A Case Study On The Emigrating Insect Sources And Migration Paths Of Brown Planthopper, Nilaparvata Lugens (Stal), In Hunan Province

Exotic predicting is an important content of migratory pest’s occurrence predicting and the simulations and predicting of its migration trajectory is a kind of ideal exotic predicting method that can reflect preferably the spatio-temporal dynamic patterns of the pest’s migration. Brown Planthopper, Nilaparvata lugens (Stal), is one of the important migratory pests of rice production in China. The accurate forecasting of its migration trajectory can provide a scientific basis for the early warning and effective prevention of its catastrophes.1. In order to select comparatively the application of some pest migration trajectory models with good accuracy, high resolution and easy popularization, in this paper, a great event of BPH’s northward migration which occurred in Hongjiang City of Hunan Province at the beginning of July in 2006 was screened as a typical case from the analysis of BPH’s lighting catches in China. Weather Research and Forecast Model, a popular meso-scale wearher research and forecast model at home and abroad, was used to simulate and output the atmospheric background fields in high resolution combined with reanalyzed meteorological data from the National Center of Environment Predicting of the USA (NCEP). The forecasting variable fields output by WRF model were used to coulpe with the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) Model and the Flexible Particle (FLEXPART) Dispersion Model respectively for reckoning backward the migration trajectories of BPH’s populations landfalled in Hongjiang City of Hunan Province. The differences between the two models in the trajectory parameters such as the pest sources, migration paths, migration heights, migrating speeds and migration distance were compared according to the backward calculation results. At last, the observed data of BPH’s habitat, feeding condition and the three dimensional atmospheric airflow field were used to verify the accuracy and precision of their simulations and calculations. The result showed as follow:(1) There are the basic consistenceies between the two trajectory models on the calculations of the pest sources, migration paths (migration azimuth and flight direction, migration heights, migrating speeds and migration distance, but there are still some differences between them and the ups and downs of WRF-FLEXPART model on the migration parameters were larger than the WRF-HYSPLIT model. (2) Although the two coupled modes had many similarities in the calling of the forecasted physical variable fields by WRF model output, WRF-FLEXPART coupled model in the processes of the trajectory calculations considered convection parameter, surface stress and a variety of terrain parameters, but WRF-HYSPLIT coupled model did not considers them. Thus, WRF-FLEXPART model had a more comprehensive and true reflection in the discribing daynamic effect of the meso-scale weather processes (specially convective weather processes) on the take off, flight in air and landfalling of the BPH’s populations and the impact of the undulating terrain, local circulation and the physical processes on land surface on the migrations of the populations than WRF-HYSPLIT model. (3) From the view point of BPH’s populations selection on the habitats and feeding conditions, the simulations of the two models on the pest sources, migration paths and landfalling of immigrating populations on the all levels were reasonable and accurate. But in terms of the distribution of three dimensional airflow field which the emigration, flight in the air and landfalling of BPH’s populations occurred, WRF-FLEXPART model in the coincidence degree between the trajectories movement directions and the horizontal prevailing airstream directions was higher remarkablely than WRF-HYSPLIT model. The vertical airflow only played a minor role on the taking off and landfalling of BPH’s populations in this event. (4) The two coulping models can be used in the professional forecasting of migratory pest occurrence as a tool.2. Based on WRF-HYSPLIT, a typical case of migration northward in Hunan Province was simulated by setting different released levels and the vertical velocity field, vorticity field, divergence field, the temperature advection, air stablity and relative humidity were selected as the influencing factors for the diagnostic analysis of the migratory height variations. The results displayed as follows:1)The pest sources of the BPH’s immigration northward situated in the south sides of the landfalled regions in general and the migration altitudes changed from 600 m to 2500 m. The horizontal airflow in the upper air is the main driving force of the long-distance migrations of BPH’s populations and the prevailing southerly on 850hPa isobaric surface is favorable to the migrations northward of the BPH’s populations.2) When BPH’populations flew to the area that the values of downdraft was large, the flying altitude was low. On the other hand, if they met updraft, they would flew upward.3)When BPH’s populations flight through a positive divergence areat, their flying altitude were low butthrough a negative divergence area their flight height would be upraised.4) When BPH’s populations passed through a positive vorticity area the flying height increased but in anegative vorticity area the flying height decreased.5)In the air, BPH’s populations like to fly in the height s with high relative humidity and large moisture fluxand they were apt to landfall in the areas where the surface relative humidity is more than 80%.3. Under the assistance of WRF-HYSPLIT coulpled model, a great event of migration southward which was a typical single immigration peak at dusk in Hunan Province was simulated by setting different released levels and the horizontal wind speedfield, vertical velocity field and the other related atmospheric physical variables were used to the diagnostic analysis on the influencing factors of the migratory height variations. The results showed as follows:1)The pest sources of the BPH’s immigration southward situated in the north of the landfalled regions in general. The migration altitudes changed from 400 m to 1250m.2) the prevailing northeasterly in the air layer between 900hPa to 925 hPa was advantageous to the migrations southwestward of the BPH’s populations.3)The temporal and spatial pattern of the maximum wind speed layer and vertical velocity played the decisive roles on the variations of BPH’s flight height in air.