Silicon-mediated Resistance Of Rice To Chilo Suppressalis (Walker) And Bionomics Of Overwintering Larvae

The rice stem borer (RSB) Chilo suppressalis Walker (Lepidoptera: Pyralidae), a worldwide insect pest of rice, is distributed in all the rice planting areas in China. To understand host and environmental adaptability of RSB larvae and the underlying mechanisms, the present thesis investigated: (1) silicon-mediated rice resistance to RSB larvae, in terms of boring behavior, survival and growth development; (2) bionomics of RSB larvae overwintering in paddy-rice and water-oat fields; and (3) influence of soil moisture on cold-hardiness of overwintering RSB larvae. The results of the current study may shed light on reasonable cultivation measures for RSB regulation.(1) Silicon-mediated rice resistance to RSB larvae. Two rice varieties, Yanfeng 47 and Shanyou 63, which are intermediate resistant and high susceptible to RSB larvae, were used in the experiments with high silicon (HS, 600 kg/hm2), low silicon (LS, 150 kg/hm2) and no silicon (NS, control) treatments. RSB larvae were exposed to these treatments and boring behavior, survival and development of RSB larvae were compared.Boring behavior was investigated against 1st and 3rd instars. For the 1st instars, compared to NS rice, HS rice reduced number of RSB larvae bored and penetrated in both varieties, and delayed time of stalk penetration in variety Shanyou 63. For the 3rd instars, significant reduction in number of RSB larvae penetrated occurred for both varieties in HS rice relative to NS rice; and penetration duration in variety Shanyou 63 was prolonged for HS rice relative to NS rice.Stalk length bored was recorded at 48 h and 96 h after exposure. For variety Shanyou 63, stalk length bored by 3rd instars were significantly reduced by 22.71% to 62.17% after 48h and 33.89% to 43.70% after 96h in HS rice relative to NS rice. For variety Yanfeng 47, difference in stalk length bored between HS and NS rice (reduction of 42%) was significant only at 96 h.Development of RSB larvae was assessed at 35 d and 45 d after exposing 1st larvae to different treatments. At 35 d, weight of RSB larvae was significantly reduced by 47.6% to 57.7% for variety Shanyou 63 and by 50.1% to 58.2% for variety Yanfeng 47 in HS rice relative to NS rice. At 45 d, Weight of RSB larvae were significant different between all treatments for Shanyou 63. Survival rate of RSB larvae at 45 d decreased significantly in silicon-added treatment than in the control for variety Shanyou63.(2) Bionomics of Chilo suppressalis larvae overwintering in paddy-rice and water-oat fields were investigated in Beijing from early-winter (middle November) to after-winter (early March). The results show that overwintering larvae are 4th instar or older, and the 6th instar ratio in water-oat fields is 17.5% higher than those in paddy-rice fields in early winter. Old larvae account for a higher percent in after-winter period than in early-winter. Female to male larvae ratios are respectively 1:2.0 and 1:1.7 in early-winter and after-winter periods in paddy-rice fields, and 1:1.4 and 1:1.2 in water-oat fields. Larvae duration is longer in early than after-winter, and not significantly different between in paddy-rice and water-oat fields in one period. However, larvae from water-oat fields take 14.0% more time before pupation than those from paddy-rice fields in early-winter, while larvae and pupa from water-oat fields are significantly heavier than those from paddy-rice fields. Mortality rate of larvae collected in early-winter is greater than 65%. Fungi infection is the primary mortality factor in water-oat fields (i.e. over 90% total mortality), while parasitic wasp and bacteria infection constitute the primary mortality factors in paddy-rice fields (i.e. respectively over 50% and 25% total mortality). Apanteles chilonis and Eriborus terebrans are the two dominant parasitic wasps.(3) Influence of soil moisture on the supercooling capacity and associated physiological parameters of overwintering larvae of the rice stem borer Chilo suppressalis was examined by exposing larvae to soil moistures of 25, 50, 75 and 100% of saturated soil water content (SSWC) at ambient temperature for 30 d from December 2007 to January 2008 in Beijing, China. At the end of the exposure, supercooling points (SCPs) varied significantly among the treatments, the lowest being in the larvae exposed to soil moisture of 25% SSWC. Fresh weight was significantly higher in the larvae exposed to soil moisture of 100% SSWC than in those kept at 25 and 50% SSWC. Dry weight and body water content (% fresh weight) were not different among the treatments. Glucose and trehalose contents were markedly lower, and glycerol content was significantly higher in the larvae confined to soil moisture of 25% SSWC than in those exposed to the other soil moisture treatments. It is suggested that variation in body water content (% fresh weight) contributes to the differences in SCPs of the overwintering C. suppressalis larvae in all treatments, but the influence of soil moisture treatments on supercooling capacity are caused through changes in glycerol content.