Physico‐chemical studies of pesticides. III.—effect of soaps on the bioloǵical activity of DDT suspensions

Abstract

AbstractThe 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 of 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 to be too slow to produce similar effects.Toxicity and deposit appeared to 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 to the hydrophobic nature of both surfaces.