Effectiveness of dsRNA versus siRNA in RNAi mediated knock-down in western corn rootworm ( Diabrotica virgifera virgifera)

Recent reports that RNAi can be used for in-plant control of western corn rootworm (WCR) have created an interest in understanding the RNAi mechanism in this important pest.   In several insects, both long dsRNA precursors and processed siRNAs can cause knockdown of gene expression.  In this report, we studied the effectiveness of dsRNA versus siRNA targeted against the V-ATPase subunit C gene in both WCR larvae and adult beetles.  In 9-day diet feeding assays, dsRNAs of at least 50-bp resulted in high levels of larval mortality.  In contrast, 15-, 25-, or 27-bp dsRNAs and pooled 21-bp siRNAs caused no oral mortality of WCR larvae. Similarly, 174-bp dsRNA caused 100 percent mortality of adult beetles while mortality of beetles exposed to siRNA was not different from negative control mortality.  Further, when adult beetles were fed siRNA, there was no effect on the level of  V-ATPase  C mRNA at day 5, whereas WCR beetles fed with the 174-bp dsRNA exhibited ~35-fold reduction in V-ATPase C mRNA. Similar results were found with the 174-bp dsRNA/siRNA injections where we observed ~100-fold reduction in V-ATPase C mRNA level in beetles injected with the 174-bp dsRNA and no change in V-ATPase C mRNA in beetles injected with siRNA. Our results suggest that only longer dsRNA is effective in triggering knock down of V-ATPase C mRNA and causing WCR mortality.  These results have implications for optimizing plant-delivered RNAi for rootworm control.