Resistance Monitoring And Resistance Risk Assessment Of Chilo Supprressalis To Bt Toxions

The striped stem borer, Chilo supperssalis Walker (SSB) is an important rice pest in the main rice-growing areas of China. In recent years, population of SSB has increased rapidly, which causes serious damages to rice production in China. Recently, chemical control is still the main strategy to prevent the damages of SSB. However, efficiency of chemical control has declined greatly due to resistance evolution of insects to most chemical pesticides. Presently, an alternatively attractive strategy to replace the traditional control methods has been developed by introducing the insecticidal proteins-coding genes into rice. However, the insect-resistant transgenic rice may have some potential risk to the ecosystem. In this research, baseline susceptibility of SSB in different geographic areas of China was conducted; Fitness of susceptible and Cry1Ac-resistant strains of SSB were compared; Cross resistance of Cry1Ac-resistant strains and control efficacy of two toxins mixture were investigated. The research results can forecast insect resistant risk and provide the scientific basis for insect resistance management. The main results are as follows:1. Geographic variabilities in susceptibility of field-collected SSB to CrylAb and CrylCa were studied using a diet incorporation method. Colonies of SSB were established from10different geographic areas of China. Among the different populations, the LC50of Cry1Ab ranged from1.95mg/L (Jiangxi. Nancang) to6.52mg/L (Fujian. Fuzhou), and the LC50of Cry1Ca ranged from1.37mg/L (Shanghai) to3.91mg/L (Fujian. Fuzhou). The intra-specific variations in Cry1Ab and Cry1Ca susceptibility were3.3-and2.8-fold, respectively. Within a given population, the susceptibility to Cry1Ab and Cry1Ca were positively correlated.2. Larvel weight and survival, pupal weight and adult fecundity of SSB resistant strains were significantly higher than that of the susceptible stain, when they fed on Cry1Ac diet.’The larval development of the resistant strain was also faster than that of the susceptible strain,which showed that Cry1Ac resistant strain presented significantly higher fitness on Bt diet than that of susceptible strain. However, larval survival rate of Cry1Ac resistant strain was significantly lower than that of susceptible strain, when both strains fed on non-Cry1Ac diet. A certain fitness cost was exhibited in Cry1Ac resistant strain of SSB.3. The cross-resistance analysis showed that SSB strains with7.77-fold resistance to Cry1Ac demonstrated2.76-fold resistance to CrylAb. In contrast, the susceptibility to Cry2Aa and CrylCa was not obviously increased in CrylAc resistance strains. Susceptibilities to toxin mixtures (Cry1Ac/Cry1Ab, Cry1Ac/Cry2Aa, Cry1Ab/Cry1Ca, Cry1Ab/Cry2Aa, Cry1Ca/Cry2Aa) were determined for resistant and susceptible strains of SSB. All toxin mixtures demonstrated significantly synergistic toxicity against SSB. All mixtures except for Cry1Ac/Cry1Ab were more effective to Cry1Ac resistant strain than that to susceptible strain of SSB. This shows that pyramid of Bt toxins with different mechanisms will contribute to the delay of insect resistance evolution.