Development of a robust dsRNA feeding methodology in
insects that mimics the results obtainable with C. elegans
(where efficient suppression of gene expression by orally
delivered dsRNA is routine) is a prerequisite for utilization
of RNAi for crop protection against insect pests.
Turner et al. [31] provided a convincing demonstration
of RNAi effects after dsRNA feeding in larvae of the
light brown apple moth (Epiphyas postvittana). dsRNAs directed against carboxyesterases were incorporated into
an artificial diet. Gene repression was observed after two
days of feeding, and maximal repression occurred after
seven days. These genes are thought to be gut-expressed,
and thus only a local RNAi effect was required for repression.
However, in the same investigation, knockdown of a
gene expressed in the adult antenna could be achieved
through feeding dsRNA to larvae, demonstrating a persistence
of the RNAi signal throughout the larval and
adult stages and a systemic spread of RNAi signal from
the gut to the antennae. In contrast to these positive
results, an earlier report showed that midgut aminopeptidase-
N gene in larvae of the lepidopteran Spodoptera
litura was efficiently downregulated by microinjection of
dsRNA into the insect haemoceol but stated that attempts
to feed dsRNA were unsuccessful in generating an RNAi
response [28], although no details of methodology were
given. An RNAi response after feeding dsRNA has also
been reported in the bug Rhodnius prolixus (Hemiptera),
where a salivary gland transcript encoding nitroporin 2
(NP2) was targeted both by oral delivery of dsRNA and by
microinjection [32]. Both treatments produced downregulation
of NP2 expression; however, microinjection was
more effective (75% reduction in gene expression) than
dsRNA feeding (42% reduction).