Selection For Indoxacarb Resistance In Spodoptera Litura And Investigation On The Resistance Mechanisms

The common cutworm, Spodoptera litura (Fabricius), was a worldwide distributed agricultural pest. Frequent application of chemical insecticides against Spodoptera litura had led to the development of pesticide resistance. Indoxacarb was a novel insecticide belonging to the oxadiazine class with unique action mechanism, excellent insecticidal activity and low toxicity to nontarget organisms, it was therefore an ideal candidate to substitute organophosphate and pyrethroid insecticides. In this paper, the toxicological characteristics of indoxacarb against Spodoptera litura had been studied, including sublethal effects, cross-resistance and resistance mechanisms.Toxicity of insecticides including indoxacarb against Spodoptera litura were tested at the laboratory using leaf dipping method, and the indoxacarb showed the highest insecticidal activity when compared with chlorfennapyr and fipronil, which were commonly used in the control of Spodoptera litura at present. Furthermore, indoxacarb showed lower insecticidal activity when exposed to Spodoptera litura by larvae dipping method, while similar toxicity against Spodoptera litura was found using leaf dipping, artificial diet incorporation and artificial diet dipping methods, suggesting that indoxacarb is more effective as a stomach poison than as a contact insecticide.Based on the bioassay results, 3rd instar larvae of Spodoptera litura were treated with indoxacarb at concentrations corresponding to LC15,LC10 and LC5 by artificial diet dipping methods. Lower pupation rate, eclosion rate and egg hatchability were observed when compared with control, and adult longevity and eggs laid per female decreased significantly when exposed to indoxacarb at LC15 concentrations. These results indicated that indoxacarb at sublethal concentrations had a negative effect on population development of Spodoptera litura. Furthermore, the activities of esterase ( EST) , glutathione S-transferase ( GST) and mixed function oxidase ( MFO) were compared between indoxacarb treated larvae and control, and no significant difference was observed, suggested that indoxacarb at sublethal concentrations had no obvious effect on activities of these three detoxification enzymes.After selection with indoxacarb 6 times during 10 generations, a resistant strain of S. litura was achieved with resistance ratio of 15.57 compared with unselected parent strain, suggesting that S. litura had high capability of developing resistance to indoxacarb, as was confirmed by resistance risk assessment. Developmental and reproductive disadvantages in resistant strain were observed when compared with susceptible strain, and the resistant strain exhibited a fitness value of 0.73 relative to the susceptible strain. Bioassay showed that the LC50 values of phoxim, betacypermethrin, fipronil were 1.53, 2.42 and 1.52 folds higher in resistant strain than that in susceptible stain, respectively, while the LC50 values of chlorfennapyr and methomyl were 0.78 and 0.96 folds lower in resistant strain than that in susceptible strain, suggesting that indoxacarb-resistant Spodoptera litura had little cross-resistance to these tested insecticides.The activities of detoxification enzymes were analyzed in both resistant and susceptible strains. The results showed that the activity of esterase increased by 2.27-fold compared with the susceptible strain, while the activities of mixed function oxidase and glutathione S-transferase had no significant difference. Furthermore, positive correlation was observed between esterase activity in 3rd larvae of every selected generation and resistance factor. These results suggested that the esterase played an important role in the resistance development of Spodoptera litura to indoxacarb.DNA fragments of 341 bp in length in the IIS5–IIS6 region of sodium channel para-orthologous gene in both resistant and susceptible strain were amplified with a pair of specific forward primer and reverse primer by using PCR method. The result showed that there was no sodium channel mutation in the resistant strain compared with the susceptible strain.The pest resistance was one of the most important problems in integrated pest management (IPM), and understanding the resistance mechanisms was the basis of insect resistance management (IRM). The present study showed that S. litura had high capability of developing resistance to indoxacarb, and esterase played an important role in the resistance development of Spodoptera litura to indoxacarb. These results will be helpful in establishing the scientific resistance management strategies, prolonging the lifetime of indoxacarb, and making full use of indoxacarb in sustainable pests management.