Resistance Selection Of Spodoptera Exigua (H(?)bner) To Tebufenozide And Preliminary Study On The Resistant Mechanisms

The beet armyworm, Spodoptera exigua (Hubner), is a polyphagous noctuid of worldwide importance that feeds on various agricultural crops, including vegetable, cotton and ornamental. The heavy dependence on insecticides for control of this pest has resulted in the emergence of resistance to a variety of insecticides. Tebufenozide is a novel synthetic non-steroidal ecdysteroid agonist, which manifests its toxic effects via interaction with ecdysteroid receptors. Insect larvae treated with tebufenozide undergo premature lethal moulting. It has selective toxicity towards lepidopteran pests, and has no adverse effects on mammals, birds, fishes and other vertebrates, as well as various beneficial insects. The objectives of this research were to determine the toxicity of tebufenozide against Spodoptera exigua larvae, to evaluate the resistance development of Spodoptera exigua to tebufenozide after a continuous treatment with sublethal doses of tebufenozide for over 12 generations. Physiological and biochemical differences between the relative susceptible population (SS) and the tebufenozide- resistant population (RR) were also examined.The bioactivity of tebufenozide and its effects on the relative susceptible population were investigated. The results showed that tebufenozide displayed high potential of ovicidal activity on 1-day eggs of S. exigua at concentration of 200mg/L; Tebufenozide can disrupted the growth and development of S. exigua. After treating with tebufenozide in diets, the larva period was elongated and the fecundity of S. exigua was decreased, and the ratio of surviving, pupating, emerging and hatching of S. exigua were also decreased.After selection for 12 generations in the laboratory, S. exigua showed 5.468-fold resistance to tebufenozide. The resistance strain showed no cross-resistance to methoxyfenozide, emamectin, fipronil, chlorfiuazuron, cyhalothrin and chlorpyrifos, the resistance ratios were only 0.829~2.190-fold.Biochemical study showed that there was no difference in acetylcholinesterase activity of thesusceptible and resistant strains, but there were significant differences in the activities of phenoloxidase (PO) and glutathione-S-transferase (GST) between the two strains. The enzyme activities of resistantstrain were 1.31 and 1.44 times higher than that of in susceptible strain. The results suggested that theenhanced activities of PO and GST were an important resistance mechanism in Spodoptera exigua totebufenozide.Last-instar larvae of Spodoptera exigua, when fed on diet containing tebufenozide, exhibited a premature induction of chitin synthesis in the cuticle within 24h, but the amounts of chitin recover to normal level after 48h. Moreover larvae treated with high concentration (10, 20 mg/Kg) tebufenozide exhibited a inhibition of chitin synthesis. The synthesis of at least three polypeptides was induced at 24h and 48h after treatment with 20 mg/Kg tebufenozide. Treatment with 20 mg/Kg tebufenozide also induced the production of three bands of esterase within 48h.