Hyposoter didymator uses a combination of passive and active strategies to escape from the Spodoptera frugiperda cellular immune response.

An endoparasitic life style is widespread among Hymenoptera, and various different strategies allowing parasitoids to escape from the host encapsulation response have been reported. Species carrying polydnaviruses (PDVs), such as the ichneumonid Hyposoter didymator, generally rely on the viral symbionts to evade host immune responses. In this work, we show that H. didymator eggs can evade encapsulation by the host in the absence of calyx fluid (containing the viral particles), whereas protection of the larvae requires the presence of calyx fluid. This evasion by the eggs depends on proteins associated with the exochorion. This type of local passive strategy has been described for a few species carrying PDVs. Immune evasion by braconid eggs appears to be related to PDVs or proteins synthesized in the oviducts being associated with the egg. We report that in H. didymator, by contrast, proteins already present in the ovarian follicles are responsible for the eggs avoiding encapsulation. Mass spectrometry analysis of the egg surface proteins revealed the presence of host immune-related proteins, including one with similarities with apolipophorin-III, and also the presence of three viral proteins encoded by IVSPERs (Ichnovirus Structural Protein Encoding Regions).