Identification of microRNAs by small RNA deep sequencing for synthetic microRNA mimics to control Spodoptera exigua.

Beet armyworm, Spodoptera exigua, is a major pest of cotton around the world. With the increase of resistance to Bacillus thuringiensis (Bt) toxin in transgenic cotton plants, there is a need to develop an alternative control approach that can be used in combination with Bt transgenic crops as part of resistance management strategies. MicroRNAs (miRNAs), a non-coding small RNA family (18-25 nt), play crucial roles in various biological processes and over-expression of miRNAs has been shown to interfere with the normal development of insects. In this study, we identified 127 conserved miRNAs in S. exigua by using small RNA deep sequencing technology. From this, we tested the effects of 11 miRNAs on larval development. We found three miRNAs, Sex-miR-10-1a, Sex-miR-4924, and Sex-miR-9, to be differentially expressed during larval stages of S. exigua. Oral feeding experiments using synthetic miRNA mimics of Sex-miR-10-1a, Sex-miR-4924, and Sex-miR-9 resulted in suppressed growth of S. exigua and mortality. Over-expression of Sex-miR-4924 caused a significant reduction in the expression level of chitinase 1 and caused abortive molting in the insects. Therefore, we demonstrated a novel approach of using miRNA mimics to control S. exigua development.