| Rice planthopper(RPH)populations of Nilaparvata lugens(Stal)and Sogatella furcifera(Horvath)are important migratory pests on rice and periodically have erupted across Asia.Every spring,a large amount of RPH migrate into China from abroad.The occurrence of RPH in China depends largely on the immigrating number,which is affected not only by the population cardinal number but also by atmospheric conditions.Much research has been done on the impacts of short-term atmospheric dynamic processes and long-term climate changes on the migration of RPH.However,little is known about medium-term weather change and the influence of atmospheric low-frequency oscillation of 10-90 days on the migration of RPH.Our aim in this thesis was to clarify the impacts of quasi-biweekly atmospheric low-frequency oscillation(10-20d)or quasi-bimonthly atmospheric low-frequency oscillation(30-60d)on immigration of RPH in China Furthermore,the early immigration of RPH has been taken attention in this study,we examined the population dynamics and migratory paths of RPH and elucidated the reasons for the huge immigration of S.furcifera in the spring of 2012.Therefore,we not only studied the effects of low-frequency atmospheric circulation on RPH’s migration in the meteorological field,but also focused on the population itself,clarified the rules of population migration and the outbreak mechanism in recent years.We provide new information that could assist with forecasting outbreaks and implementing control measures against these migratory pests.The main results are:1.Low-frequency characteristics of early immigration of RPHStudy on the early immigration of N.lugens of 47 sites in the eastern half of China in 2007-2016 showed that the main low-frequency oscillation period in the southern parts of Guangxi and Guangdong was 10-20 days;in Nanling rice area mainly exhibited a periodic oscillation of 10-20 days or 10-30 days;in Lingbei rice area was mainly 10-20 days,but two sites had a significant low frequency oscillation signal of 30-60 days;the sites in Yanjiang rice area mainly had low-frequency oscillation period of 10-20 days,20-30 days and 30-60 days.Analysis of early immigration of S.furcifera of 20 sites in southern China in 2011-2016 showed the main low-frequency oscillation period in most sites was 10-20 days,but a few sites had 20-30 days or 30-50 days cycle.2.Impact of quasi-biweekly atmospheric low-frequency oscillation on the immigration of N.lugens in Hunan and Jiangxi provincesA case study was conducted to ascertain the influence of low-frequency atmospheric oscillation on the immigration of N.lugens in Hunan and Jiangxi provinces.The results showed the following:(1)The number of immigrating N.lugens from April to June of 2007 through 2016 mainly exhibited a periodic oscillation of 10 to 20 days.(2)The 10-20 d low-frequency number of immigrating N.lugens was significantly correlated with a low-frequency wind field and a geopotential height field at 850 hPa.(3)During the peak phase of immigration,southwest or south winds served as a driving force and carried N.lugens populations northward,and when in the back of the trough and the front of the ridge,the downward airflow created a favorable condition for N.lugens to land in the study area.In conclusion,the northward migration of N.lugens was influenced by a low-frequency atmospheric circulation based on the analysis of dynamics.3.Impact of quasi-biweekly atmospheric low-frequency oscillation on the.immigration of S.furcifera in southern ChinaThe main low-frequency oscillation period of the number of immigrating S.furcifera from March to May during 2011-2016 in southern China were 10-20 days and 20-30 days,the oscillation period of 10-20 days was more significant.There was a significant positive correlation between low-frequency zonal wind,meridional wind,geopotential height field and low-frequency immigrating number of S.furcifera.Analysis of the quasi-biweekly low-frequency wind field showed that during the rising period of immigration,south China was mainly controlled by the prevailing south winds,and then the southwest wind gradually increased until it reached the peak during the peak phase of immigration,indicating that the southwest winds played a key role to help the north migration of S.furcifera.Study on the quasi-biweekly low-frequency geopotential height field can be found when the study area was in the low-frequency positive anomaly,the downward airflow was conducive to the landing of S.furcifera;and when the study area under the negative anomaly situation,it was not conducive to the migration and the amount of immigration decreased.4.Impact of quasi-bimonthly low-frequency atmospheric oscillation on the migration of N.lugens in cropping area of JianglingThe daily data of light trapped catches of N.lugens in cropping area of Jiangling in 2007 were collected to clarify the influence.The results showed as follows:(1)The immigrating amount of N.lugens in the summer of 2007 mainly exhibited a quasi-bimonthly periodic oscillation of 30 to 60 days.(2)The wind field and geopotential height field at 850 hPa were the key factors that affected the low-frequency oscillation of the immigrating amount.(3)On the northern hemisphere,the longitudinal northward propagations of quasi-bimonthly low-frequency atmospheric mainly embodied in its transferring from the tropical region to the high latitude area and the zonal eastward propagations reinforced the northward propagations.It was conducive to the northward migration from the rice-growing region of southern China and resulted in immigration during the middle ten days to the last ten days in July.(4)After the analysis of the low frequency atmospheric circulation on the different phases in 2007,it was found that in the early summer,the westerly trough controlling or affecting the eastern China weakened and propagated eastward,the subtropical high pressure on the Western Pacific Ocean gradually strengthened,extended westward and moved northward.Meanwhile,the southeastern China was in the front of the trough and the back of the ridge,and the warm damp flows from the southwest direction enhanced and transported from the South China Sea to the Jiangling rice-growing region and it was extraordinary favorable for the migratory populations moving from the southern China to this area.(5)The source areas of N.lugens at the oscillation peak were mainly located Guangxi,Guangdong and the southern part of Jiangxi by analyzing the backward trajectories.5.Early immigration of N.lugens and insect source analysis in southern parts ofGuangxi and GuangdongDaily light-trapped data of N.lugens of 8 representative sites in southern Guangxi and southern Guangdong during March-May,2007-2016 were collected to analyze the early immigration population dynamics and identify their source areas.It can be found that:(1)The average initial arrival period of N.lugens in most stations was between late March and early April.(2)In recent years,the number of immigration N.lugens has dropped slightly compared with that of the earlier years.Generally,the immigration started in March,a few immigration peaks appeared in April and.gradually increased in May.May was the main period of immigration.(3)The main sources of the overseas insects moved to the south of Guangdong and Guangxi in the early stage were mainly in central Vietnam and central Laos,and a small amount in northeastern Thailand.The source areas in China were mainly in Hainan province and the southern parts of Guangdong and Guangxi.From March to May,source areas prevailed southwest wind,especially during the main period of migration the southwesterly wind was stable.The steady southwest wind is favorable for N.lugens migrating from Indochina Peninsula into southern China.6.Analysis of early immigration of S.furcifera in southern China and their source areas,and reasons for the huge immigration occurred in the spring of 2012Sogatella furcifera is a migratory rice pest that periodically erupts across Asia,and early immigration is an important cause of its outbreak.The early immigration of S.furcifera into southern China showed evident annual fluctuations.In the spring of 2012,the huge size of the immigrant population and the large number of immigration peaks were at levels rarely seen prior to that year.In this study,the light-trap data for S.furcifera in southern China and Vietnam in 2011-2016 were collected,and the trajectory modeling showed that the early immigrants to southern China came from the northern and central Vietnam,Laos and northeastern eastern Thailand.Analysis of the development of the population,the migration process and meteorological factors revealed the reasons for the huge size of the early immigration:1)the expansion of the source area could be seen as a precondition;2)the large size of the returned population in the last autumn and the warm temperature of the overwintering area southern Vietnam and Laos in the last winter increased the initial populations;3)the sustained strong southwest winds were conducive to the northward migration of the population during the major immigration period in early May.Therefore,the large-scale immigration of S.furcifera to southeern China in the spring of 2012 resulted from the combined effects of several factors involved in the process of round-trip migration. |
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