The rice leafroller, Cnaphalocrocis medinalis Guenée, is a major crop pest that causes serious damage to rice production. Because Xing'an, in northeast Guangxi Municipality, is the only annual migratory route for C. medinalis migrating back and forth between southern and northern China, a better understanding of the population dynamics and migration trajectory of this pest in Xing'an will be beneficial for the long-term control of the rice leafroller in China. We studied C. medinalis in Xing'an in 2010 using light traps, systematic field surveys, and dissection of female ovaries. The spatiotemporal distribution of source areas of C. medinalis migration peaks was analyzed using HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory), a software package that simulates migration trajectories, and GrADS (Grid Analysis and Display System), a program for analyzing and displaying the synoptic meteorology background during the migration periods.
The results showed that: (1) the population dynamics of C. medinalis varied seasonally in Xing'an. C. medinalis had six generations a year. Immigrants arrived in Xing'an at the end of April, and migration occurred mainly from the end of May until the middle of August. The autumn return of C. medinalis to Xing'an was small in 2010. Each of the three annual rice crops (early double-crop, middle-season single-crop, and late double-crop) experienced three generations of C. medinalis, with some overlap of leafroller generations onto more than one crop. The population density of C. medinalis in the early double-crop rice field was significantly higher than in other two rice fields.
(2) The population characteristics of C. medinalis as observed in light traps was similar to that in field observations. The ratio of mated females and the ratio of females with matured ovaries indicated that the second and third generations (16 Apr.-20 Jun.), which mainly infested early rice, were immigrants. The fourth generation (21 Jun.-20 Jul.), which also mainly infested early rice, emerged locally and had the highest density of all generations. This generation could not emigrate long distances because of the long-lasting rainy weather and instead settled down and infested middle-season rice. The fifth generation (21 Jul.-20 Aug.) mainly infested middle-season rice and were mostly locally emergent, but with some immigration between 29 Jul.-7 Aug. and some emigration from 8-20 Aug.. The sixth generation (21 Aug.-20 Sep.) was mostly emigrants. The early sixth generation (21-28 Sep.) infested middle-season rice, while the late sixth generation (29 Sep.-20 Oct.) infested late rice. The seventh generation (21 Sep.-28 Oct.), mainly infested late rice and were basically emigrants.
(3) The major immigration peaks of C. medinalis were 5-6 May and 17-19 Jun. and the major emigration peak was 14-15 Oct.. Most of the immigrants came from Guangdong Province: from Yangjiang and Zhanjiang during early May followed by a massive immigration in mid-June that probably came from the Leizhou Peninsula. Analysis of weather conditions during the migration peaks showed that strong low-level jets carried massive numbers of C. medinalis into Xing'an, while heavy rainfall caused mass descent. Local C. medinalis carried by northeast air currents may arrive at Beihai of Guangxi Municipality in the middle of October.