The Chemical Ecology of Rapid Ohia Death

Rapid ʻōhiʻa death (ROD) is a disease complex caused by two Ceratocystis fungi, C. lukuohia and C. huliohia, that is devastating the keystone tree of the Hawaiian Islands, ʻōhiʻa lehua (Metrosideros polymoropha). The causal agents of ROD were identified in 2015 and I began researching entomological aspects of the complex in 2016. Much like other Ceratocystis diseases, my colleagues and I suspected that beetles and frass might be involved in the system. Together, we identified four species of invasive ambrosia beetles (Coleoptera: Curculionidae) that contribute to the spread of ROD: Xyleborinus saxesenii, Xyleborus affinis, Xyleborus ferrugineus, and Xyleborus perforans. Both ROD-Ceratocystis fungi and the ambrosia beetles inhabit the xylem of ʻōhiʻa. When these beetles create their home galleries, they produce frass particles infested with resting chlamydospores that can be transported in the environment through the soil, wind, and water. Secondly, the beetles are capable of vectoring the fungi directly to stressed trees via viable propagules attached to their exoskeleton. The natural progression of this research was to investigate the chemical ecology of the system, therefore building the foundations for management strategies to reduce the spread of ROD. In addition, I satisfied my curiosity to explore the fungal mutualisms of these beetles through the use of phylogenetics.

In Chapter 1, I review the literature describing ROD and the four ROD-associated ambrosia beetle species. I report all of the research to date regarding ROD, including current monitoring and management strategies. Then, I introduce ambrosia beetles and the Xyleborini tribe, focusing on the life history of the ROD-associated beetles and current literature describing the use of semiochemicals to control them.

In Chapter 2, I determine the volatile organic compounds associated with the ROD Ceratocystis – ʻōhiʻa pathosystem and the response of the associated beetles to those compounds. I investigated the volatiles produced by C. lukuohia and C. hulihia in culture in addition to when inoculated into ʻōhiʻa seedlings. Then, I describe olfactometer assays to determine if the ROD-associated beetles are attracted to the volatiles emitted from ROD-Ceratocystis in culture.

In Chapter 3, I investigate semiochemicals for attracting and repelling ambrosia beetles in ʻōhiʻa forests. I describe separate trapping experiments, first, testing the attraction of beetles to 100% ethanol and 1:1 methanol ethanol. Second, we investigate the use of two beetle repellent products, one with verbenone and the other with verbenone + methyl salicylate active ingredients.

In Chapter 4, I describe the testing of the repellent, verbenone, in the SPLAT^® Verb formulation, to deter ambrosia beetle attack from both healthy ʻōhiʻa trees and trees infested with ROD-Ceratocystis. Over two field seasons, we monitored ambrosia beetle attacks on trees treated with verbenone and measured the abundance of verbenone released from the repellents over time during the first season.

In Chapter 5, I investigate the ambrosia fungi of the ROD-associated beetles and native Hawaiian ambrosia beetles on the Island of Hawaiʻi. We isolated a dozen fungal symbionts from the mycetangia of ambrosia beetles, most of which are first reports in Hawaiʻi, and use phylogenetics to investigate putative new species of Raffaelea and Ambrosiozyma.

Finally, in Chapter 6, I synthesize the results and future directions of the aforementioned chapters. Together, these dissertation chapters provide insights into ambrosia beetle monitoring and management strategies in Hawaiʻi and beyond. I describe the groundwork for understanding the pathosystem from a chemical ecology perspective and touch on the understudied world of Hawaiʻi fungi and potential pathogens.