Water-seeking behavior in insects harboring hairworms: Should the host collaborate?

We explored the idea that hosts infected with manipulative parasites might mitigate the costs of infection by collaborating with the parasite rather than resisting it. Nematomorphs are usually considered to be manipulative parasites of arthropods because they cause hosts to seek an aquatic environment, which is needed by the adult parasite. We placed infected cricket hosts in situations of forced noncompliance and compared some fitness parameters (life expectancy, gonad development, and reproductive behaviors) in noncompliant hosts and hosts allowed to express parasite-induced behavior. Compared to uninfected controls, reduced survival was observed in both males and females from the two categories of infected hosts, collaborative or not. A substantial proportion of collaborative females produced eggs or had developed ovaries while such phenomena were never observed among noncollaborative ones. Collaborative females retained a nymphal phenotype, but adult males nevertheless courted and produced spermatophores to such females. However, collaborative females had difficulties mounting males, taking spermatophores and/or ovipositing. In contrast to females, all males were entirely castrated by the parasite regardless of their behavior, collaborative or not. Thus, bringing the parasite into water does not effectively mitigate the costs of infection for the host.