Susceptibility of mountain pine beetle (Dendroctonus ponderosae Hopkins) to gene silencing through RNAi provides potential as a novel management tool

The mountain pine beetle (MPB), Dendroctonus ponderosae, is an eruptive endemic forest pest that is undergoing substantial range expansion in response to recent climatic changes, breaching geographic barriers, exploiting novel hosts, and affecting millions of hectares of conifer forests in western North America. Current management approaches have been unable to keep pace with MPB population outbreaks, and novel and aggressive management responses are required as MPB's range expansion progresses. Gene silencing through RNA interference (RNAi) is an emerging pest management approach that is being developed for agricultural pests, and has also been shown to be effective against some xylophagous forest pests, including the southern pine beetle (SPB), D. frontalis. When essential genes are targeted, RNAi can cause rapid insect mortality; here we focus on evaluating its effectiveness in MPB. We identified reference genes for quantitative real-time PCR (qPCR) and validated RNAi responses in MPB by analyzing gene expression and beetle survival. Using an adult bioassay that combined oral ingestion and dermal absorption of dsRNAs targeting three genes (hsp, iap, and shi), we measure gene expression and demonstrate silencing, as well as insect mortality, following dsRNA exposure. All three genes were silenced and all treatment beetles died within 7 d. This validates reference genes for expression analyses and demonstrates that MPB, similar to the congeneric SPB, has a highly sensitive RNAi response. Additionally, we document sex-specific differences in gene expression for one of the three target genes, hsp; any differences in gene expression and subsequent mortality based on sex must be considered as this technology progresses as a pest management tool. RNAi causes rapid insect mortality when essential genes are targeted, is highly specific to the target pest, and has no environmental contamination risks, making it an attractive approach for further development in forest pest suppression.