It is becoming increasingly clear that insecticides must be formulated with due regard to the insects to be dealt with, the types of surfaces to which they must be applied, the degree of "weathering" they must withstand, and other local factors that will influence their efficacy.Laboratory tests have shown that a wettable powder is a most promising type of formulation for application to absorptive mud walls of native-type houses in Africa for the control of adult mosquitos (Hadaway & Barlow, 1949). Surface deposits on mud are greater from water suspensions of wettable powders than from solutions or emulsions.Studies of the properties of wettable powders have been continued and are described below. Particular attention has been given to the relation between particle size and insecticidal efficiency. These investigations are related primarily to the development of improved formulations for application to absorptive surfaces for adult mosquito control but the fundamentals involved are of interest and importance in other fields. Materials and Methods. Test surface.An absorptive surface which is less variable and less likely to interfere with chemical analysis than mud was needed for general use in testing insecticides as liquids or dispersions. Accordingly plaster of paris was chosen. The ordinary commercial material was not satisfactory but a better type procured from Messrs. British Drug Houses Ltd. was white in colour, varied only slightly from batch to batch and had no free chloride to give high blank values in the determination of chlorinated hydrocarbon insecticides.The plaster was mixed with water in the proportion of 5 : 2 by weight and allowed to set in moulds consisting of iron or brass rings 7-5 cm. in diameter and 1 cm. deep. They were air dried at 25°C. for at least four days. The final treatment before spraying was to smooth and flatten the surface by scraping with a palette knife. The porosities of the blocks used throughout this work-were occasionally checked and found to be 36-37 per cent., changing slightly from one delivery of plaster to another. Within a given batch the variation was much less. Use of less water than the ratio mentioned gave considerably lower porosities. Spraying tower.Some of the particles of wettable powders were too large to be sprayed with the usual type of atomising gun. It was necessary, also, to use a dip tube rather than a gravity feed and to spray the whole of a given sample of suspension in order to avoid irregularities due to differential sedimentation of particles of varying size.A tower of the type described by Webb (1947) was available. The top box containing the window was removed and the square lid screwed centrally over the remaining cylindrical portion, thus giving a tower about 3 ft. high and 10 ins. in diameter. The base was not altered. (865) K

Formulations of DDT have been applied to standard mud blocks. Toxicities to Glossina palpalis and Aëdes aegypti, and the proportions of insecticide recovered from an outer layer of approximately 0·1 mms. thickness have been compared.Absorption of DDT in oil solutions was considerable, and amounts recovered from the outer layer were only from 8–19 per cent, of the total dosage applied. Adsorption of the insecticide from the oil on to mud occurred. There was some correlation between the capillary rise of the solvent and the extent of absorption. Toxicities of the blocks to test insects were low.Using concentrations of 2 per cent., 5 per cent, and 10 per cent. in power kerosene, there was no increase in the proportion held on the surface. Application of excessively heavy dosages increased the amounts in the outer layers but there was a trend towards greater absorption, and therefore to greater waste, at the high dosages. Repeated applications similarly built up a larger dosage in the outer layer but did not increase the proportion there.Emulsions were intermediate between solutions and wettable powders as regards absorption and toxicity.Up to 77 per cent, of the insecticides applied as wettable powders was recovered from the outer layer, and toxicities were correspondingly high. When benzene hexachloride wettable powder was used there was rapid loss of benzene hexachloride from the surface by volatilisation. After fifteen days the dosage had decreased considerably and the percentage in the outer layer had fallen from over 70 per cent, to 20 per cent.Loss of DDT by penetration of the carrier oil through the leaf cuticle may occur when an oil solution is sprayed on to vegetation. The extent of penetration varies with different plants.There is an indication that small amounts of DDT are transferred from the inside of treated leaves to other untreated parts of young coffee and Avocado pear plants.Deposits of DDT on leaves exposed to ordinary climatic conditions remained toxic to tsetse flies for a longer period when applied as an emulsion than as an oil solution. Deposits from a water suspension of a wettable powder were washed off readily by rain.There is some evidence that continuous exposure to sunlight produced some chemical change in, and reduced the toxicity of, DDT deposits applied to glass plates as a solution in kerosene.

Factors affecting the persistence of deposits from aqueous suspensions of insecticides have been studied.There is an inverse relationship between particle size of insecticides and the initial contact toxicity to mosquitos. The influence of particle size on effectiveness decreases as the intrinsic toxicity of the insecticide increases. The compounds investigated can be arranged in order of immediate contact toxicity to mosquitos (Aëdes aegypti): dieldrin ≥ gamma-BHC > aldrin > DDT.

Investigations have been made on the influence of environmental conditions on the contact toxicity to adult mosquitos (Anopheles stephensi List.) of two kinds of insecticidal deposits of importance in sprayed houses, superficial deposits from wettable powders and insecticides sorbed on dried soils.Increasing the relative humidity from 43 to 80 per cent, during the contact period had no effect on the toxicity of wettable-powder deposits on plywood of dieldrin, Sevin and O-methyl O-(2,4,5-trichlorophenyl) ethylphosphoramidothioate. On the other hand, the availability of DDT, dieldrin and 3-isopropylphenyl N-methylearbamate sorbed on dried soils increased considerably as atmospheric humidity increased. The logarithm of the median lethal exposure time was linearly related to atmospheric humidity, and decreased by a factor of 2·8, 2·2 and 2·1 for the carbamate, dieldrin and DDT, respectively, for each 10 per cent, increase in relative humidity. There was also a linear relationship between the logarithm of the water uptake of a given soil and humidity.Temperature affected the action of insecticides during both the contact period and the post-treatment period. Median lethal doses of dieldrin, Sevin and the phosphoramidothioate decreased, and that of DDT increased, as the post-treatment temperature increased from 20 to 30°C. These effects of the post-treatment temperature were also found when treatment was by exposure to residues. When these effects were eliminated it was found that the contact toxicity of superficial deposits on plywood and of insecticides sorbed on dried soils increased as the temperature during the exposure period increased. A simple relationship between median lethal time and exposure temperature was indicated.

Previous work (Hadaway & Barlow, 1952) has shown that solid particles of insecticides deposited from aqueous suspensions could disappear very rapidly from the external surfaces of blocks made of some tropical soils. This physical process was thought to be one of adsorption and involved complete loss of biological activity of those compounds, such as DDT and dieldrin, which possess little or no fumigant action. On the other hand, it extended the effective life of fumigants such as y BHC and aldrin. This paper collects together some miscellaneous items of work on this phenomenon. The Sorption Process.Experiments described by Hadaway & Barlow (1952), in which particles of y BHC were moved away from the surface of the mud blocks, clearly showed that the sorption of this insecticide by the mud was from the vapour phase and loss was not due to a migration of molecules from the solid over the surface of the sorbent. As the rates of sorption of particles of DDT, aldrin and dieldrin of the same size, compared with y BHC, were approximately in the ratio of their vapour pressures, these compounds are also presumably adsorbed by mud from the vapour phase.If the disappearance of insecticides from lateritic mud surfaces is due to adsorption it should also happen on other adsorbents. It was not easy to see if this was so on aluminium oxide or silica gel because insecticide particles were difficult to observe on these materials. Charcoal, being black in colour, enabled particles on its surface to be readily seen and, on blocks of respirator charcoal, DDT disappeared at about the same rate as it did on Uganda mud. This provides an illustration of the high sorptive power that the lateritic soils possess. Activated iron oxide gels prepared as described by Lambert & Clark (1927) were even more effective than charcoal. Crystals of DDT less than 10 microns in size were sorbed in about two hours as compared with two to three days, 10-20 micron particles of y BHC in about one hour compared with overnight, while a flat plate of aldrin, 10 by 30 microns, lying flat on the surface, disappeared in about ten minutes. It is of great interest that iron oxide can be so active in a physical sense because the most active soils, the laterites, contain high proportions of this substance.The highest rates of sorption were obtained when the insecticide particles were in close contact with the sorbent. Movement away from the surface greatly reduced the rate. This was so even if the distance was only the width of a few particles, for if particles were piled on top of one another the bottom ones were sorbed first. Also, if single crystals were observed microscopically, it was obvious that the molecules were not being removed from all over the crystals but from the side next to the sorbent. The crystals became thinner and thinner and before finally disappearing were just ghost-like outlines of their original selves. Thus the forces which cause the rapid transfer of molecules from the solid are only of short range.Normally such compounds as DDT evaporate ...