Studies On The Cold Hardiness Of Mythimna Separata(Walker)

The armyworm(Mythimna separata),which belongs to Noctuidae,Lepidoptera,is a kind of overeating,seasonal long-distance migrant pest.Armyworm widely distributed in several countries such as Australia and Asia etc,which covered the tropical,subtropical and temperate regions.The disaster caused by armyworm outbreak frequently,which resulted in serious economic loss.The armyworm has no character of diapause,that means,they usually have migrant round trips to avoid the low temperature,however,only did the pupa and mature larva survive the winters in partial regions.The variation of cold hardiness has determined their reproductions,diffusion,distributions and the dynamic occurrence in the next season,which is an important prerequisite of the armyworms in terms of existence and development of the population,The major factors that affect the cold hardiness are seasonal variation,geographical location,humidity,cold acclimation,and individual development,etc.The study on cold hardiness not only makes us clear their overwinter conditions but also provides us a scientific basis for analyzing the dynamic occurrence and the geographical distribution of the armyworm in the following year.So the study on cold hardiness of armyworm plays an important role in the prediction,prevention and the division of the overwinter regions.In order to provide basic data and the forecast of armyworm,this thesis do some research of cold hardiness on armyworm.The main results were as follows:1.The cold hardiness of armyworm in different development stagesThe armyworm has four states:egg,larva,pupa and adult.Different development stages always have differences in morphological and physiological characteristics,so the cold hardiness of the armyworm is varying.The survey in the field of overwintering indicates that the pupa and mature larva are overwintering stages.Measuring the SCP and FP of pupa and mature larva,the results show that the SCP and FP of pupa are minimum,the SCP had extremely significant difference in different development stages,the SCP and FP of pupa have extremely significant difference,and the water contents of armyworm have significant differences in the different stages.So the distinctions of cold hardiness in development stages of armyworms are very obvious,the pupa is the strongest,which is in accord with the field survey.2.The cold hardiness of armyworm in different photoperiodsThe diapause is a strategy of insects to resist the harmful environment,which is usually induced by the photoperiod.Although the armyworm has no character of diapause,photoperiod as signal of seasonal variation,always along with the coming of the low or high temperature.In order to seek for whether the photoperiod has influence on cold hardiness of armyworm or not,I make the following experiment to measure the SCP of armyworm in different photoperiods.The results show that the SCP of armyworms in different photoperiods have significant differences.The photoperiod has influence on the SCP of 3rd instars larva and pupa,but,it doesn't work for the SCP and of 5th instars larva.The influence of the photoperiod hasn't caused the obvious tendency of variation,for example,the SCP of armyworms is higher in long-day,the SCP of armyworms is lower in short-day.The photoperiod has influence on the water content of pupa and 3rd instars larva,however,it has no influelce on that of the 5th instars larva.When the photoperiod is for L14:D10,the body water content of pupa is the highest,which has a obvious difference compared with the other three groups.(P<0.01).All above shows that the photoperiod can make a difference of the cold hardiness of armyworm,but it is not the main factors,may be as a secondary factor.3.The cold hardiness of armyworm at low temperaturesTemperature is one of the important factors that affect the survival and reproduction of insects,the insect resistance to low temperature is the most important part on the study of cold hardiness.The survival of the overwintering insects usually depend on the ambient temperature,extremely low temperature or continuous low temperature often make a big difference of insect,so the mortality of the armyworms in extreme low temperature and continuous low temperature also can be used as the index of their cold hardiness.To explore the cold hardiness of armyworm in low temperature,the mortality of armyworm were measured in different low temperature and in different processing time.The research results show that the low temperature makes a difference on cold hardiness of armyworm.When the temperature is 0?,the LT50 and LT99.9 of mature larva respectively are 6.50days,11.50days,the LT50 and LT99.9 of pupa respectively are 9.00days,12.67days;when the temperature is 5?,the LT50 and LT99.9 of mature larva respectively are 10.77days,17.02days,the LT50 and LT99.9 of pupa respectively are 14.24days,20.55days;when the temperature is 10? the LT50 and LT99.9 of mature larva respectively are 15.75days,28.43days,the LT50 and LT99.9 of pupa respectively are 24.02days,35.19days.Removing from low temperature,we say in normal temperature,especially in 10?,the part of mature larva can be pupated.At the same temperature,compared with the pupa,the LT50 and LT99.9 of mature larva is lower,so the cold hardiness of pupa is higher.Under the same time,the mortality of mature larva and pupa decreased as the temperature increases.The LT99.9 for pupa were 12.67 days while the temperature was 0?,which indicated that armyworm was not able to overwinter in areas where temperature below 0?exceed 12.67 days.Therefore,low temperature and continuous low temperature have great influence on the mortality of armyworm,which provides the basic data of the cold hardiness as well as the research of overwintering.