Development of RNAi in Insects and RNAi-Based Pest Control

In agricultural systems, insect pests can cause crop damage mainly through loss in yield or quality resulting in a loss in profits for farmers. Worldwide pests cost billions of dollars due to damage and use of pesticides. Chemical pesticides are still the major approach for controlling insect pests, but they are associated with significant hazards to the environment and human health. The alternative commercial biotechnological system relies mostly on the expression of Bacillus thuringiensis insecticidal proteins (Cry toxins). Its effectiveness however is threatened by the development of resistance in some species such as Ostrinia nubilalis (Lepidoptera, Pyralidae) and Heliothis virescens (Lepidoptera: Noctuidae) (Ferre and Van Rie, 2002; Baum et al., 2007). As a result, there is an urgent need to develop economically and ecologically sound alternatives for pest control. Gene silencing has been suggested as one of the new alternatives to reduce damage from insect pests. RNA interference (RNAi) is first described by Fire et al. (1998), and its mechanism lies in that a double-stranded RNA (dsRNA) introduced in an organism has the capacity to silence post-transcriptional genes (Hannon, 2002; Geley and Muller, 2004). RNAi is highly conserved in eukaryotic organisms (Fire, 2007). It is considered as a specific type of defence mechanism (Terenius et al., 2011). Four different types of RNAi have been described including short interfering RNAs (siRNAs), piwi-interacting RNAs (piRNAs), endogenous siRNAs (endo-siRNAs or esiRNAs), and microRNAs (miRNAs) (Terenius et al., 2011). To date, RNAi has been proven promising for research on gene function determination and gene knockdown in eukaryotes and medical control of cancers and viral disease (Huvenne and Smagghe, 2010). In insects, studies have mainly targeted the understanding of the RNAi mechanism, and the function, regulation and expression of genes. Introduction of dsRNA into an organism has been tested by using different techniques such as microinjection (Bettencourt et al., 2002;