Abstract: Appearance and population of Cnaphalocrocis medinalis(Guenée)at the rice fields in the Karst region in Guizhou was studied.By using light traps and systematic field surveys,the variations on the pest population in 5rice producing areas were monitored during 2010 and 2011.The results revealed that the population size varied significantly from year to year and from area to area.The initial migration of C.medinalis occurred in early May of2010 in Daozhen,Jinping,Sinan and Sandu.However,it changed to early June in 2011 in Jinping and Sandu,and late May in 2010 and 2011in Huishui.In the paddy fields,there were generally 3to 4overlapping generations of the pests in a year.The 2nd and 3rd generation C.medinalis were the major pestilent generations with damages to the crops peaked in July.It appeared that the topography,altitude,temperature and diversity of rice development were the main factors affecting the occurrence pattern of C.medinalis in Guizhou.