Deluded Zombies: Induced Behavioral Modification in a Cobweb Spider Does Not Increase the Survival of Its Parasitoid Wasp

Behavioral modifications induced by parasites have been extensively documented across multiple taxa. However, a major challenge is to experimentally determine whether such manipulations confer an adaptive advantage to the parasite. Behavioral alterations in spiders attacked by ichneumonid wasps are characterized by the construction of modified webs. These modified webs can enhance the survival chances of the parasitoid by reducing mortality due to natural enemies and environmental factors during the pupal stage. Additionally, some modified webs offer extra protection by keeping a centrally attached leaf shelter, originally used by the spider as a refuge, where parasitoids can build their cocoon. In these webs, it is possible that web modifications are not critical for the survival of parasitoid pupae. We evaluated the hypothesis that modifications made by parasitized spiders in a web with leaf shelter do not improve the survival of the parasitoid and presented details of behavioral modifications induced by the wasp Zatypota alborhombarta (Ichneumonidae) in its host spider Cryptachaea migrans (Theridiidae). We observed that modified webs built by parasitized spiders had silk thread around the web shelter, reduced vertical lines, and an increase in forked distal ends of lines. However, these changes in the host web did not improve the survival of parasitoid pupae. Our results indicate that the benefits of host behavioral modification for the parasitoids may vary across different host species. In addition, we suggested that the effects of altered web designs may depend on specific host-created web features and environmental factors like predation pressure. Significance statement Behavioral manipulations of hosts induced by parasites are often suggested as an adaptive trait, increasing the fitness of the parasitic organism. Still, the influence of host characteristics on the parasite’s fitness is rarely experimentally assessed, raising questions about the extent of the adaptive nature of manipulation. Here, we demonstrate for the first time that behavioral modifications induced by Ichneumonidae wasps in a cobweb spider do not increase the survival of the parasitoids. We argue that the presence of shelters in these webs, used by parasitoids in their pupal stages, provides a safe environment for development, independent of other modifications. Furthermore, we suggest that the behavioral modification reflects the maintenance of a phylogenetically conserved trait.