Current Status of Insecticide Resistance and Virulence to Resistant Rice Varieties in Asian Rice Planthoppers

Insecticide susceptibility and virulence to resistant rice varieties were determined and evaluated in the brown planthopper (BPH) and the whitebacked planthopper (WBPH), collected from Asian countries (Japan, China, Taiwan, northern Vietnam, southern Vietnam, and Philippines) in 2006. Insecticide susceptibility was determined by a topical application method using four insecticides. Virulence to resistant rice varieties was evaluated using five differential rice varieties carrying different planthopper resistance gene(s). Species-specific changes in insecticide susceptibility were found in Asian rice planthoppers (i.e. imidacloprid resistance in BPH and fipronil resistance in WBPH). The topical LD50-values for imidacloprid in the BPH strains in East-Asia (Japan, China, and Taiwan) and Vietnam were significantly higher than those in the Philippines, suggesting that insecticide resistance in BPH against imidacloprid occurred in East Asia and Indochina, but not in the Philippines. Almost all the WBPH populations collected had extremely large LD50-values for fipronil, suggesting that insecticide resistance in WBPH against fipronil occurred widely in East and Southeast Asia. The virulence of Asian BPH strains was classified into three groups: (1) The BPH strains in East Asia and northern Vietnam were virulent to Mudgo (Bph1) and ASD7 (bph2) but avirulent to other three varieties, (2) The BPH strains in Southeast Asia (the Philippines) were virulent to Mudgo, ASD7 and also partially virulent to Babawee (bph4), and (3) The BPH strains in southern Vietnam were highly virulent to Babawee in addition to Mudgo and ASD7. The varieties Rathu Heenati (Bph3, Bph17) and Balamawee (Bph9) still have a broad spectrum of resistance against all the Asian BPH strains tested.