Migration Of Rice Planthoppers In East Asia: Patterns,Processes And Meteorological Backgrounds

Rice planthoppers(Hemiptera:Delphacidae)(RPH),Sogatella furcifera(Horvath)(WBPH)and Nilaparvata lugens(Stal)(BPH),are the most serious insect pests of rice,which had caused heavy losses and economic damage in Asian rice production.In general,the RPH in East Asia migrate regularly northward in spring and summer and southward in autumn(i.e.,return).So there have been a long-time thought that the hoppers perform a migration circuit between lndo-China Peninsula and mainland of China.However,as the main immigration region of massive returned PRH populations from southern China,North Central Vietnam experienced about 3-month rice-free fallow from November to January that disrupted the life cycles of returned RPH by depriving food and refuges.Consequently,the returned populations collapsed and only very few individuals could be seen on the ratoon rice and the seedling of the falling grain.We hypothesized that the hoppers should be able to re-colonize paddies in North Central Vietnam at the next cropping season during late February and March to establish the source population for southern China.Questions were proposed here.Firstly,did rice planthoppers actually conduct a perfect and annual migration circuit?Secondly,where were the source areas for re-colonized population,and how is the process of re-colonization?Thirdly,how did the immigrant populations that invading into China reform in early spring?Fourthly,what climate parameters would influent the first arrival dates of rice planthoppers in southern China?Fifthly,what atmospheric factors would determinate the spatio-temporal distribution and massive landing of rice planthoppers?Last but not the least,what were the lying mechanisms of the topographic terrain on the landing of rice planthoppers?In this study,a high-resolution frame work of three-dimensional trajectory modeling with Weather Research and Forecasting(WRF)Model was described to disentangle how the RPH population re-colonize and to reconstruct spatial and temporal migration patterns between ’Source and Sink’ of rice planthoppers in early spring.Statistical correlation analysis was employed to test impact of climate parameters on the first arrival dates of rice planthoppers and to select short-term forecasting and prediction factor.Meanwhile,sensitivity experiments were carried out using reduced topography to investigate how topography affects rainfall and induces huge light-trap catches of rice planthoppers.Hence,the full map of ’Source area’and ’Fallout site’ would be reconstructed,the migration patterns of rice planthoppers in Indo-China Peninsula from February 21st to March 20th would be clear and the knowledge gap about landing mechanisms would be enriched.We obtained the following results:1.Validation and estimation of the trajectory analysis based on WRF model.(1)The outputs in high spatial and temporal resolution and benifiting from the three-dimensional trajectory program which contains flight behavior parameters of migratory insects were accurate and it is of great practical values to trace source areas of migratory insects and predict their fallout sites.(2)Helicoverpa zea population taking off from Lower Rio Grande Valley(LRGV)On March 20th to 22nd,1995 mainly landed in eastern Texas,USA.The fallout sites were in the regions which were influenced by the Low-level jets,which also played a key role in migration process of Helicoverpa zea.2.Short-distance dispersal of windborne rice planthoppers and the role of meso-scale vortex on their long-distance migration in Mekong River Delta in February and March.(1)Owing to the prevailing easterly winds with low speed,in February and March from 2010 to 2014,emigrant rice planthoppers from southeastern Indo-China Peninsula primarily undertook the short-distance dispersal to the local paddies,part of them moved into Cambodia,central Vietnam,southern Laos and southern Thailand,a few individuals could fly over the sea then land into south Myanmar and very small part of populations migrated northward to North Central Vietnam as well as Hainan Island,China.(2)It is showed very significant correlations among the number of light-trap catches of rice planthoppers in Mekong River Delta,Quang Nam(South Central Vietnam)and Nghe An(North Central Vietnam).And the correlation between Phu Yen(South Central Vietnam)and Quang Nam is significant.(3)Under the influence of single or multiple vortex(cyclone or anticyclone)over central peninsula,southern Indo-China Peninsula prevailled easterly wind,while the wind in the central peninsula blew northwestward then changed into southwesterly wind.The vortex airflow in the Indo-China Peninsula enabled long-distance northward migration of the rice planthoppers in Mekong River Delta.3.The spring re-colonization of rice planthopper population in North Central Vietnam.(1)From last November to January,only ratoon rice and the seedling of the falling grain were found in fallows in North Central Vietnam,and the returned populations collapsed and very few individuals could survive because of lacking of food and refuges.(2)In North Central Vietnam,rice plants are transplanted in late February and will be at seeding to tillering stage in mid March.Meanwhile,rice plants are at tillering to jointing stage and the rice plants in southern Laos are at jointing to booting stage.These regions with all-year-round rice could provide source populations to North Central Vietnam.(3)The prevailing easterly wind in the central Indo-China Peninsula spread westward or southwestward,then blew northward above Cambodia and Thailand,which provided extremely favorable condition for the transportation of rice planthoppers into North Central Vietnam from South Central Vietnam,Laos and Isan of Thailand.4.’Source and Sink’ migration patterns in Indo-China Peninsula.(I)Windborne rice planthoppers in Indo-China Peninsula primarily undertook the short-distance dispersal.Rice planthoppers emigrating from South Central Vietnam mainly landed into Cambodia,part of the population could migrate into Central Highlands in Vietnam and Laos;Rice planthoppers taking off from Central Highlands migrated westwards or northwestwards and had close relationships with Cambodia,Thailand and Laos;Rice planthoppers in Southeastern Vietnam and Mekong River Delta mainly migrated into Cambodia,while rice planthoppers in Mekong River Delta also dispersed to Thailand,southern Laos and southeastern Vietnam;Rice planthoppers in southern Laos could arrive Thailand and North Central Vietnam;the mainly fallout sites of migrating rice planthoppers in Isan of Thailand were local regions in Thailand,Laos and North Central Vietnam;most of the rice planthoppers in eastern Thailand were relocated in eastern Thailand,partly in Cambodia and Myanmar;Rice planthoppers in Tonle Sap Lake Region of Cambodia mainly migrated to Thailand,Laos and Myanmar.(2)There were direct relationships among rice planthoppers in Thailand,Cambodia and Vietnam.In specifically,in the south of 13°N easterly wind determined the ’source area and fallout site’ relationship among Mekong River Delta,Central highlands and southern Vietnam,eastern Thailand and central Cambodia,while in the north of 13°N,winds were various,southwesterly wind prevailed in Isan of Thailand and southern Laos,and southeasterly wind prevailed in South Central Vietnam were the necessary conditions for immigration of rice planthoppers in North Central Vietnam.In addition,temperature also determined the migration dynamic of rice planthoppers in Indo-China Peninsula.5.Effects of climate parameters on first arrival date of RPHs in southern China and short-term forecasting factors.(1)The average first light-trap catches of RPHs in Guangxi and Guangdong emerged in late March to early April but showed great annual differences.(2)There was a significant negative correlation between the first arrival dates of RPHs in Hepu,Guangxi and the monthly mean monthly Sea Surface Temperature(SST)in 10-20°N region in March.(3)The first arrival dates of RPHs in Xinyi had negative correlations with monthly surface temperature in March in Isan of Thailand,southern Laos and northern Vietnam,same negative correlations with monthly 850 hPa temperature in Isan of Thailand,southern Laos and North Central Vietnam;The first arrival dates of RPHs in Xinyi advanced as monthly surface temperature in March in South Central Vietnam increased;The first arrival dates of RPHs in Hepu had negative correlations with monthly surface temperature in March in North Central Vietnam,with monthly 850 hPa temperature in Guangxi,northern Vietnam,Hainan,southern Laos and North Central Vietnam;The first arrival dates of WBPHs in Yangchun were negatively correlated to the monthly surface temperature in northern Vietnam and Hainan,China in last December and BPHs had negative correlations with monthly surface temperature in January in Isan of Thailand,southern Laos and North Central Vietnam,with monthly surface temperature in February in Hainan,with monthly 850 hPa temperature in January in Isan of Thailand.6.Landing mechanisms of WBPH and the effect of topographic terrain.(1)Three main synoptic systems,including the East Asian Trough,West Siberia Cold Vortex,and Western North Pacific Subtropical High were found to trigger the outbreak of S.furcifera in south-central China.This study also revealed that the low temperature,downdrafts,wind convergence zone,convection,rainfall,low-vortex shear and the southwesterly low level jet in this region can also act as other key factors of concentration and landfall of S.furcifera.(2)Two sensitivity experiments were carried out using reduced topography with the WRF model.Topographic terrain mainly affected the concentration and landfall of WBPH from three aspects:(ⅰ)warm and humid southwesterly airflow were forced to climb the windward of the slope mechanically because of the blocking and uplifting effects of mountainous terrains,then condensed to rainfall.The latent heat generated by the precipitation could reduce the air pressure,which further stimulated the more intensive convection and enhanced the intensity of the rainfall;(ⅱ)mountainous terrains could adjust the spatial distribution and intensity of precipitation by blocking the northerly cold airflow on the leeward side to invade directly and forcing it to flow around the mountain or just through the valley,and current convergence and wind shear line occurred in the north of the mountains;(ⅲ)WBPHs already in flight were forced to land when the temperature decreased as the height of the terrain got higher.