Significance of absorption, oxidation, and binding to toxicity of four ecdysone agonists in multi-resistant cotton leafworm

Treatment of last-instar larvae of multi-resistant cotton leafworm Spodopte ra Littoralis with four dibenzoylhydrazines, methoxyfenozide (RH-2485), teb ufenozide (RH-5992), halofenozide (RH-0345), and RH-5849, resulted in prema ture molting leading to death. Methoxyfenozide was the most toxic followed by tebufenozide, halofenozide, and RH-5849. To explain differences in toxic ity, especially between multi-resistant and laboratory strains, absorption in the body tissues and oxidative metabolism were tested with C-14-labeled ecdysone agonist and a Lineweaver-Burk assay, respectively. Then to address different compound potencies in multi-resistant strains, the potency of th e four ecdysone agonists was measured based on their ability to mimic the n atural insect molting hormone, 20-hydroxyecdysone (20E) by inducing evagina tion in isolated imaginal wing discs. Using monoclonal antibody 9B9, the pr esence of ecdysteroid receptors in imaginal discs in vitro was confirmed. I n parallel, Scatchard plot analysis with whole imaginal wing discs cultured with different concentrations of H-3-Labeled ponasterone A indicated no si gnificant difference in affinity and in number of target sites for binding between multi-resistant and susceptible laboratory strains. The four compou nds tested caused the effect as agonists of 20E in vitro, and typically the order of their toxicities (LC(50)s) corresponded with that for evagination -induction with whole imaginal discs. (C) 2001 Wiley-Liss, Inc.