Dipping trials with two supelcooled melt formulations of DDT, in which the DDT concentrations were raised progressively to four times the usual strength, showed that whilst more DDT was deposited both on the hair of calves and on bush ticks (Haemaphysalis bispinosa Neumann) as the DDT concentration was increased and the dip aged, the toxicity to ticks of both formulations increased only during the early stages of each trial. Clearly the size and shape of the DDT crystals which developed in both dips and the degree of crystal aggregation were important factors which influenced the deposition of DDT and its toxicity. Of the forms of DDT present, the toxicity is attributed mainly to microcrystals of colloidal dimensions. Additions of formulation would serve to maintain but not increase the concentration of these microcrystals. Differences in performance of the dips is ascribed to the composition of the soaps (i.e. surface active agents) in the formulations. The experiments emphasize the importance of the nature of the deposit, and indicate that increases in dip concentration will not necessarily give better biological performance.

The beliaviour of several relatively insoluble pesticides (y-BHC, carbophenothion, coumaphos, diazinon, fenchlorphos and malathion) solubilised in aqueous soap (sodium dodecyl sulphate, cetyltrimethylammonium bromide and dodecyltrimethylammonium bromide) solutions have been examined by in vitro dipping experiments with flour beetles (Tribolium colzfisum Duval).Median lethal concentrations (C) ,and saturation solubilities (C,) of the pesticides increased with soap concentration whilst their ratios (C/C,) which could be regarded as approximating their thermodynamic activities, decreased. The soaps, alone, whilst being sorbed from solution by the insects, removed cuticular material as did some common solvents (ether, hexane, methyl alcohol and methyl cyanide) but the effects were not sufficient to explain the changes in biological activity of the solubilised pesticides.Changes in surface tension and viscosity of the solutions and the amounts of liquid retained by the beetles after dipping were small and did not appear t o affect the toxicities of the pesticides tested. Biological response appeared t o be determined by the rate of uptake of pesticide by the insect cuticle and hence was affected by the concentration of the soap micelles.

Bush ticks (Haemaphysalis bispinosa Neumann) were used to compare the biological efficiency (toxicity) and amount of DDT deposited from suspensions of four formulations of DDT normally used for dipping cattle. The effects of aging and the addition of bentonite clay and of salt on the suspensions were measured over periods of 28 days: three formulations which were of the supercooled melt type changed in toxicity and deposit on aging whilst the fourth formulation-a dispersible powder-did not alter appreciably. It is suggested that the low concentration of soaps (i.e. surface active agents) in the latter made it relatively immune to aging and the effects of bentonite and salt. In the supercooled melt formulations, wherein the concentrations of soaps were very much greater, the changes are ascribed to DDT crystal growth and aggregation, which in turn are governed by the nature and amounts of soaps present. The formulation containing sulphated butyl oleate gave the greatest deposit, with a maximum occurring at about 14 days, whereas the toxicity increased progressively on aging. The two other supercooled melt formulations contained undisclosed mixtures of anionic and non-ionic soaps; although the deposits increased, no maximum occurred within the aging period. The amounts deposited were similar to but not as large as those from the first formulation. The addition of bentonite and salt lowered both deposits and toxicities. It is suggested that bentonite adsorbed the soaps, thereby retarding crystal growth, and also became adsorbed on the DDT surfaces, thereby stabilizing the suspensions. The concentration of added salt was such that it stabilized the suspensions and resulted in DDT crystal sizes giving lower deposits and hence lower toxicities. The effect was most marked with the formulation containing sulphated butyl oleate. It is suggested that the hydrophobic surface of the tick is important in the attachment of DDT particles, the amount retained on dipping depending largely upon crystal size and degree of aggregation. Correlation between the in vitro tests and field performances of the formulations is discussed.

The effects of various synthetic soaps on the biological efficiency of a DDT/clay/lignin* sulphonate suspension and of a colloidal DDT suspension have been examined by in vitro dipping experiments with flour beetles (Tribolium confusum Duval).Additions o f soap up to the c.m.c. (critical micellar concentration) decreased the toxicity in both types of suspension. This is attributed to adsorption of the soap on both DDT and cuticular surfaces, with consequent reduction in the probability of adhesion when DDT particles collide with the cuticular surface.When the soap concentration was increased beyond the c.m.c., the toxicity of the DDT/clay/ligninsulphonate suspension remained unchanged, whilst the toxicity of the colloidal DDT suspension increased. Crystal growth of DDT in the colloidal suspension was observed above the c.m.c. and is considered to be the cause of the increased toxicity. The soap micelles did not dissolve sufficient DDT to influence toxicity directly, but were responsible for crystal growth. With the DDT/clay/ligninsulphonate suspension, crystal growth of the DDT protected by clay and ligninsulphonate appeared t o be too slow t o produce similar effects.Toxicity and deposit appeared t o be closely related and were influenced by the duration and degree of agitation during dipping, indicating that deposition of DDT on the cuticle did not attain equilibrium. Liquid retention appeared to be a function of surface tension. Affinity between DDT and insect cuticle is ascribed t o the hydrophobic nature of both surfaces.