Susceptibility to insecticides of brown planthopper Nilaparvata lugens in the lower Yangtze Valley

Adult brown planthopper (BPH), more than 50 females of each population, have been collected from ricefields of Anqing (Anhui Province) and Yangzhou (Jiangsu Province) and multiplied with rice in the laboratory at 25°C, 16 h photoperiod since 1989. Assays were conducted using female adults of F 2 -F 3 within 5 d after emergence by the topical treatment method. Technical grade insecticides were dissolved in analytical grade acetone and five or more diluted concentrations were prepared. For each concentration, 50 macropterous females were treated with 0.05 μL of solution applied by microsyringe on the thoracic dorsum. Monitoring was done 48 h after treatment. Dose-mortality regressions were estimated by probit analysis. LD 50 s were considered to be indicative of resistance and their differences among years were tested by t test.