756 Invasive Stink Bugs and Related Species (Pentatomoidea)
yielded proteins with activity on other groups of insects, including plant bugs (Baum et al. 2012). These
advanced Bt genes remain under testing agreements as on-going research validates and qualifies perfor-
mance in the field and laboratory.
16.4.2.2 RNA Interference
RNA interference (RNAi) could provide an alternative strategy for the management of insect pests
(Gordon and Waterhouse 2007, Price and Gatehouse 2008). This post-transcriptional gene silencing
phenomenon employs a conserved pathway found in eukaryotic cells by which exogenously applied and
endogenously expressed double-stranded (ds) RNAs direct the degradation of complementary endog-
enous messenger RNA (mRNA) transcripts within the cell, resulting in sequence-specific gene sup-
pression (Fire et al. 1998, Hannon 2002, Tomari and Zamore 2005). This conserved RNAi machinery
includes RNAse III-like proteins referred to as Dicer or Dicer-like proteins that process long dsRNAs
to 21- to 24-bp silencing (si) RNA duplexes. These siRNA duplexes are loaded into a multi-protein com-
plex called the RNA-induced silencing complex (RISC) where the passenger (sense) strand is removed
and the guide (antisense) strand remains to target mRNA for silencing. The guide strand in the RISC
enables base pairing of the complex to complementary mRNA transcripts that are then subject to enzy-
matic cleavage by a class of proteins referred to as Argonaute proteins, resulting in arrest of mRNA
translation.
RNAi-mediated suppression of essential genes in insects via ingestion of dsRNA can lead to increased
mortality (Baum et al. 2007, Whyard et al. 2009). In sensitive insects, this environmental RNAi response
requires a ≥ 21 nucleotide match between the dsRNA sequence and the target gene mRNA transcript
(Whyard et al. 2009, Bolognesi et al. 2012, Bachman et al. 2013). In addition, multiple barriers exist
that block the environmental RNAi response in non-target organisms, including mammals and other
vertebrates (Petrick et al. 2013). The aggregate of these factors results in a technology that has potential
for great selectivity towards susceptible insect species, more so than any insecticidal agent conceived
to date. Commercial development of next-generation rootworm-protected corn hybrids that combine an
RNAi-based trait with multiple Bacillus thuringiensis insecticidal protein-based traits for corn root-
worm pest management is in progress (Kupferschmidt 2013).
The introduction of dsRNA into nymphs and adults via injection is generally effective in triggering an
RNAi response in hemipteran species (e.g., Mutti et al. 2006, 2008; Walker and Allen, 2011; Zha et al.
2011; Yao et al. 2013). Target gene-specific silencing following ingestion of dsRNA has been reported in
the triatomine bug, Rhodnius prolixus (Stål) (Araujo et al. 2006); the pea aphid, Acyrthosiphon pisum
(Harris) (Shakesby et al. 2009; Whyard et al. 2009; Mao and Zeng 2012, 2014; Sapountzis et al. 2014);
the peach aphid, Myzus persicae (Sulzer) (Mao and Zeng 2014); the cotton aphid, Aphis gossypii (Glover)
(Gong et al. 2014); the brown planthopper, Nilaparvata lugens (Stål) (Chen et al. 2010; Li et al. 2011);
the potato–tomato psyllid, Bactericera cockerelli (Šulc) (Wuriyanghan et al. 2011); the corn planthop-
per, Peregrinus maidis (Ashmead) (Yao et al. 2013); and the grain aphid, Sitobion avenae (F.) (Zhang et
al. 2013) among others (Christiaens and Smagghe 2014). Dietary concentrations of dsRNA required for
silencing and/or lethal phenotypes in hemipteran species via feeding vary widely and tend to be at least
three orders of magnitude higher than effective concentrations observed with sensitive coleopteran spe-
cies (Baum and Roberts 2014). Furthermore, different groups working with the same species and gene
targets have reported conflicting results, suggesting that the response of hemipteran species to ingested
dsRNAs is neither robust nor consistent (Christiaens et al. 2014). Among hemipteran species, the [sap-
sucking] sweetpotato whitefly, Bemisia tabaci (Gennadius), appears to be one of the more responsive to
ingested dsRNAs. Both siRNAs and dsRNAs exhibit activity in whitefly bioassays employing an artifi-
cial diet (Upadhyay et al. 2011), and transgenic tobacco plants expressing a Bemisia v-ATPase A dsRNA
exhibit protection from whitefly feeding damage in controlled environment tests (Thakur et al. 2014).
Recent studies, however, reinforce the conclusion that this is the exception to the rule and that hemipteran
species, in general, are either unresponsive to ingested dsRNAs targeting essential genes or require high
dietary dsRNA concentrations in the 50 –1,000 ppm range for a significant effect on mortality, growth
inhibition, or fecundity (Christiaens et al. 2014, Gong et al. 2014, Sapountzis et al. 2014, Wan et al.
2014). Rather than target essential genes, opportunities may exist to restore sensitivity to insecticides