Abstract:The toxic activity of five insecticides to the 3rd instar nymphs of the small brown planthopper (SBPH), Laodelphax striatellus (Falln), was determined by using rice-stem-dipping method, and impact on the productivity of SBPH was tested with the LC50, LC75 and LC90 of insecticides. Compared with the amount of the second generation of SBPH treated with water, the amounts of SBPH treated with fipronil and imidacloprid were decreased, the amounts of SBPH treated with chlorpyrifos, triazophs and cyhalothrin were increased. The amount of the second generation of SBPH treated with LC50 of cyhalothrin was 1.533 times of that treated with water, which was significantly different at 0.01 probable level. The numbers of SBPH treated with fipronil (39.9mL/667m2) and imidacloprid (53.2mL/667m2) only were 0.335 and 0.646 times of that treated with water, and the numbers of SBPH treated with chlorpyrifos (59.9mL/667m2) triazophs (98.6mL/667m2) and cyhalothrin (99.8mL/667m2) had increased by 1.73, 1.35 and 1.93 times in the separated rice-wheat intercropping system in the second year.