Significance of Absorption, Oxidation, and Binding to Toxicity of Four Ecdysone Agonists in Multi-Resistant Cotton Leafworm

摘要

Treatment of last-instar larvae of multi-resistant cotton leafworm Spodoptera littoralis with four dibenzoylhydrazines, methoxyfenozide (RH-2485), tebufenozide (RH-5992), halo- fenozide (RH-0345), and RH-5849, resulted in premature molt- ing leading to death. Methoxyfenozide was the most toxic followed by tebufenozide, halofenozide, and RH-5849. To explain differences in toxicity, especially between multi-resistant and laboratory strains, absorption in the body tissues and oxida- tive metabolism were tested with 14C-Iabeled ecdysone agonist and a Lineweaver-Burk assay, respectively. Then to address dif- ferent compound potencies in multi-resistant strains, the po- tency of the four ecdysone agonists was measured based on their ability to mimic the natural insect molting hormone, 20- hydroxyecdysone (20E) by inducing evagination in isolated imaginal wing discs. Using monoclonal antibody 9B9, the pres- ence of ecdysteroid receptors in imaginal discs in vitro was con- firmed. In parallel, Scatchard plot analysis with whole imaginal wing discs cultured with different concentrations of 3H-Iabeled ponasterone A indicated no significant difference in affinity and in number of target sites for binding between multi-resistant and susceptible laboratory strains. The four compounds tested caused the effect as agonists of 20E in vitro, and typically the order of their toxicities (LC5os) corresponded with that for evagi- nation-induction with whole imaginal discs. Arch. Insect Biochem.