The Argentine stem weevil (Listronotus bonariensis) is an economically significant pasture pest in New Zealand, primarily controlled by Microctonus hyperodae; a parasitoid wasp biological control agent. M. hyperodae parasitises adult weevils and develops within the host, killing it as the parasitoid larvae emerges. Initial parasitism rates were as high as 80% but are now declining significantly, resulting in severe pasture damage totalling an estimated NZ$160M per annum. This is the first reported example of resistance in a biological control system worldwide, and a genetic basis for this parasitism resistance is suspected. Resistance to parasitism is most likely to be either a barrier preventing the parasitism event itself, or a host-mediated immune response towards the oviposited parasitoid egg.
Dual-species RNAseq was used to investigate the possibility of a host immune response toward the parasitoid egg. Results show that while parasitism causes changes in expression in a small subset of genes in the L. bonariensistranscriptome; these are involved in metabolism, regulation of fertility, and the innate immune system (against potential bacterial infection). Dual-species transcriptomic techniques also allowed for gene expression analysis of the simultaneously developing parasitoid egg, revealing changes to metabolism, epigenetic regulation and the initiation of nervous system development. The absence of genes involved in the cellular immune system, responsible for immune response and subsequent encapsulation of parasitoid eggs in Drosophila melanogaster, and the similarity of parasitism-induced changes with hosts of model-parasitoid Nasonia vitripennis, which do not display parasitism resistance, implies resistance is unlikely to be a post-parasitism immune response.