Double-stranded RNAs(dsRNAs) targeted against essential genes can trigger a lethal RNAi response in insect pests.However,although expression of dsRNAs targeted against insect genes in transgenic plants has impaired growth and development,complete protection of the plants and efficient killing of the insects were not achieved due to two possible reasons:1) 21 bp siRNAs,the major processing products of dsRNA cleavage by Dicer,had little or no effect when fed to insects.the presence of an endogenous RNAi pathway in plants that processes dsRNAs into short interfering RNAs(siRNAs).2) Insects cannot amplify RNA silencing signals due to RNAdependent RNA polymerase(RdRP) genes are absent from the genomes of insects.