A finely tuned strategy adopted by an egg parasitoid to exploit chemical traces from host adults.

Scelionid egg parasitoids can obtain reliable information on the presence of host eggs by discriminating host gender on the basis of chemical footprints of their co-evolved hosts, with a strong preference for the footprint left by host females. Based on the concept of dietary specialization and infochemical use in natural enemies, it could be predicted that host gender discrimination in specialist species belonging to the genus Trissolcus is further tuned to specific cues from distinctive chemical traces left by host females as a consequence of copulation and/or oviposition. To test this hypothesis we used the system Murgantia histrionica - Trissolcus brochymenae. Our results showed that the females of the egg parasitoid search intensely on chemical traces left on the substrate by host females that had mated but had not yet laid host eggs compared with the chemical traces left by virgin or parous host females. This preference for mated females that had not yet laid host eggs was strictly related to the transfer of sperm and associated substances from males to females during copulation. The compounds that mediated the arrestment response of T. brochymenae females are part of the host cuticle, and those that play a role as gender-specific cues seemed to be present in the legs of the host adult. This result represents an interesting new piece of information regarding the exploitation of indirect host-related cues by egg parasitoids. It reveals the existence of a finely tuned strategy that allows the parasitoid to find newly laid host eggs, as chemical traces left by mated host females that have not yet laid eggs are strongly correlated with the moment of oviposition.