| The striped stem borer Chilo suppressalis Walker, a serious pest in the world, is in the course of diverging into rice population and water-oat population. However, it still needs further study to explore more differences between the two populations, which will contribute to a better understanding of the mechanism of host population formation and analysing its origin. C. suppressalis overwinters as a diapause larva in the field and diapause is an important stage in its life history. In this thesis, the biology and physiology of overwintering larvae in rice and water-oat fields were compared, the diapause induction and cold hardiness of rice and water-oat populations were also compared in controlled condition. The main results are as follows:1. Comparison of biology of overwintering larvae in rice and water-oat fieldsBody weight of overwintering larvae in the water-oat field (22.7-252.4mg/larva) was greater than that of larvae in the rice field (14.3-149.5mg/larva). The survival rate of overwintering larvae in the two fields was different. Parasitic wasps was one of the main factors contributed to overwintering mortality in the rice field, parasitism rates were14.25%-23.29%in2009-2010and21.15%in2010-2011in the rice field, however, the rate were0.90%-6.55%in2009-2010and2.28%in2010-2011in the water-oat field. Five species of parasitic wasps were identified, and Chelonus munakatae (30.00%-64.00%) was the dominant parasitic species. Trends in data suggested parasitism rate in the overwintering larvae with smaller body weight was higher. Among all the larvae that parasitized, percentages of larvae below38.5mg were87.27%,91.23%,93.33%in Jan, Feb, Mar in2010and93.50%in Dec,2010in the rice field, and ratios of larvae below70.5mg is100.00%,50.00%,86.67%in Jan, Feb, Mar in2010and90.00%in Dec,2010in the water-oat field. The weight of3-day-old male and female pupae, and the number of eggs carried by adult females from the larvae overwintering in the rice field were significantly lower than those from larvae overwintering in water-oat field. The average weight of male and female pupae was41.8mg and55.3mg, average numbers of impregnated eggs of adults were223.7in the rice field. However, the average weight of male and female pupae was65.5mg and98.5mg, average numbers of impregnated eggs of adults were341.8in water-oat field. After transferring to25℃and LD16:8h, average days required for pupation of overwintering larvae from the two fields were different, indicating the diapause intensity of larvae in the two fields were different. After transferring larvae to laboratory in Mar,2010, average days required for pupation were17.1days in water-oat field and22.7days in rice field, indicating larvae in the water-oat field terminated diapause earlier than larvae in the rice field. The supercooling point and freezing point of larvae collected from rice field were significantly lower than those of larvae from water-oat field. For example, the supercooling point was-10.0℃,-9.4℃,-7.6℃in larvae from rice field and-7.2℃,-6.9℃,-6.7℃in larvae from water-oat field in Jan, Feb, and Mar in2010. The body water content and glycerol content of larvae in two fields also differed.2. Comparison of diapause induction and cold hardiness of rice and water-oat populationsThe critical photoperiods of the rice population of C. suppressalis-fed on rice and water-oat were both about12h30min in25℃, indicating the rice population can adapt to water-oat, and the physiological character of photoperiodic response can be expressed. The diapause rate was100%of water-oat population of C. suppressalis fed on rice under the condition of25℃and LD15:9h, suggesting that water-oat population had little adaptability to rice and strong specialization ability to water-oat. Regardless of the host that C. suppressalis fed on, diapause ratios of water-oat population were above50%, always higher than rice population. The water-oat population had no significant critical photoperiod, however, the critical photoperiod of rice population is12h30min, indicating the two populations have been diverged in photoperiodic response. Weight of the same instar larvae of rice and water-oat populations fed on the two hosts didn’t differ much. For instance, the weight of6th instar larvae of rice population and water-oat population fed on rice were44.2mg and44.3mg, respectively. Weight of the same instar larvae of the same population fed on water-oat was always higher than that fed on rice. Results of GLM indicated that host and instar were the determinant factor influencing larval body weight whereas population did not play an important role. The supercooling point, body water content and glycerol content of diapause larvae of rice and water-oat population fed on the two hosts differed. Results of GLM indicated that host can impact the supercooling point of the two populations whereas population is the main determinant of cold hardiness (supercooling point, body water content and glycerol content).In conclusion, the biology and physiology of overwintering larvae from different host plant fields differed significantly, and population differentiation of C. suppressalis was the main determinant of photoperiodic response of diapause induction and cold hardiness. In addition, host plant was the main factor determing the larval body weight. These results clarified the differences of overwintering biology and physiology of the host populations of C. suppressalis, providing more evidence for population differentiation, which will contribute to a further understanding of the mechanism of host population formation and analyzing the origin. |
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