Volatiles Induced from Hypolepis punctata (Dennstaedtiaceae) by Herbivores Attract Sclomina erinacea (Hemiptera: Reduviidae): Clear Evidence of Indirect Defense in Fern

SIMPLE SUMMARY: Plants have developed many kinds of indirect defense mechanisms against herbivorous insects, through which the natural enemies are attracted by herbivore-induced plant volatiles to prey on these phytophagous species. There have been many reports regarding this mechanism in spermatophytes, but little is known in ferns. In this study, the relationship between the fern species Hypolepis punctata and the predatory insect Sclomina erinacea was studied. Employing field observations, plant volatile analysis, electrophysiological experiments, and behavioral experiments, we found that not only the plant can attract the assassin bug in the field, but compared with the healthy individuals, the fern being damaged by herbivorous insects also emitted several novel volatile organic compounds, which were strongly attractive to the assassin bug. The results indicate that ferns may also have indirect defense mechanisms using volatile organic compounds, and it is the first report of an indirect defense mechanism in fern. ABSTRACT: Plants have evolved various self-defense mechanisms against insect feeding. There are many reports regarding both direct and indirect defense mechanisms in seed-plant. However, only direct defenses on ferns were considered and the indirect defense mechanism has never been reported. In this study, it was observed that the fern Hypolepis punctata can attract the assassin bug Sclomina erinacea in the field. We collected and analyzed volatiles from H. punctata healthy individuals and the ones wounded by Bertula hadenalis, using dynamic headspace and GC-MS. We recorded the electroantennogram responses of antennae of S. erinacea to different standards of volatile compounds identified from the GC-MS analysis. We also analyzed the behavior of male and female S. erinacea adults in response to volatiles collected from H. punctata using a Y-tube olfactometer. The results showed that a number of volatile compounds were produced when the fern was damaged by B. hadenalis. Electroantennography and Y-tube olfactometer results showed that some herbivore-induced volatiles and volatiles from undamaged leaves could attract S. erinacea. Our research suggests that H. punctata can attract insect predators by releasing herbivory-induced volatile organic compounds, and for the first time we found ferns may also have indirect defense mechanisms using volatile organic compounds.