Brown planthopper (BPH), Nilaparvata lugens(Stål), is one of the most important migratory rice pests in China. The northeast of Guangxi Zhuang Autonomous Region, which is not an overwintering area for BPH, is situated in a double-cropping rice region and is near the "Xiang-gui Corridor", the major pathway for the seasonal northward and return migration of BPH in China. In recent years, infestations of the 7^th generation of BPH, which is regarded as a secondary devastating generation, have frequently broken out in this region, leading to higher production costs and a serious loss of rice yield for farmers. The migration of BPH to China in spring has increased with the increased backward migration of BPH towards the Indo-China Peninsula during the previous autumn. However, research on the migratory patterns of the 7^th generation of BPH is seldom reported. An investigation into the migrating and source areas of this generation has been initiated as a first step towards developing procedures for forecasting and suppressing significant infestations of BPH. Our studies were conducted between 2007 and 2010 in Xing'an county, northeastern Guangxi Zhuang Autonomous Region, with a view to providing reference data for the forecasting and control of BPH. A Jiaduo light trap and a searchlight trap were used to monitor the migratory periods of BPH. The migration process of BPH and the atmospheric conditions during those periods were analyzed with HYSPLIT, a trajectory analysis system, to simulate the migration trajectories of migratory BPH, and also with GrADS software. From this, we ascertained the migration rule of the 7^th generation of BPH and their source areas. Furthermore, the relationship between migration of BPH and atmospheric conditions was examined.
There were two or three peak migratory periods for the 7^th generation of BPH every year, which generally occurred between late September and early October, in northeastern Guangxi Zhuang Autonomous Region. There are significant differences in immigratory periods, emigratory periods and migratory amount for different years. Most of the immigrant BPH in Xing'an county apparently originated in the mid-west of Jiangxi Province and the mid-east of Hunan Province. Nevertheless, the emigrant BPH from Xing'an county would land mainly in the central part of Guangxi Zhuang Autonomous Region if they flew for 12 h. After 24 h or 36 h of flight, they would land in the south of Guangxi Zhuang Autonomous Region, the north of Vietnam, and on Hainan island which were overwintering areas for BPH. They mainly flew in warm air at about 925 hPa pressure. Our earlier study demonstrated that the critical temperature for flight was a daily average of 20℃. A northeasterly airflow at the 925 hPa pressure level was the major impetus for the return migration of the 7^th generation of BPH. They tended to migrate when it was not raining and there was a weak pressure drop. In conclusion, the searchlight trap can trap BPH effectively in any migratory period, so when combined with the Jiaduo light trap, it can help determine the migratory characteristics of BPH. We suggest that plant protection stations use the searchlight trap to monitor and control BPH.