The In Vitro and In Vivo Fungal Volatile Organic Compounds Associated with Rapid 'Ōhi'a Death and the Response of Xyleborine Ambrosia Beetles to those Compounds.

Abstract

Rapid 'ōhi'a death (ROD), caused by fungal pathogens Ceratocystis lukuohia and Ceratocystis huliohia, is devastating 'ōhi'a (Metrosideros polymorpha), the keystone forest tree species of Hawai'i. Ceratocystidaceae fungi produce fruity volatile organic compounds (VOCs) and sticky sexual spores, suggesting they are adapted for insect dispersal. In this study, we explored chemical signaling of the causal agents of the ROD disease complex and how those VOCs mediate ambrosia beetle (Coleoptera: Curculiondiae: Scolytidae)-plant interactions. Four species of ambrosia beetles (Xyleborinus saxesenii, Xyleborus affinis, Xyleborus ferrugineus, and Xyleborus perforans) are direct vectors of the ROD pathogens in addition to spreading the fungi indirectly through contaminated frass. To further understand the extent to which semiochemicals mediate relationships between agents of the ROD pathosystem, we identified fungal VOCs both in vitro and in vivo. We then tested behavioral responses of the ROD-associated ambrosia beetles to the odor of the fungi using still-air olfactometer assays. Similar fruity and banana-like fusel alcohols and acetates were produced by C. lukuohia and C. huliohia in culture and when colonizing 'ōhi'a seedlings, which could be detected one-day post-inoculation, nine days before symptom onset. X. affinis was attracted to both C. lukuohia and C. huliohia, while X. ferrugineus positively responded to C. lukuohia and X. perforans to C. huliohia. These findings suggest a semiochemical association and potential support for the hypothesis of coevolutionary history between Ceratocystis fungi and insects.