Ecdysone agonists - mechanism of action and application onSpodopteraspecies

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.