The insecticidal potency of γ-BHC and the chlorinated cyclodiene compounds and the significance of resistance to them

Abstract

The insecticidal potencies of γ-BHC and a range of chlorinated cyclodiene compounds have been measured for normal and resistant strains of both Musca domestica L. and Lucilia cuprina (Wied.). Apart from minor discrepancies, the relative potencies presented similar pictures in the two species, and the resistance spectra were also similar.Moderate to high insecticidal potency is shown by a variety of compounds with a chlorinated bicyclo-heptene nucleus in the molecule, including Diels-Alder condensation products and chlorinated camphenes. Various additions and sub-stituents on the nucleus affect the potency quantitatively but not qualitatively. In contrast, γ-BHC is almost unique among poly-halogenated cyclohexanes.When resistance develops to any of the chlorinated cyclodiene compounds, it invariably extends to all others and to γ-BHC and chlorinated adamantane. The resistance level, in most cases, is proportional to the original potency of the compound; this is apparently because resistance is usually complete, so that the more toxic compounds have more to lose. There are, however, a few puzzling exceptions.The consistency of the cross-resistance spectrum to all members of the group implies a common defence mechanism. This suggests a common mode of toxic action, resistance being due to insensitivity at the site. A common mode of action for the cyclodiene series is understandable; the anomalies are γ-BHC and chlorinated adamantane. Their conformity with the cyclodiene insecticides was investigated by the following observations with normal insects: (a) signs of poisoning; (b) relative potency to different species; (c) tests with mixtures. The general conclusions were not unfavourable to the hypothesis of a common mode of action, though secondary features of the intoxication process somewhat complicated the results.