An egg parasitoid assesses host egg quality from afar using oviposition-induced plant volatiles

Parasitoids of herbivores exploit inducible plant volatiles to find plants with potential hosts from a distance, whereas at close range they typically use host-derived cues to pinpoint and identify suitable hosts. Here, we show, however, that the egg parasitoid Trichogramma japonicum assesses host egg quality far more efficiently by remotely using oviposition-induced plant volatiles (OIPVs). In olfactometer assays, female T. japonicum wasps showed a strong preference for the odor of rice plants carrying 2-day-old eggs of the rice leaf folder Cnaphalocrocis medinalis over the odor of plants with younger or older eggs, a preference that correlated with higher parasitism rates. In accordance with the preference-performance hypothesis, the offspring of T. japonicum showed superior performance in 2-day-old eggs, including shorter development times and higher eclosion rates. Volatile analysis revealed significantly increased emission of D-limonene and α-pinene from plants with 2-day-old eggs, and we found that synthetic versions of these two monoterpenes were highly attractive to the wasp. Knockout rice plants deficient in D-limonene and α-pinene synthesis lost their appeal to the wasps, but attraction could be restored by dispensing synthetic versions of the attractants alongside the knockouts. These findings reveal a novel and highly efficient host-assessment strategy in egg parasitoids, whereby plant-provided cues inform the wasps about host quality from afar. This discovery is illustrative of the clever strategies that have evolved out of plant-insect interactions and offers fresh ideas to optimally exploit plant traits for biocontrol approaches against C. medinalis, a major rice pest.