Studies On The Differences Of Tolerance Between Chilo Supperssalis Walker And Sesamia Inferens Walker To Bt Rice

The striped stem borer, Chilo supperssalis Walker(SSB) and the pink stem borer, Sesamia inferens Walker (PSB) are the important rice pests widely distributed in all rice growing regions in China. In recent years, SSB and PSB populations have increased gradually, because of changes in rice cultivation, use of new hybrid varieties and global climate and the occurrence of pest resistance to chemical insecticides. The development of transgenic rice lines expressing Bacillus thuringiensis (Bt) insecticidal protein provide the new strategy for the control of SSB and PSB. However, the ecological safety is the main factor for transgenic insect-resistant rice not being approved to be commercial application. Generally speaking, protein types of Bt insecticidal crystals are different, so are their insecticides spectrums. The Cry protein has showed high virulent effect on SSB, but low toxicity on PSB with similar niche to SSB. A preliminary field test also showed that the insecticidal Cry protein expressed in transgenic rice has effective control on SSB, but its efficacy on SSB decresed, which might change the component proportion of major pests in transgenic rice field. Therefore, researches on artifical rearing technique of PSB, the effect of Bt rice on the survival and nutritional utilization of SSB and PSB, baseline susceptibility of SSB and PSB to different Bt toxins were conducted. The main results were as follows:1. An artificial diet for PSB was formulated with soybean powder, corn powder, wheat germ powder and fresh water bamboo as major components. Rearing technique for mass and successive production of PSB was developed using fresh water bamboo (natural food source) for young larvae (neonates-2rd instar) and the artificial diet for older larvae (3rd instar and beyond). The performance of PSB reared on this fresh water bamboo/artificial diet regimen was evaluated over 3 consecutive generations by comparison with individuals fed on fresh water bamboo only for the whole generation. The fitness traits of larval durations, pupal weight, larval survival rate, pupation rate, eclosion rate and number of eggs laid per female were similar between these two diet regimens. Additionally, the rearing technique with fresh water bamboo/artificial diet combination could lower the cost by saving considerable amount of labor and time and significantly reduce the rate of insecticidal pathogen contaminations. The formulated diet and rearing technique were suitable for the mass and successive production of PSB.2. Effects of two transgenic Bt rice lines, TT51 and Cry2A, and their untransformed parental cultivar, Minghui63, at four different growing stages on the survival and nutritional utilization of SSB and PSB were conducted. The results indicated that both transgenic rice lines showed higher resistance to SSB than that to PSB at four different stages. The corrected mortalities of SSB feeding on both transgenic lines for 2 day and 4 day are significantly higher than that of PSB at different stages, respectively. Indices, Corrected relative growth rate, corrected relative consumption rate and corrected approximate digestibility, for larval consumption and digestibility indicated that both transgenic lines showed significant inhibition on the larval growth and development of SSB and PSB. CRGRs for SSB feeding on TT51 and Cry2Aa at different stages were lower than or similar to that for PSB on both lines, While CRCRs and CADs for SSB were higher than or similar to that for PSB.3. Baseline susceptibilities of SSB and PSB to different Bt toxins were established. The results of bioassay showed that the LC50 values of PSB to Bt toxins were very higher than that of SSB to Bt toxins. The ratio values of PSB and SSB LC50 values were 20.21 (CrylAc),39.13 (Cry1Ab),3.2 (Cry1Ah),1.85 (Cry1Ca) and 1.57 (Cry2Aa), which indicated that PSB showed higher tolerant to Bt toxin compared with SSB. Geographic variability in susceptibility of field-collected PSB to CrylAc and CrylAb was also studied using a diet incorporation method. Colonies of PSB were established from 6 different geographic areas of China. Among the different populations, the LC50 of CrylAc ranged from 138.09 mg (A.I.)/L to 1521.76 mg (A.I.)/L, and the LC50 of CrylAb ranged from 17.25 mg (A.I.)/L to 173.08 mg (A.I.)/L. The intra-specific variations in Cry1Ac and CrylAb susceptibility were 11- and 10-fold, respectively. Within a given population, the susceptibility to CrylAc and CrylAb were positively correlated and CrylAb was more susceptive to PSB than CrylAc. The results provided much needed groundwork for establishing susceptibility baseline and monitoring Bt tolerance of PSB.