Pathways and vectors across the South Pacific and Aotearoa New Zealand: The Threat of Rapid Ōhi’a Death (ROD) to Metrosideros spp.

Invasive pathogens and insect pests are a current and existential threat to nations in the South Pacific. These invasive species can have potentially devastating and costly negative impacts, whether environmental, economic or cultural. In Hawaii, the emergence of rapid ‘ōhiʻa death (ROD), caused by two related pathogens Ceratocystis lukuohia and C. huliohia, has resulted in widespread death of the ‘ōhiʻa lehua tree Metrosideros polymorpha. Metrosideros spp. growing in Aotearoa NZ (e.g., pōhutukawa, rātā) and the wider South Pacific, are potentially susceptible to ROD. Trials undertaken by the USDA-ARS in Hilo, Hawaiʻi, are testing the susceptibility of Aotearoa New Zealand Metrosideros species to C. lukuohia. Ceratocystis lukuohia, the more aggressive of the two ROD pathogens. These are maximum challenge tests. Interestingly, preliminary testing has also indicated M. collina, a species found in the South Pacific may be susceptible to ROD.

The vectors and pathways for spread of ROD in Hawaii are generally well understood including bark beetles, ungulates, contaminated equipment, humans, and wind. However, the potential pathways and vectors for invasion in other South Pacific nations including Aotearoa NZ less understood.

Bark beetles are a major vector of the two pathogens in Hawai’i, and several native and exotic bark beetle species are found across the South Pacific Islands including Tonga, Fiji, Papua New Guinea, Solomon Islands, French Polynesia and New Caledonia. Ceratocystis fimbriata is the only known species in Aotearoa New Zealand, so while these Ceratocystis pathogens causing ROD in Hawaii are not currently found in Aotearoa New Zealand, a proactive strategy is to understand the risks they may pose should they reach the country. Therefore, our research aims are to understand what bark beetles and Ceratocystis spp. are in Aotearoa New Zealand; Identify new Ceratocystis strains; and provide an early warning of risk if ROD was to reach the country.

Traps have been deployed near Metrosideros species to determine which bark beetle species are associated with the trees. These beetles are also swabbed to detect Ceratocystis species that may already occur in Aotearoa New Zealand and other plant pathogens. We found at least seven bark beetle species were associated with Metrosideros spp. in Aotearoa New Zealand. This included the ambrosia beetle Xyleborinus saxeseni Reitter (Curculionidae: Scolytinae), known to vector ROD in Hawaii. To date, no new Ceratocystis strains have been identified.

There are benefits of this research, as it allows time to implement strategies to act quickly to eradicate or mitigate spread should ROD reach the country. For the South Pacific, a coordinated approach to understanding pathways and vectors, especially where Metrosideros species and related Myrtaceae occur, can provide an early warning of the arrival of ROD or new Ceratocystis stains (which are an emerging global threat). Overall, the initiative provides opportunities to more effectively protect our important native or economically important plant species.