Studies On The Resistance Mechanisms Of Spodoptera Exigua (Hübner) To Spinosad

The beet armyworm (Spodoptera exigua Hubner) is a polyphagous pest that damages numerous kinds of cultivated crops. The continuous chemical control on beet armyworm had led to its resistance to many kinds of insecticides. Spinosad is a novel insecticide and showed good activities against beet armyworm. However, it has been observed that some beet armyworm populations became resistant to spinosad. The physiological and molecular mechanisms of beet armyworm resistances to spinosad were studied in this paper. Main results of this study were showed in the follows.With leap-dipping method the resistant strain of beet armyworm to spinosad was selected. After five generations of selection, the level of spinosad-resistance of the beet armyworm population increased 345.4 times compared with the susceptible strain, which indicated that the resistance of beet armyworm to spinosad developed rapidly. There was no cross resistance between spinosad and fenvalerate, phoxim, methomyl, abamectin, and cyfluthrin in the beet armyworm population. Piperonyl butoxide, triphenyl phosphite, S,S,S-tributyl- phosphorothioate and diethylmaleate in vivo synergized the toxicity of spinosad by 0.7-, 0.5-, 1.0- and 0.6-fold in the susceptible strain and 9.8-, 1.5-, 2.6- and 1.5-fold in the resistant strain, respectively. The activities in vitro of microsomal-O-demethylase and glutathione S-transferase in the resistant strain were 5.2- and 1.0-fold of the susceptible strain, respectively. The results implied that microsomal-O-demethylase is involved in the resistance to spinosad in the beet armyworm.With a pair of degenerate primers designed according to the conservative amino acid sequences of CYP4 genes in insects, 2 new cDNA sequences encoding cytochrome P450 were amplified from the fifth instar larvae of the resistant strain of the beet armyworm by RT - PCR. The result of sequence analysis showed that these 2 new cDNAs belonged to the CYP4S and CYP4G subfamilies and they were named with Secyp4s8-like and Secyp4g31-like gene, respectively. The length of Secyp4s8-like gene fragment was 441 bp and the length of its putative amino acid sequence was 146 residues. The length of Secyp4g31 -like gene fragment was 456 bp and the length of its putative amino acid sequence was 151 residues.The expression levels of mRNA of these 2 cytochrome P450 genes in different strains of beetarmyworm were studied by real-time quantitative PCR. With new primers for real-time PCR, the copy numbers of these 2 genes in different strains of beet armyworm were detected by absolutely quantitative method. The results showed that the expressing level of Secyp4s8-like and Secyp4g31-like gene were 1.74E+04 and 3.70E+02 copies/|ig total RNA in resistant strain, respectively, and 1.45E+03 and 2.69E+02 copies/ug total RNA in susceptible strain, respectively. Accordingly, the over-expressed times of Secyp4s8-like and Secyp4g31-like gene in resistant strain were 12.0 and 1.38 compared with susceptible strain, respectively, while it implied that the over-expression of Secyp4s8-like gene in beet armyworm could be involved in resistance to spinosad.