Wax moth larvae demonstrate a high level of humoral immunity after envenomation by parasitoid Habrobracon hebetor

Abstract

A successful development of parasitoids on their insect hosts is determined by various factors, including the host's physiological response and proliferation of concomitant microbes. However, such interactions have not been sufficiently studied from the point of view of host immunity. In this report, we studied parameters of immunity in Galleria mellonella L. (Lepidoptera: Pyralidae) larvae after envenomation by Habrobracon hebetor Say (Hymenoptera: Braconidae), infection with the entomopathogenic fungus Metarhizium robertsii JF Bisch, SA Rehner et Humber (Hypocreales) and combination of these treatments. We utilized qPCR to analyze the expression of genes encoding antimicrobial peptides (AMPs), metalloproteinase inhibitors (IMPI), as well as reactive oxygen species-, stress- and apoptosis-related genes in the fat body and integuments of the larvae. The parasitoid's venom upregulates the expression of host genes coding for AMPs, NADP-H oxidase 4 (nox-4), impi, and heat shock proteins (hsp 70). Surprisingly, the envenomated insects responded actively to fungal infection by upregulation of galiomycin, gallerimycin, gloverin, cecropin-like, nox-4, and inhibitor of apoptosis genes in the fat body and/or integuments. We conclude that envenomated wax moth larvae retain a high level of humoral immunity in response to the infection. We discuss that these reactions could be beneficial for the parasitoid, as they prevent the development of opportunistic infections. However, an increase in these immune parameters does not defend wax moth larvae from infection caused by entomopathogenic fungi.