The effect of aging on the biological efficiency of some DDT cattle-dipping formulations.

Abstract

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.