Oral delivery of double-stranded RNA induces prolonged and systemic gene knockdown in Metaseiulus occidentalis only after feeding on Tetranychus urticae.

Metaseiulus (=Typhlodromus or Galendromus) occidentalis is an important biological control agent. Functional genomic studies on this predator have been hampered by the lack of reverse genetic tools such as RNA interference (RNAi). In the current study, we evaluated possible RNAi responses in M. occidentalis females by feeding double-stranded RNA (dsRNA) of RpL11, RpS2, RpL8, or Pros26.4 genes in 20 % sucrose solution. Females needed to subsequently feed on two-spotted spider mites (Tetranychus urticae) to elicit a nearly complete loss of egg production. The corresponding gene knockdown was robust, long-term, and was observed in the very few eggs produced (systemic or parental RNAi). Interestingly, dsRNA-mediated gene knockdown could not be induced if these predators were provided only the sucrose diet after ingesting dsRNAs; T. urticae had to be provided to elicit the RNAi response. However, the spider mite diet was not needed for sustaining the dsRNA-mediated gene knockdown once it commenced. Oral delivery of dsRNA will be a valuable tool for efficient genome-wide functional screens in this important predatory mite.