The meadow moth, Loxostege sticticalis L. is an important outbreak pest in northeastern China. For a long time, the spring populations in northeastern China were considered to be immigrants from a “permanent breeding area” in northern China. However, the population dynamics during the 1996~2007 outbreak were not consistent with this traditional view. In some years, the peaks of the spring population in northeastern China occurred more than 10 days before those in northern China, and ovarian development in many of the females was only at the transparent stage, i.e. there were no oocytes in the ovary, which means the moths are newly emerged. It therefore appears that there must be some other source area for the spring immigrants in northeastern China.
In this paper, the source areas of the spring populations in northeastern China were identified for each of the years 1997-2007 by back-trajectory analysis, using wind field simulated by using the PSU-NCAR mesoscale non-hydrostatic numerical forecast model MM5 and starting on the days of immigration peaks. The results indicated that the population in the “permanent breeding area” were able to migrate into northeastern China but did not make a crucial contribution to the outbreaks there, and that weather systems were generally adverse for movements from northern China to northeastern China in most years.
On the other hand, high density cocoons of the beet webworm have frequently been found in autumn in northeastern China since 1996. The trajectory analyses indicate that most of the meadow moths in northeastern China in spring emerged from cocoons that had overwintered either locally or in neighboring countries, i.e. Mongolia and Russia. The development of overwintering at these higher latitudes coincides with an increased frequency of second-generation larvae, and both phenomena can perhaps be attributed to a regional climate change associated with global warming.