Formamidine pesticides: octopamine-like actions in a firefly.

The formamidine pesticide chlordimeform and its N-demethylated metabolites cause the light organ of the firefly Photinus pyralis L. to glow brightly. Monodemethyl chlordimeform is active at doses as low as 5 nanograms per insect when applied topically. This action is postsynaptic and probably involves membrane-bound receptors since cyproheptadine blocks the glows induced by both monodemethyl chlordimeform and octopamine, the putative neurotransmitter in the light organ. The pesticidal and pestistatic properties of the formamidines may result from actions on octopaminergic systems.