RNA interference (RNAi) has been used for agricultural insect pest control based on silencing of targeted insect genes. However, the effectiveness of RNAi and its applications in insect pest control remain challenging. Here we review factors that may affect the effectiveness of RNAi application, including the variability in RNAi efficacy among different insect species, a limited understanding of double-stranded RNA (dsRNA) uptake and systemic RNAi mechanisms, and the effective delivery of dsRNA in field conditions. Furthermore, we summarize recent progress in RNAi strategies for crop protection, discuss the advantages and disadvantages of RNAi-based insect control, and propose potential strategies to increase the effectiveness of RNAi in insect control.