Protecting Hawaiʻi’s only native palm from a new threat

April 15, 2025

By Jon Letman, Bulletin Editor

Close your eyes and think of the Hawaiian Islands. What do you see? Perhaps a coconut palm swaying in the breeze? Coconut palms in Hawai‘i are so common, they almost go unnoticed, even as their towering arched figures evoke images of tropical paradise—pacific, serene, and abundant. Given their ubiquity, many are surprised to learn that Cocos nucifera or niu in Hawaiian may, in fact, not be native at all.

It is the Pritchardia, loulu in Hawaiian, that is Hawai‘i’s only native palm genus. Currently, there are at least 23 species of loulu endemic to Hawai‘i, with four additional species known from other Pacific Islands¹.

Hawai‘i is the epicenter of loulu diversity and it is fitting that NTBG houses the largest and most diverse collection with 22 of the 23 endemic species. Loulu are found at all four NTBG Hawai‘i gardens, but they are most numerous in McBryde Garden—more than 700 trees.

Dr. Seana Walsh, conservation scientist and curator of living collections, explains that NTBG prioritizes the collection and conservation of rare loulu species including Pritchardia flynnii, a Kaua‘i single island endemic with an estimated 400 trees left in the wild. The loulu collection is not only an invaluable resource for research, it serves as a living seed bank for a genus whose seeds are not ideal for dried or frozen storage. It’s also a source for genetic material to be shared with other botanical institutions.

NTBG’s loulu conservation dates back to at least the late 1980s when longtime NTBG botanists Steve Perlman and Ken Wood explored far and wide across some of Hawai‘i’s most remote and inaccessible habitat including steep valleys and high sea cliffs where they used an innovative webbing-line method that synched around loulu trunks to climb and collect seeds. Pritchardia conservation gained momentum with the establishment of the Hawai‘i Plant Conservation Center at NTBG in 1989.

NTBG staff and partners have successfully collected and are growing two extremely rare loulu species endemic to east Maui—P. woodii and P. arecina. Rare loulu species have long grown in and around Limahuli Garden and Preserve as well as Kahanu Garden and Preserve, contributing greatly to the genetic diversity, health, and vigor of the overall collection.

Invaluable research assets

NTBG’s loulu collection also provides research material for scientists and students like University of Hawai‘i masters student Makoa Elgin who is studying hybridization patterns and breeding systems. The collection is also the focus of Dr. Susan Fawcett, an NTBG postdoctoral researcher and botanist at the University of California, Berkeley and Jepson Herbaria. Susan spent much of 2024 on Kaua‘i modeling the origin and evolution of twelve Hawaiian species including loulu.

Armed with Pritchardia pollen records, Susan is investigating the dominant role of loulu in Hawai‘i’s ancient coastal forests. “It’s hard to overstate what an important role ecologically loulu had in the history of Hawai‘i,” she says, pointing to the loulu’s sturdy fronds and trunk which are used for construction and thatching material, for dye, flower lei, spears, mats, fans, and pahu (drums).

Loulu have also been used for altars or small heiau dedicated to specific deities and to make honorific floral decoration called ho‘oulumāhiehie, a practice that continues today. The sweet inner flesh of seeds can be eaten like young coconuts. Research indicates that it was the loulu, not the coconut palm, that was most common in pre-Western contact Hawaiian homesteads. 

Susan says she first became interested in studying loulu after encountering what she calls “NTBG’s spectacular collection.” As a scientist, this diversity allows her to familiarize herself with hundreds of palms as distinct individuals, closely examining them for subtle variations in height, length of petioles and inflorescence, fruit and flower size, color, appearance, growth habits and more. Because each tree is tagged with data, she can study loulu in a way not possible elsewhere, gaining a better understanding of the adaptive advantages of evolving on a remote island or the role of fruit size in seed dispersal.

The loulu at NTBG provide the tools needed to make taxonomic assessments (determining if species should be lumped together or split apart), which, Susan notes, has serious implications for conservation policies and environmental law.

“There’s no way that I’d be able to do this if I was working only with herbarium specimens,” Susan says. “I don’t think I would have been able to get to know these plants in the way that I can when I walk among them.” As one example, she points to a photo of a loulu species P. glabrata which includes 89 individuals she can easily examine in the garden.

Because loulu have thick, large fronds, they are difficult to collect as dried herbarium vouchers and optimum long-term seed storage conditions are not well known, growing collected seeds into plants is our best option until more is known. This makes NTBG’s living collection akin to a library, a classroom, and a living seed bank. It is the collection that also allows for the perpetuation of cultural knowledge and illustrates why the seeds need to be collected, protected, and grown for outplanting in restoration sites where some species are down to single-digit populations².

Today, few people ever see loulu growing in the wild but they can be found in Hawai‘i’s forests, albeit in habitat that has been degraded by rats, pigs, and feral ungulates which disrupt the understory and alter forest composition. In lower elevations and along the coast, humans have developed areas once populated by loulu.

When the first humans settled the Hawaiian Islands, they most likely encountered forests and groves of loulu. But with humans came Rattus exulans—the Pacific rat, a ravenous pest that can rapidly and dramatically alter ecosystems. Rats quickly developed a fondness for loulu seeds, leading to greatly reduced wild regeneration.

New Pest in Town

Rats, pigs, and other animals have fed on native Hawaiian plants for centuries, but there’s a new pest in town, one that threatens Hawai‘i’s most cherished plants including the common coconut palm and the loulu. Named for its favorite food, the coconut rhinoceros beetle³ (CRB) is native to East Asia, but began moving through the Pacific in the 1940s, spread by human activity, on vessels and vehicles, and inside organic material like mulch and potting soil.

In 2007, CRB was detected in Guam, and in the Northern Mariana Islands a decade later. CRB was found in Hawai‘i at the end of 2013 and has since invaded Vanuatu and the Marshall Islands where it has proven to be fatal to trees causing major cultural and economic impacts.

In 2023, CRB was discovered on Hawai‘i Island and Maui⁴, but no further spread has been detected in almost a year or more. After CRB was found near Kaua‘i’s airport in May 2023, NTBG took quick action to hang and monitor a pheromone-baited trap designed for early CRB detection. In January 2024, the first coconut rhinoceros beetle was found in a trap at NTBG headquarters on Kaua‘i and by early March it had spread into the Lāwa‘i Valley.

Loulu is on the menu

Dr. Mike Melzer, principal investigator with the University of Hawai‘i’s College of Tropical Agriculture and Human Resilience’s Coconut Rhinoceros Beetle Response program, describes CRB as one of the most impactful invasive insects in the Pacific, warning that Kaua‘i today is where O‘ahu was at roughly five years ago.

Mike says that although CRB have a limited flight range and are generally “lazy” insects content to spend their days in a mulch pile eating and breeding, they pose a major threat to coconut, loulu, and other palms with a diameter greater than six inches. He notes that if coconut or loulu are unavailable, CRB have been known to eat hāpu‘u tree ferns, hala (Pandanus), kalo (taro), mai‘a (bananas), and other crops. Even if loulu is not the coconut rhinoceros beetle’s preferred food, Melzer says, “loulu is certainly on the menu for CRB, unfortunately.”

When the beetles burrow into the center of a palm, they feast on the sap, destroy the central meristem, leaving the tree weak and vulnerable to infection or killing it outright. 

These voracious feeders and stealthy hitchhikers can expand their territory through the movement of potted plants, green waste, compost, and mulch where eggs and larvae thrive and so, Mike says, early detection and response is vital. That means setting and monitoring traps, carefully managing potential breeding sites, and watching for signs of beetle damage.

Mike says that on O‘ahu the threat to coconut palms has garnered the most attention, but loulu also face great risk. He calls this an “all hands on deck moment.”

As CRB Response encourages greater awareness and scientists look for innovative ways to combat this threat, Mike says, “the more people we have doing treatments, the better,” adding that O‘ahu’s greatest success in fighting CRB has been at the landscape-scale level where full—not patchwork—treatment has been possible.

Look closely, act fast

Recognizing the grave threat to its living collections, NTBG staff have had no choice but to quickly familiarize themselves with CRB early detection and rapid response. From the first trap that was set until the present moment, NTBG horticulture and ground staff have had a crash course in monitoring for CRB damage, shredding mulch and compost management, and taking precautions to avoid spreading any material that could serve as a breeding site.

Ever since the first CRB detection at NTBG headquarters in 2024, McBryde Garden foreperson Leslie Matsumoto and horticulture technician Laura Mansor have spent countless hours checking traps, inspecting trees and painstakingly wrapping netting as a protective barrier around the crowns of priority loulu species. Additionally, they have been bagging flower buds with nets and, when the loulu seeds reach the size of olives or small plums, they switch to mesh wire cages to protect the seeds from rats. 

Defending these valuable seeds from beetles and rats, ensures more seeds are available for propagating palms that can be managed ex situ (in the garden), outplanted in the wild, and shared with conservation partners.

Over the last year, NTBG has sent more than 600 loulu seeds representing six species to institutions including Lyon Arboretum, Singapore Botanic Gardens, San Diego Zoo, and the Montgomery Botanical Center, as well as NTBG’s own Kampong in Miami and Kahanu Garden on Maui.

Fighting beetles with dogs and drones

In the months since CRB reached the Lāwa‘i Valley, McBryde Garden has welcomed Conservation Dogs of Hawai‘i, an environmental nonprofit that visits suspected or infested sites with canine sleuths trained to detect CRB. Over several visits, the dogs have not detected CRB larvae, but the search benefits both the dogs and the collections.

Meanwhile, some of the garden’s coconut palms (but not the more sensitive and rare loulu) have been treated with an injected systemic insecticide which has proven effective but moves slowly through the palm. 

Last December, NTBG partnered with the Aloha ‘Āina Drone Company which visited McBryde Garden with a drone equipped with a spray nozzle that safely and surgically applied a contact insecticide to the crowns of over 90% of the targeted palms on NTBG’s south shore properties. The difference between treating loulu by hand using a lift boom and spraying with a drone was tremendous. The drone was able to reach the crown of even the tallest palms, in hard-to-reach areas under dense canopy and along steep cliffs, reaching dozens of palms in minutes rather than days. In less than a week, the drone treated nearly 650 loulu palms.

Hawaiian palms are Hawaiian heritage

Why is protecting NTBG’s loulu collection so urgent? “The vast majority of the diversity of Pritchardia is in Hawai‘i,” explains Seana Walsh. “They are our only native palm genus so getting more native loulu on the landscape and in horticulture should happen.”

Tobias Koehler, director of South Shore gardens, points to the cultural importance of loulu. He says that while much of the CRB conversation has understandably been focused on the better-known coconut palm, loulu are a unique life form, without which Hawai‘i is incomplete. “We’re here to sound the alarm that loulu are special and irreplaceable,” says Tobias. “We need others to join us to rally to the cause of preservation of this Hawaiian palm.”

While the battle against the beetles continues, Leslie is cautiously hopeful that CRB can be contained. With a dozen traps on NTBG’s south shore properties, Garden staff are catching between one and up to eight beetles a week and while there has been damage observed, to date, no palms have been killed by CRB in the Lāwa‘i Valley.

CRB has been found on the north shore of Kaua‘i, but as of February 2025, it has not been detected at Limahuli Garden or Kahanu Garden on Maui. Staff at both gardens continue to take preventative measures (quarantining and wrapping) to protect these sites as they are both home to rare and single-island endemic loulu, many from wild collected sources. 

If CRB gets out of control on Kaua‘i, that could severely impede or prevent NTBG’s horticulture center in the Lāwa‘i Valley from sending plants to Maui or Limahuli. Kahanu Garden and Preserve director Dr. Mike Opgenorth says the potential disruption underscores the need to build capacity at each garden rather than being dependent on centralized operations.

Meanwhile, NTBG has taken every preventative measure recommended, from careful monitoring and management, safeguarding plants from rats and beetles, and an aggressive effort to promote the study, conservation, and expansion of collections of loulu, Hawai‘i’s one and only native palm.

[1] P. thurstonii, P. pacifica, P. mitiaroana, P. tahuatana, native to Fiji, Tonga, the Cook Islands, and French Polynesia

[2] For example, Pritchardia viscosa

[3] Oryctes rhinoceros (Scarabaeidae)

[4] As of early April 2025, CRB has not been detected on Moloka‘i or Lāna‘i