| The rice stem borer(RSB),Chilo suppressalis,one of the main pests of rice, overwinters as a diapaused mature larva.Cold hardiness of different populations of the over-wintering RSB larvae were compared and mechanisms underlying the changes in cold hardiness in different populations were investigated in this thesis.The major results are as follows.In age structure,the RSB larvae over-wintered mostly as sixth instars,comprising more than 50%.Mortality of the different geographical populations of overwintering RSB larvae changed differently with the season.Mortality of the Fuzhou population decreased all the way in the whole winter,while that of the Beijing population increased from 18.4%in the early winter to 22.3%in the deep winter and then decreased to about 11%in the early spring.There were three factors responsible for the mortality of the over-wintering RSB larvae,i.e.,pathogenic fungus,pathogenic bacteria,and parasitic wasp.In different geographical populations,factors accounting for mortality were different:in Fuzhou population,it was principally parasitic wasp;while in Beijing population,they were parasitic wasp and pathogens equally.The activities of stress-tolerant enzymes(CAT,POD,SOD,LDH and ATP) were analyzed for diapaused and diapause-terminated Chilo suppressalis(Walker) larvae from different geographic populations.The results showed that the activities of the five enzymes were not significantly different among diapaused larvae from different geographic populations,but increased in diapause-terminated larvae with the decrease of latitude of the geographic populations.The enzyme activities of Changsha population were obviously higher than those of Harbin population.The activities of CAT,POD and SOD were higher in diapaused larvae than in diapause-terminated larvae,while the activities of ATP and LDH were the other way round.Tests of the activities of the five enzymes in mature larvae,prepupae and pupae showed that CAT,POD and SOD were less active in mature larvae than in pupae,while ATP and LDH were more active in mature larvae than in pupae.From October to February,the supercooling points(SCPs) of Beijing larvae were -8.01℃,-8.65℃,-10.59℃,-8.72℃and -6.13℃,respectively,and the freezing points, -3.68℃,-4.27℃,-5.98℃,-4.69℃and -2.91℃,respectively.The SCPs of Fuzhou larvae were-7.12℃,-10.02℃,-11.794℃,-9.21℃and -6.98℃,respectively,and the freezing points -3.45℃,-4.31℃,-5.93℃,-3.66℃and -2.21℃,respectively.Changes in SCPs and freezing points correlated with variations of environment temperature,but SCPs fluctuated more greatly than the freezing points.Body water content of Beijing populations decreased from 68.73%in October to the lowest 61.53%in December and then increased to 65.68%in February.Fuzhou population showed the same trend of change in larval water content.These results suggest that the larvae reduce body water content during over-wintering and the changes in body water content is closely correlated to SCPs.The variations of glycogen and fat content from October to February were different from those of trehalose and glycerol.Glycogen and fat content increased with the decrease in temperature from October to November,but decreased with the further decrease in temperature in December,which reveals a process from accumulation to metabolism.Tehalose and glycerol contents in hemolymph increased from the early to the deep over-wintering stage.Cold acclimation had a significant effect on the SCP,freezing point,body water content,larval fat and glycogen contents,and glycerol and trehalose contents in the larvae's hemolymph.After cold acclimation,body water content decreased,contents of glycerol,trehalose,glycogen and fat all increased,and SCP and freezing point decreased.The above results show that there are significant differences in suercooling capacities between different populations of over-wintering RSB larvae,the differences are closely related to the variations in body water content and contents of the small molecular carbonhydrate compounds and fat.
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