Activity and fate of chlorfluazuron and diflubenzuron in the larvae of Spodoptera littoralis and Heliothis virescens

Abstract

Larvicidal activity, inhibition of chitin synthesis, and metabolic fate of chlorfluazuron and diflubenzuron were investigated in the larvae of Heliothis virescens and two strains of Spodoptera littoralis (normal senstive (sens.) and organo-phosphate resistant (OP-R)). When injected, chlorfluazuron was more toxic than diflubenzuron by two orders of magnitude. No cross-resistance was apparent in the S. littoralis, OP-R strain. Both compounds were similarly potent inhibitors of chitin synthesis in vivo 2 hr after injection but this inhibition persisted with chlorfluazuron only. The in vivo half-lives of chlorfluazuron and diflubenzuron were estimated to be about 50 and 5 h, respectively, indicating that different rates of detoxification could account for the different persistence of chitin synthesis inhibition and in turn also for the toxicological differences in insects. Detoxification of the two compounds occurred primarily by metabolic degradation. As a result of its fast metabolism, the larvicidal activity of diflubenzuron was highly dependent on the mode of application. After continuous exposure, the LD 50s were found to be low compared to the effect of an application early in the instar and during a short period of time only. A failure to ecdyse was observed with high doses, while at low doses moulting was apparently normal, but the larvae ceased their development after ecdysis and starved to death. Differential susceptibility of the chitinous organs might be the reason for this effect.