Cuticle immunity and susceptibility to infection in Galleria mellonella larvae envenomed by the parasitoid wasp Habrobracon hebetor.

The ectoparasitoid wasp, Habrobracon hebetor, targets a range of Lepidopteran larvae and plays an important role in biological control efforts of insect pests. The venom of H. hebetor contains a variety of components with paralytic, neurotoxic and immune-modulatory properties. Herein, we explored the interactions between the entomopathogenic fungus (EPF) Metarhizium robertsii and the integument/cuticle defence reactions of wax moth larvae, Galleria mellonella, (Lepidoptera: Pyralidae) post-envenomation by H. hebetor to gain much needed insight into the mechanisms of disease susceptibility. Envenomation of G. mellonella led to a dramatic decrease in phenoloxidase (PO) enzyme activity and the transcription of proPhenoloxidase (proPO) cascade genes in the integument. Concurrently, M. robertsii expressed several virulence factors that were more active on the cuticle of immobilized (envenomed) insects compared to control insects. Interestingly, G. mellonella larvae envenomed by the parasitoid, and infected with the fungus, could still maintain high levels of antimicrobial gene expression in the cuticle. We present data on the complex interplay between an insect host, an entomopathogenic fungus, and the venom of an ectoparasitoid wasp.