Studies in selective toxicity. III. —Benzenesulphonanilides as DDT‐synergists

Benzenesulphonanilides with halogen or methyl substituents in the phenyl radicals, with or without N ‐alkyl substituents, were assayed as DDT‐synergists against DDT‐resistant houseflies. Contrasting test methods provided a guide to the intrinsic biological activity of the synergists, and to their ‘availability’ for insects by pick‐up from residual deposits and by absorption through the cuticle. The homologous series of N ‐ n ‐alkyl‐4‐chloro‐ and ‐4‐bromo‐benzenesulphon‐4′‐chloro‐anilides were studied. In both series the biological response to topical application of dilute toxicant solutions was highest for the N ‐methyl compounds. The ‘availability’ from deposits appeared highest in the intermediate range C 4 to C 8 . In this range the ‘availability’ appeared to oscillate between homologues having an odd number and those having an even number of carbon atoms. The homologues in the medium range showed a high solubility in kerosene. Homologues above C 9 showed no biological activity. The possible modes of action and penetration of the benzenesulphonanilide synergists are discussed.