Effects of organosilicon pyrethroid-like insecticides on nerve preparations of American cockroaches and crayfish.

Quaternary organosilicon pyrethroid-like ethers (five compounds) and alkanes (three compounds) were used for neurophysiological tests. Their activities in inducing repetitive firing in the central nervous cord of the American cockroach (Periplaneta americana) were evaluated by an extracellular recording method. The ethers were more active than the corresponding alkanes. The ability of the compounds to cause conduction blockage was also measured using the same nerve preparations, but the effects were too weak to allow definitive activity values to be determined. The compounds prolonged the sodium tail-current in the crayfish giant axon under voltage clamp conditions. The rate of decay of the tail-current changed in parallel with the reported insecticidal activity against P americana. These findings indicated that tail-current activity was the most useful nerve parameter in predicting insecticidal activity. Regression analysis of the numerical data together with those reported for other alkanes revealed that the higher the tail-current activity, the higher the insecticidal activity when a structural feature and the hydrophobicity were considered separately. The insecticidal activity of the ethers was about seven-fold higher than that of the alkanes with equivalent tail-current activity and hydrophobicity. Variations in insecticidal activity were parabolically correlated with the hydrophobicity [(log P)opt = 9.1] when other factors were similar.