Improving parasitism success of a weakly virulent parasitoid strain.

Endoparasitoids possess a whole set of virulence factors to counter the immune response of their host, among which can be found venom, endosymbiotic viruses and ovarian proteins. Depending on the species, some factors are expected to be less necessary than others. Notably, venom is reported as inessential in some parasitoids bearing viruses. We investigated the virulence factors of Cotesia typhae, a gregarious endoparasitoid of Sesamia nonagrioides, using highly and poorly virulent strains (CtV+ and CtV-). We tested if virulence of CtV- toward a reference strain could be improved by superparasitism (two females per host) or by injection of CtV+ virulence factors (venom and/or ovarian fluid). The parasitism success of CtV- could be increased by superparasitism with one female CtV+ confirming the importance of the virulence factors. Superparasitism with at least one CtV+ female decreased each female's reproductive success and increased the number of non-viable offspring revealing larval competition. Parasitism order impacted the offspring proportion of each strain in the progeny, suggesting adaptation of egg-laying behavior in response to an already parasitized host. Injection of CtV+ venom or ovarian fluid alone maintained CtV- success unchanged, but their combination increased it to the one of CtV+. Altogether, this study hints towards larval competition, potentially specific to CtV+ strain, and suggests that C. typhae females can adapt their oviposition behavior based on the parasitic status of their host. It also demonstrates that, like several other Cotesia species, C. typhae resorts to a combination of venom and ovarian fluid to overcome the host's immune responses.