The brown planthopper (Nilaparvata lugens) is among the most destructive pests of rice in Asia. However, control of this pest is hampered because of the emergence of strains with resistance to standard chemical pesticides. Herein, we investigated the use of RNA interference (RNAi) to knock down genes in the immune system of N. lugens as a possible approach to the control of the pest. We suppressed the expression of the peptidoglycan recognition protein–LC gene and Toll-1 gene by RNAi-mediated knockdown in N. lugens infected with live bacteria. Mortality rates were compared between insects with and without RNAi-mediated gene knockdown. We found that suppression of peptidoglycan recognition protein–LC, which encodes a factor in the immune deficiency pathway, resulted in a significantly increased mortality rate in insects infected with Escherichia coli (gram-negative bacteria) or Micrococcus luteus (gram-positive bacteria). Our results indicate that RNAi knockdown of genes in the immune deficiency pathway will be of value for identifying candidate genes that can be used to control this pest species.