Biological characteristics of different geographical populations of the small brown planthopper Laodelphax striatellus Infection and spread of Rice black streaked dwarf virus in the digestive system of its insect vector small brown planthopper Analysis of QTLs for Resistance to Small Brown Planthopper(Laodelphax striatellus Fallen)in Rice(Oryza sativa L.) Using an F2 Population from a Cross between Mudgo and Wuyujing 3 Expression of Rice Defence Genes under Small Brown Planthopper Stress