|
Brown planthopper(BPH), Nilaparvata lugens (Stal) is a major pest of rice in China and various other countries and regions in Asia. Its seasonal northward and southward migrations, landing and endangering abruptly result in serious damage to rice production in China. BPH have broken out frequently in China since 1970s, but the overall decreased since the early 1990s owing to using pesticides such as buprofezin and imidacloprid, BPH outbreaks have increased again since the beginning of the 21st century, because of some factors (such as change of rice cropping system, rice varieties in Southeast Asia and China, and the resistance of pesticides), especially successive outbreaks from 2005 to 2007, which resulted in serious damage to rice production. Therefore, the population dynamic of BPH under current cultivation system and weather condition is an important project worthily to be studied and discussed again. In this paper, the population dynamics, population characteristics, distributions of mid & small scale source areas and take-off behavior of this pest in Guangxi Autonomous Region (breeding place of initial immigrant source in spring) and Anhui Province (emigrant area of migration source in autumn) were studied through the light trap catches of BPH, systematic investigation both in fields and cages, female ovarian dissection, trajectory analysis by using HYSPLIT, and analysis of the historical data. The results were summarized as follows.The light trap data of BPH during March to June in Longzhou, Hepu, Yongfu since 1977 were analyzed, it showed that significant positive correlation and highly significant positive correlation existed between the logarithm of light trap catches of BPH in Longzhou, Yongfu and the national occurrence extent of BPH, respectively (r= 0.373*,.P=0.035; r= 0.496**, P=0.005). The light trap catches in Longzhou, Yongfu could also reflect the immigration status of BPH’s population from abroad to some extent. Therefore, the mass enhancer of the immigrant light trap catches in Longzhou, Yongfu, might be the important reason for successive BPH outbreaks.The population dynamics and characteristics of the pest in double-cropping early-season rice fields in Nanning and Yongfu from 2007 to 2008 were studied through systematic investigation and female ovarian dissection. The results showed:The population density of BPH in the late matured variety of early rice was significantly higher during each rice growing stage than in that of the early matured variety, and the period of occurrence and crop damage were also longer in the late variety, which might provide an adequate source for emigrant population outbreaks in northern China later in the season. The peaks of emigration displayed in middle and late June in Nanning, but from early July to late July in Yongfu.Simulations of migration trajectories during the major immigration peaks in Guangxi, 2007 indicated that most of the immigrants came from the centre part of Vietnam or Laos (18°N-20°N,102-106°E) from late April to early May; from the northern part of Vietnam or Laos (20°-22°N,104-107°E), and also from Hainan Province and Leizhou Peninsula during middle May to late May; while the massive immigration to northern Guangxi Autonomous Region in early June probably emigrated from the north-east part of Vietnam (21-22°N, 106-108°E) and the south-west part of Guangxi Autonomous Region. The harvest seasons of winter-spring season rice in centre and north part of Vietnam were different to 1 and 2 months, which formed the various source areas of BPH in different time periods in Guangxi Autonomous Region. Case analysis was studied for the super occurrence of BPH in northwest part of Guangxi in middle June,2008. The results showed that there were the similar weather conditions for migration and landing of BPH either in the northwest or northeast parts of Guangxi, but there were the significant differences of BPH population densities and proportions of macropterous between the southwest and southeast parts of Guangxi which was proved to be the mid or small scale corresponding emigrant source areas of the northwest and northeast immigrant areas of Guangxi, respectively.According to the analysis of field investigation results in 2007 and 2008, the occurrence of BPH in single-cropping rice was earlier and serious than in double-cropping late-season rice, and BPH in single-cropping rice began to emigrate or transfer after late August, this mixed cropping system was beneficial for the occurrence and damage of BPH. According to the result of female ovarian maturity in 2007 and 2008, the peak of emigration was from late August to middle September in single-cropping rice, but in October in double-cropping late-season rice, and the single-cropping rice could provide an adequate source for posterior immigration at Yangtze River Delta rice areas. Simulations of migration trajectories during the major immigration peaks in Anqing,2007 indicated that most of the immigrants came from the centre and south parts of Guangxi from late June to early July, and from lingnan rice districts during middle July to early August, the major immigration peaks were consistent with the yellow ripeness and harvest period of double-cropping early rice of emigrant source areas. According to case analysis of the corresponding migration trajectories between the posterior immigration in 2007, Shanghai and the emigration in south rice region of Anhui Province, Huaining, it was showed that change of rice planting system mainly with the expending of single-cropping rice paddy area will provide huge amount of macropterous adults of BPH emigrants during late August to middle September, and cause the mass occurrence of posterior immigration at Yangtze River Delta rice areas. In addition, the amount of BPH in posterior immigration was influenced by the population density in corresponding source areas, and proper meteorological conditions during migratory process.The phototactic rhythm and take-off behavior of BPH were observed accurately in Yongfu County, Guangxi and Qianshan County, Anhui in 2008, the results showed that:the take-off behavior of this pest responsed to light rhythm significantly, presenting a bimodal crepuscular type, and the take-off peak at dusk was obviously greater than at dawn, the take-off peak occurred 10-20min later after sunset at dusk, lasted for a few minutes and then quickly ended in twilight, but the peak of take-off occurred 5-15min before sunrise at dawn. BPH had expressed the phototactic rhythm obviously, which could be caught by light trap during either the immigration or emigration period. The peak time period responsed to light trap were occurred between 20:00-21:00 h while the highest peak occurred 10-20min late to twilight, and the light trap catch peaks usually occurred about half hour late to the take-off peaks in the field. The amount of landing and time schedule of the large-scale dependent immigration (concentrated immigration with peaks) was different with the emigration peak as well as the independent immigration (dispersed immigration with no peaks), which had showed close relations with rainfall. The catch amounts raised rapidly after 0:00h and the dawn peak occurred unconspicuously.The developmental schedule of female ovarian for field population and light trap population can be used expediently to distinguish the source properties of the immigration population with the emigration population. There was mainly immature ovary (Ⅱgrade) during immigration, but immature ovary (Ⅰgrade) during emigration for light trap population, But was mainly matured ovary (Ⅲ-Ⅴgrade) during immigration, and immature ovary (Ⅰgrade) during emigration for field population.
|