Neonicotinoids, such as imidacloprid, are nicotinic acetylcholine receptor (nAChR) agonists with potent insecticidal activity. Since its introduction in the early 1990s, imidacloprid has become one of the most extensively used insecticides for both crop protection and animal health applications. As with other classes of insecticides, resistance to neonicotinoids is a significant threat and has been identified in several pest species, including the brown planthopper,Nilaparvata lugens, a major rice pest in many parts of Asia. In this study, radioligand binding experiments have been conducted with whole-body membranes prepared from imidacloprid-susceptible and imidacloprid-resistant strains ofN. lugens. The results reveal a much higher level of [³H]imidacloprid-specific binding to the susceptible strain than to the resistant strain (16.7 ± 1.0 and 0.34 ± 0.21 fmol/mg of protein, respectively). With the aim of understanding the molecular basis of imidacloprid resistance, five nAChR subunits (Nlα1–Nlα4 and Nlβ1) have been cloned fromN. lugens.A comparison of nAChR subunit genes from imidacloprid-sensitive and imidacloprid-resistant populations has identified a single point mutation at a conserved position (Y151S) in two nAChR subunits, Nlα1 and Nlα3. A strong correlation between the frequency of the Y151S point mutation and the level of resistance to imidacloprid has been demonstrated by allele-specific PCR. By expression of hybrid nAChRs containingN. lugensα and rat β2 subunits, evidence was obtained that demonstrates that mutation Y151S is responsible for a substantial reduction in specific [³H]imidacloprid binding. This study provides direct evidence for the occurrence of target-site resistance to a neonicotinoid insecticide.

RNAi is a potent technique for the knockdown of target genes. However, its potential off-target effects limit the widespread applications in both reverse genetic analysis and genetic manipulation. Previous efforts have uncovered rules underlying specificity of siRNA-based silencing, which has broad applications in humans, but the basis for specificity of dsRNAs, which are better suited for use as insecticides, is poorly understood. Here, we investigated the rules governing dsRNA specificity. Mutational analyses showed that dsRNAs with >80% sequence identity with target genes triggered RNAi efficiently. dsRNAs with ≥16 bp segments of perfectly matched sequence or >26 bp segments of almost perfectly matched sequence with one or two mismatches scarcely distributed (single mismatches inserted between ≥5 bp matching segments or mismatched couplets inserted between ≥8 bp matching segments) also able to trigger RNAi. Using these parameters to predict off-target risk, dsRNAs can be designed to optimize specificity and efficiency, paving the way to the widespread, rational application of RNAi in pest control.