former the insects are present at the time of spraying. and the poison kills by penetration through the integument of the insect and into its tracheal system. I n this case it is not hard to demonstrate that the presence of a wetting agent is desirable. Wilcoxon and Hartaell (16) found that the toxicity of free nicotine in water to aphids could be inoreased by adding surface active agents to the spray. It was also shown t h a t solutions with a high spreading coefficient were more likely to penetrate the tracheal system of insects. Hence, in the use of contact sprays, good wetting and spreading properties are more important than a high deposit of toxicant. Residual sprays present a somewhat different situation. Here a deposit of the toxicant is intended t o remain on the foliage for some time, and a high initial deposit is needed, as well as resistance t o rain and weathering. The effect of added agents on the initial deposit and on resistance to weathering of a material such as lead arsenate has been frequently investigated. Graham and Richardson (4) in 1942 reviewed the results of previous workers and noted that four of them claimed beneficial results from the use of a casein spreader, whereas eleven others obtained no benefit from its use. Graham and Richardson tested soybean flour, lime casein, and a commercial spreader with lead arsenate for t h e control of codling moths. No benefit from any of these materials as used in their experiments could be shown by statistical analysis. I n 1943 Isely and Horsfall (9) published the results of experiments in which bean plants were sprayed with lead arsenate to which various other materials had been added. The addition agents used by them included Bordeaux mixture, hydrated lime, wettable sulfur, soybean flour, mild powder, blood albumen, talc, four commercial spreaders, potash fish oil soap, and a white summer oil. The deposit of lead arsenate was determined by analysis of the leaves, and the results submitted to statistical analysis. None of the added materials increased the initial deposit of lead arsenate, and some of them reduced it t o less than half of its value for the toxicant alone. An interesting method for obtaining a high deposit of lead arsenate has been developed in the state of Washington (10). This is the so-called dynamite spray in which a n oil and a material such as triethanolamine oleate are incorporated in the spray with the lead arsenate. Under suitable conditions the lead arsenate, which is normally preferentially wetted by water, becomes preferentially wetted by the oil. Under these conditions a large deposit of the arsenical may be obtained.
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RESISTANCE OF RESIDUAL SPRAYS
Resistance to rain and weathering is also a factor of importance in the use of residual sprays. The manner in which added agents affect this property in lead arsenate was studied recently by Garman (3). I n experiments on sprayed glass slides which were submitted to a standard washing procedure, many added substances exhibited a n optimum concentration for rain resistance, when the concentration of lead arsenate was increased. Among the materials found t o increase rain resistance were bentonite with skim milk or casein, aluminum hydroxide gel, and certain oils. A number of years ago a n electrical theory of the adherence of spray materials was put forward by Moore (11). He maintained t h a t leaves in contact with aqueous solutions or suspensions were negatively charged, and that in order to obtain good adherence, the particles of spray suspension should be positively charged. In 1936 Hoskins and Wampler (8)made a further investigation of this point and found that when increasing amounts of aluminum ion were added t o a lead arsenate suspension, the deposit of arsenic adhering to a surface of beeswax increased, reaching a maximum when 1millimole of the electrolyte per liter was present. They explained this by the fact that the added aluminum ion reduced the negative charge of the lead arsenate more than t h a t of the beeswax surface. At higher concentrations both became positive and the adherence decreased. These experiments did not involve resistance t o rain, but as mentioned previously, aluminum hydroxide has been found by Garman (3) to be a n excellent sticker in certain concentrations. The effect of added agents on the initial deposit and adherence of sulfur to rose foliage was investigated by White (14). The amendments .tested by him included a salt of a sulfonated carboxylic acid, a coconut fatty acid soap, sodium lauryl sulfate, sodium oleyl sulfate, powdered skim milk, calcium caseinate, commercial wheat flour, waste sulfite liquor, and gum arabic, as well as four proprietary products. Only three materials increased the initial retention of the sulfur. These were wheat flour, waste sulfite liquor, and gum arabic. Wheat flour and gum arabic were the best also from the point of view of rain resistance. A paper by Holland, Dunber, and Gilligan (6) gives data on the effect of additives to a copper spray made from a basic copper sulfate. Among the materials tested were sugar, wheat flour, glue, a soybean'fish oil preparation called Laucksite, calcium caseinate, castile soap, rosin-fish oil soap, and Wilkinite. Only wheat flour increased the initial deposit, and none of the materials contributed much to rain resistance; ordinary lime Bordeaux was superior t o all of them in this respect. A recent development in the field of adhesives for fungicidal and insecticidal sprays is the use of a latex of polyethylene polysulfide (7), which it is claimed dries down to a rubbery weblike film and imparts rain resistance to the toxicant in the spray. CONCLUSIONS
I
A few conclusions may be drawn from the very large and often conflicting literature on foliage spray addition agents. The spreading coefficient is a fairly satisfactory index of the ability of the spray t o wet and spread over the surface of leaves and fruits, but a high spreading coefficient involves a low initial retention of spray if conditions are such that runoff takes place. I n spite of this, surface active agents are useful additives to contact sprayi. Many of the proposed addition agents to residual sprays do not increase the initial deposit, and those which do seem to be of a type which increase the viscosity of the spray as has been noted by Woodman (16). Regarding rain resistance, materials such as oils, bentonite with casein, and aluminum hydroxide gel promote resistance t o
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rain, but there is often a n optimum ratio of adhesive to toxicant that is required to obtain best results. Although spreading coefficients and viscosity measurements may serve to predict the performance of a spray in regard to wetting power and initial retention, rain resistance seems t o require an actual rain, or washing test, for its evaluation. LITERATURE CITED (1) (2) (3) (4)
Cupples, H. L., IND. ENG.CHEM.,27, 1219-22 (1935). Cupples, H. L., J . Agr. Research, 63, 681-6 (1941). Garman, Philip, Conn. Agr. Expt. Sta., BUZZ.485 (1945). ~ ~L. T.,and ~ Richardson, h ~c. H,, J~. E ~, ~ ~~ t ~ ~35,. ~ 911-14 (1942).
W. D.. and Feldman. . , Harkins. 2665-85 (1922).
(5)
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(6) Holland, E. B., Dunbar, C . O.,and Gilligan, G. M., Mass. Am. Expt. Sta., Bull. 252 (1929). (7) Hopperstead, S. L., Agr. Chemicals, 2, 24-6 (1947). (8) Hoskins, W. M., and Wampler, E. L., J . Econ. Entomol., 29,13443 (1936).
(9) Isely, D., and Horsfall, W. R., Ibid., 36, 751-6 (1943). (10) Marshall, James, Wash. Agr. Expt. Sta., Bull. 350 (1937). (11) Moore, William, Minn. Agr. Expt. Sta., Bull. 2 (1921). (12) O’Kane, W. C., Westgate, W. A., Glover, L. C., and Lowry, P. R., N. H. Agr. Expt. Sta.. Tech. BUZZ.39 (1930). (13) Vermorel, V., and Dantony, E., Compt. rend., 154, 1300-2 (1912). (14) White, R. P., N. J. Agr. Expt. Sta., Bull. 611 (1936). Frank, and Hartzell, Albert, Contrib. BoWe Thompson ~ (15) l ,WilCoxon, , Inst., 3, 1-12 (3931). (16) Woodman’R * ” Ind‘, 49193-8T (1930)* RECEIVED November 22, 1947.
Biological Methods of Testing Insecticides Harold H. Shepard Insecticide Testing Laboratory, United States Department of Agriculture, Washington, D . C. Biological methods of testing insecticides are utilized to discover promising new materials in screening operations, to develop specific uses and formulations for practical application, to make fundamental studies of relative toxicity and mode of action, to provide quality control in commercial production and governmental regulation, ‘and to supplement chemical analyses with biological assays. Because of the variables which must be controlled when the reactions of living organisms are measured, a thorough understanding of insect biology is necessary for the most effective biological testing of insecticides.
I
N R E C E N T years intense interest has been aroused by the
successful development of synthetic organic insecticides capable of being produced in large quantities by the chemical industry. As a result tens of thfousands of new compounds are now being scrutinized by biological methods for possible insecticidal value. SCREENING
Biological methods of testing insecticides may be classified appropriately on the basis of the objectives t o be gained in their use. Given a series of new compounds entirely unknown as t o their potentialities in this field, it is necessary fist t o screen out those which show sufficient promise t o be studied further. This process of screening is accomplished by a variety of techniques, many of which are tests directed toward specific practical applications rather than ltoward insecticidal activity in general. As a consequence, it is likely t h a t valuable insecticidal chemicals are overlooked because the screening test applied is not sufficiently broad. The injection of test fluids into the blood stream of 8 common large insect such as the American cockroach is a recommended preliminary or basic screening technique. This method eliminates the influence of differences in surface activity, penetrability, and other properties of the body wall, digestive tract, or respiratory system of difTerent kinds of insects. Such factors are largely responsible for t h e specificity of insecticidal action and in the early stages of studying a series of compounds should be bypassed. Equipment for injection of insects may be a tuberculin syringe and a calibrated micrometer head, or it may be a calibrated capillary tube drawn out t o a fine tip and equipped with a rubber tube or bulb for the application of pressure (fa).
Another approach t o t h e screening of potential insecticides is a sorting scheme made up of a series of combinations of cage tests which indtcate fairly well whether a compound has much promise as a stomach poison, a contact insecticide, a fumigant, or a repellent, or various combinations of these (23). The steps in the scheme read like a miller’s flow sheet. I n the end one can say for what particular purpose a promising compound should be tested further. Settling tower methods are commonly utilized for either dusts or sprays, both in preliminary screening operations and in the more fundamental study of practical formulations where carriers and additives may all influence the behavior of the primary toxicant compound. I n the use of a settling tower the object is t o produce as uniform a cloud of dust or spray particles as possible, so t h a t a number of test objects (such as leaves, glass slides, or insects) will receive simultaneously a fairly uniform deposit of insecticide. The insecticide is dispersed by means of a n air line opening sometimes at the top of the tower ( 2 4 , sometimes at the bottom (12). I n all cases a check on dosage is made by weighing pieces of glass or paper of known area exposed t o the same deposit as the‘ test leaves or insects. By modifying the equipment and standardizing the procedure it is possible t o reproduce deposits within reasonable errors between different sprayings or dustings, or t o vary dosage in a predetermined manner. Considerable advantage is claimed for the horizontal or Hoskins type of spray tunnel. I n this apparatus a mist is intrcduced at one end, while below the other end is attached a cage of insects upon which the spray settles. Being sprayed horizontally, t h e insecticide appears t o fall in a manner resembling actual practice more than when a perpendicular tower is employed. For this type of work results possess a high degree of uniformity (18). For t h e sake of even distribution of insecticides many methods involve turntables (f4)and conveyers upon which test objects are exposed t o a spray from precision sprayers installed in B fixed position (9). Results, however, are usually not so reproducible as in a settling tower. Many organic compounds have been tested by the Siegler appleplug technique, whereby newly hatched codling moth larvae must penetrate a spray residue if they are t o eat their way into their normal food, the fruit of the apple (81,26). Because of differences in the susceptibility of various groups of these larvae,
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