Effects Of East Asian Summer Monsoom And Local Circulation On The Occurrence Of Cnaphalocrocis Medinalis(Guenée) In Southern China

As an important agricultural pest,rice leaf roller（Cnaphalocrocis medinalis Guenée:C.medinalis）is an important agricultural pest threatening rice production in China.Its migration activities are closely related to the atmospheric background.On the one hand,affected by the East Asian monsoon,C.medinalis migrates to and fro every year in the different rice-growing areas of eastern China.On the other hand,the outbreaks of C.medinalis are intermittent,sudden and have very strong regional differences.The complex topography of southern China formed a variety of local microclimates,which often lead to the outbreaks of C.medinalis in the area.In order to provide a scientific basis for the prediction and prevention of C.medinalis,in this study,the medium-term migration patterns and short-term migration-landing mechanisms of C.medinalis were discussed from the large-scale monsoon circulation and the small-scale local circulation respectively.Firstly,statistical analysis was made on the occurrence characteristics of C.medinalis in southern China from 2000 to 2016.The results showed as follows:（1）The occurrence duration of C.medinalis was shortened from south to north.The longest occurrence duration of C.medinalis in the rice-growing region of South China was about 210—230 days,while the occurrence duration of C.medinalis in northern of the rice-growing region between the Yangtze River and the Huaihe River and the rice-growing region in Southwest China was shorter,48—70 days on average per year.（2）According to the statistics,there were 146immigration events with the daily trapped catch more than 500 heads,among which 4—6generations of C.medinalis outbroken most frequently and brought about a wide range of infestation.The frequency of immigrating events was high and the events lasted for a longer duration in the rice-growing region between the Yangtze River and the Huaihe River,the rice-growing region in Southwest China and northern part of the rice-growing region between the Nanling Mountain and the Yangtze River,while the occurrence frequency of immigration events was the lowest in eastern of the rice-growing region between the Nanling Mountain and the Yangtze River.Secondly,the relationship between the monsoon and the migration of C.medinalis in the main rice growing regions of southern China was analyzed by using the north boundary index of the East Asian Summer Monsoon（EASM）.The results showed as:（1）The northward migration of C.medinalis was closely related to the northward advancing of the EASM.The first appearance and second appearance of the northern margin of EASM had certain indication significance for the beginning of C.medinalis northward immigration.The peak events of northward immigration generally occured in the control areas of EASM.（2）The period from mid-June to early August,when the north margin of EASM moved to the north of30°N,during which more peak events of northward immigration usually with large populations took place in the main rice-growing regions of southern China.In the year when the north margin of EASM fairly north,the occurrence amount of C.medinalis was more in the rice-growing region between the Yangtze River and the Huaihe River,but less in the rice-growing region in South China,the rice-growing region in Southwest China and the rice-growing region between the Nanling Mountain and the Yangtze River.（3）The mid-late August was the transition period of winter and summer monsoon in East Asia,and also the"mixed migration"period of C.medinalis.At this time,the first batch of southward migration peaks came in the rice-growing region in Southwest China and the rice-growing region between the Yangtze River and the Huaihe River;From the beginning of September,the prevailing northeast wind with the attribute of winter monsoon moved southward and the population of C.medinalis moved southward simultaneously.（4）The atmospheric backgrounds for the outbreak of C.medinalis in the rice-growing region between the Yangtze River and the Huaihe River in 2007 were investigated:At first,the strong southwest monsoon during the period from the end of June to July increased the immigrating amount of C.medinalis in the rice-growing region between the Yangtze River and the Huaihe River significantly and it laid a foundation for the large-scale migration of C.medinalis from the rice-growing region along the valley of Yangtze River to the rice-growing region between the Nanling Mountain and the Yangtze River.Secondly,the anomalous low precipitation in the south of the Yangtze River and South China in July led to the further accumulation of hygrophilous C.medinalis populations in the rice-growing region along the Yangtze River and the rice-growing region between the Yangtze River and the Huaihe River.Thirdly,the delaying of EASM retreating in August and the relative warm of the north China and the region between the Yangtze River and the Huaihe River in September detained C.medinalis populations in these areas.At last,the beginning dates and the peak dates of their migration southward were unusually late.Lastly,the migration and landing processes of C.medinalis under three typical topographic conditions were simulated by the HYSPLIT model,and the atmospheric background that affect the migration-landing process of C.medinalis were analyzed.The results showed that:（1）The insect source in southeastern Guizhou as a typical Mountainous terrain was mainly from western Guangxi.In the region,the upward movement of airflow in the northern part of Guangxi was beneficial to the migration of C.medinalis from low altitude to high altitude.The southwest wind on 850 hPa and 925 hPa were the main driving forces for the migration of C.medinalis.There was no obvious subsidence airflow in the landing area of C.medinalis,and the terrain stres played an important role in the landing processes of C.medinalis:The weakening of horizontal wind field and the terrain blocking made the airflow’s transport effect on C.medinalis decrease.In addition,Rongjiang is located in the valley area surrounded by mountains on three sides,which makes C.medinalis centrally landing in the area.（2）As a typical immigration case of C.medinalis occurred in the environment of valley topography during 6^th to 7^th,September in 2005,the source of insects moving into Tongcheng and Guichi came from the central part of Anhui Province.The northerly wind at 925hPa was the main driving force for this migration.There was no rainfall and the downward movement of air flow in the vertical direction was the main driving force for the landing.Distribution of topography and mountain ranges hads an important impact on the landing area.Under the combined influence of northerly wind and downdraft,populations of C.medinalis mainly landed in Tongcheng and Guichi,the windward side of the mountains,while no insect landed on the leeward side.（3）As the typical cases of the immigration of C.medinalis in a land-sea border area which occurred in Yangchun of Guangdong Province during 22^th to 23^th,Ocotober in 2009.The two-day retrograde trajectories showed that the source of insects in Yangchun cames from the central and western parts of Guangdong Province.The northerly wind in the coastal area of South China was the main driving force for the two days migration events of C.medinalis.There was no precipitation in Guangdong during the two days.The subsidence air was the main driving force for the landing.The land breeze at night was one of the main reasons for the centrally landing of C.medinalis in Yangchun area.