Methoxyfenozide (RH-2485) was found to be 7.5-fold less toxic in terms of LD50 values against last-instar larvae of a greenhouse-selected strain of the beet armyworm Spodoptera exigua (Hübner) that was collected in July 2001 in an experimental greenhouse for resistance at Murcia in southern Spain, in comparison with a laboratory susceptible strain. To date, the compound is the newest member of this new group of moulting hormone accelerating IGRs to reach the marketplace against Lepidoptera. To understand this different potency in the greenhouse-selected S exigua, oxidative metabolism and acetylcholinesterase activities were measured in last-instar larvae and adults. In addition, we determined, by the use of 14C-labelled methoxyfenozide, the pattern of absorption in body tissues and excretion via faeces in last-instar larvae of the greenhouse-selected strain and compared the results with those from the laboratory susceptible strain. It was striking that the rate of excretion was about twice as high in the greenhouse-selected strain, resulting in a more rapid clearance of insecticide amounts from the insect body. Data are discussed in relation to mechanisms of lower toxicities for this new group of IGRs.
Treatment of last-instar larvae of multi-resistant cotton leafworm Spodoptera littoralis with four dibenzoylhydrazines, methoxyfenozide (RH-2485), tebufenozide (RH-5992), halofenozide (RH-0345), and RH-5849, resulted in premature molting 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 oxidative metabolism were tested with 14C-labeled ecdysone agonist and a Lineweaver-Burk assay, respectively. Then to address different compound potencies in multi-resistant strains, the potency 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 presence of ecdysteroid receptors in imaginal discs in vitro was confirmed. In parallel, Scatchard plot analysis with whole imaginal wing discs cultured with different concentrations of 3H-labeled 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 (LC50s) corresponded with that for evagination-induction with whole imaginal discs.
Laboratory assays using artificial diet demonstrated that tebufenozide (RH‐5992) and the new structural analogue, RH‐2485 (proposed common name methoxyfenozide), possess strong ecdysone‐like activity against last‐instar larvae of the beet armyworm, Spodoptera exigua, and the cotton leafworm, Spodoptera littoralis, leading to precocious lethal moulting. LC50 values showed that the activity of RH‐2485 (0.38 mg AI litre−1) was about twice that of tebufenozide (0.60 mg AI litre−1) in S. exigua, whereas in S. littoralis respective LC50 values were 1.15 mg AI litre−1 and 9.51 mg AI litre−1. The retention‐fate curves of 14C‐radiolabelled ecdysone agonist could not explain the differential toxicity values between species and compounds. Ingestion of the oxidase inhibitor piperonyl butoxide (PB) synergized the toxicity of the ecdysone agonist, indicating the importance of oxidative detoxifcation in Spodoptera larvae, and may raise the possible use of PB as synergist for this group of insecticides, or for monitoring resistance due to increased oxidation. © 1999 Society of Chemical Industry
Ring-substituted dibenzoylhydrazines are well known as nonsteroidal ecdysone agonists that cause precocious larval molt leading to death. Among them, tebufenozide (RH-5992) is used in practice as insecticide to control lepidopteran pests selectively. Recently, a new dibenzoylhydrazine, methoxyfenozide (RH-2485), with a higher activity for Lepidoptera, has been discovered. To obtain insight into the molecular mechanisms involved in the insecticidal selectivity of the dibenzoylhydrazine family, we measured the in vivo toxicity of these dibenzoylhydrazines against larval stages of the beet armyworm, Spodoptera exigua, and their action on in vitro cultured imaginal discs. We found that both nonsteroidal ecdysone agonists induced premature and lethal molting, and caused the same effect as 20-hydroxyecdysone in vitro. Furthermore, we measured the larvicidal activity against S. exigua of a series of dibenzoylhydrazines, in which ring-substituents were varied in a range of halogen, lower alkyl, OCH 3 , SCH 3 , Ph, CN, NO 2 , and CF 3 . The substituent effects on the larvicidal activity against S. exigua were very similar to that for another lepidopteran insect species Chilo suppressalis, suggesting that the molecular mechanism of action of these compounds is similar within both lepidopteran species. Arch. Insect Biochem. Abbreviations used: EcR = ecdysone receptor; EC 50 = evagination of wing discs; IC 50 = hormone binding activity to wing discs; LD 50 = larvicidal toxicity; QSAR = quantitative structure-activity relationship; PonA = ponasteroneA; RH-5992 = tebufenozide; RH-2485 = methoxyfenozide; 20E = 20-hydroxyecdysone.
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