Native trees connect us to our forests. This is especially true of the ‘ōhi‘a lehua (Metrosideros polymorpha) — a native tree in Hawai‘i that grows naturally nowhere else on Earth. As one of the first trees to grow in land cleared by a lava flow, it builds forest ecosystems that provide habitat for hundreds of species, including 18 threatened or endangered birds. By intercepting rainfall, ‘ōhi‘a canopies let water seep into underground aquifers that supply water for downstream communities.
The ‘ōhi‘a embodies cultural values too. From the wood to the blooms, the ‘ōhi‘a is interwoven into traditional medicine, tools, woodworking, dance (hula) and lei — a wreath of natural objects worn in celebration or greeting.
Unfortunately, invasive fungi are threatening this biocultural tree and all it brings to the islands.
Invasive Fungi Cause a Deadly Disease
How these invaders first came to Hawai‘i remains a mystery. What is known — these two related fungal pathogens (Ceratocystis lukuohia and Ceratocystis huliohia) are not native to the islands. Once the fungal spores get inside an ‘ōhi‘a, the fungus grows and multiplies. It blocks the vascular system that transports water from ground to crown.
After symptoms of infection appear – brown leaves, black staining in wood, fruity odor in cut wood – death comes quickly. Often within a few months. This quick decline is why the resulting disease is called Rapid ‘Ōhi‘a Death. Today, this disease has killed over 1 million ‘ōhi‘a trees, mostly on Hawaiʻi Island, but with detections on Maui, Oʻahu, and Kauaʻi as well. At present, there is no known cure.
To battle this fungus, a collaborative group with university, nonprofit, state and federal partners, including the USDA Forest Service, continue to work together. Helicopters, insect investigations and educational festivals lead the response.
Aviation Maps the Spread
Getting a sense of the scale of the disease informs efforts to reduce Rapid ‘Ōhi‘a Death. And since 2016, helicopter surveys have been doing just that.
“Each island has a pilot and at least three people who are trained to conduct the surveys. These are folks that know the forests well,” said Brian Tucker, Rapid ‘Ōhi‘a Death Data Manager & GIS Specialist with the University of Hawai‘i Pacific Cooperative Studies Unit.
Every island gets a full-island survey twice a year, six months apart, and some islands do additional surveys of priority areas. For the Big Island, where the disease is most extensive, additional surveys focus on fringe areas where the disease has not taken hold. On Kauaʻi, where the fungus continues to spread, areas of expansion are surveyed every three months. Frequency is important because Rapid ‘Ōhi‘a Death impacts trees quickly – within a few months infected trees will be leafless.
“The crews look for the early phase, which is really short — a yellowing that quickly turns a brighter red and then goes into brown. These colors really stand out in the sea of green,” Tucker said.
While flying a few hundred feet above the canopy, the aerial survey specialists use tablets with digital maps that move in sync with the flight. With this digital mobile sketching map system developed by the Forest Service (also used to record tree mortality within the contiguous United States each year), they capture the location and degree of ‘ōhi‘a mortality.
“Throughout the year, I take in their data and work it into our tools that inform field operations,” said Tucker.
In this behind-the-scenes work, Tucker combines the inflight observations with high-resolution aerial imagery to create interactive maps that display where Rapid ‘Ōhi‘a Death is present and the severity of the associated ‘ōhi‘a mortality. Discoveries of a new outbreak may lead to specialists putting boots on the ground to gather samples from the trees suspected to have the disease. If DNA analysis of tree samples leads to a Rapid ‘Ōhi‘a Death diagnosis, the infected trees may be removed.
“With the digital mobile sketch mapping, which helps us keep an eye on the forests, we’ve been learning about this disease and exploring the best ways to mitigate it.”
Insects and Transmission
Forest Service entomologists and plant pathologists dig into the details of how the invasive fungi infiltrates trees and spreads. Some recent studies confirm that invasive ambrosia beetles are both direct and indirect agents of transmission.
“We saw ROD (Rapid ‘Ōhi‘a Death)-associated ambrosia beetles going to healthy trees and wondered if they could be involved in direct vectoring by carrying the fungus on their bodies,” said Kylle Roy, a Forest Service entomologist with State and Private Forestry, based in Hilo.
Roy and a few collaborators carried out a study that confirmed that these insects can carry fungal spores from an infected tree directly into a healthy tree’s vascular system. So they asked — how can we protect healthy trees from these woodboring beetles?
“After we figured out which beetles were involved with the spread of ROD, we wanted to investigate how to manage them. And right now, that's testing different repellents,” said Roy.
Roy has been looking into the potential use of repellants like pheromones. These chemicals send a message telling ambrosia beetles “this tree is full,” similar to work being done in California for some bark beetle species.
Roy’s work cross-pollinates with that of Marc Hughes, a research plant pathologist at the Pacific Southwest Research Station, working out of the Institute of Pacific Islands Forestry in Hilo. He led a study that confirmed a long-held belief – ambrosia beetles indirectly transmit the disease.
“The fungus is kind of locked in the tree itself. You need agents to release it, and that’s where ambrosia beetles play a role,” said Hughes. “They are drawn to the dead or dying trees infected with Rapid ‘Ōhi‘a Death and create frass as they bore in.”
The fungal spores within the dead trees mix into the sawdust-like frass as they tunnel. That frass then blows where the wind goes, including into tree wounds or onto the soil. From there, hiking boots can carry the disease into other forested areas. But Hughes’ research discovered something else.
“Kylle was part of this study with me, which confirmed the fungus in the frass can survive in certain environments for at least six months,” said Hughes.
“This is why it is important to clean your boots with 70% isopropyl alcohol” Roy added.
In other words, even half a year later or longer, fungal spores in frass can infect new ‘ōhi‘a if carried by the wind, clothing or vehicles to a tree wound.
Hughes is also part of an ongoing project to cultivate resistance to the disease. The ‘Ōhi‘a Disease Resistance Program includes the Forest Service’s Institute of Pacific Islands Forestry, Akaka Foundation for Tropical Forests and other partners. By collecting cuttings and seeds from survivor trees, the team grows seedlings in greenhouses, then inoculates them with the fungus searching for a natural genetic resistance.
“The goal is to find and build up disease resistance. We’re sampling in different geographies throughout the state. With these collections of locally adapted seed sources, we can build a diversity of resistant seed families for reforestation,” said Hughes.
Celebration Connects the Community
A final piece in the puzzle is ‘Ōhi‘a Love Festivals, creating awareness through community celebrations of the ‘ōhi‘a.
“The intention of the festivals is to not only to increase understanding about Rapid ‘Ōhi‘a Death, it's to celebrate the plant itself. Because it is a very culturally important plant species here in Hawai‘i,” said Ambyr Mikaye, Rapid ʻŌhiʻa Death Statewide Outreach Coordinator with the University of Hawaii at Manoa. “We provide information on the disease and how it spreads, but we also celebrate the ‘ōhi‘a with hula and music.”
As the lead organizer of the ‘Ōhi‘a Love Festivals, Miyake mixes fun with education. Events typically open with hula and include games and crafts, with specialists like Roy and Hughes staffing booths or giving presentations. Attendees at one event entered via a boot brush and spray station. The resulting questions taught the importance of decontaminating shoes, equipment and vehicles before entering a forested area. And another festival offered free ‘ōhi‘a trees if you learned five new things about conservation by talking with specialists.
Community outreach also happens in the classroom. These discussions dovetailed with a recent youth-led campaign to designate the ‘ōhi‘a lehua as the official Hawai‘i State Endemic Tree.
“That's probably one of the most rewarding things I've done,” said Miyake.
“It's extremely important to build community awareness. We can each do things that are more what we call here in Hawai‘i pono or ‘more right.’” I think empowering the people to make those decisions for themselves allows them to become part of something that’s trying to protect what they value.”
To learn more, click on these linked resources:
Rapid ‘Ōhi‘a Death 101 – University of Hawaiʻi
“Ambrosia beetles (Coleoptera: Curculionidae) can directly transmit the fungal pathogens responsible for Rapid ʻŌhiʻa Death” (2023) – Kylle Roy, et.al
The use of semiochemicals for attracting and repelling invasive ambrosia beetles (Coleoptera: Curculionidae) in ʻōhiʻa (Metrosideros polymorpha) forests (2023) – Kylle Roy, et. al
“Ceratocystis lukuohia-infested ambrosia beetle frass as inoculum for Ceratocystis wilt of 'ōhi'a” (2022) - Marc Hughes, et. al.
ʻŌhiʻa Disease Resistance Program: Combatting Rapid ʻŌhiʻa Death in Hawai‘i (akakaforests.org)