The Current Geographic Distribution of the Hala Scale, Thysanococcus pandani Stickney, throughout the Hawaiian Islands
Posted on Jul. 1, 2025 / Invasive Species, Native Species, Plant Pests, Arboriculture / Subscribe 0
By: Mason Russo and Zhiqiang Cheng*
Introduction: The hala scale, Thysanococcus pandani Stickney (Halimococcidae) was first detected in Hāna, Maui in 1995 on hala trees (Pandanus tectorius). These palm scale insects were initially described from specimens collected in Java and Singapore on trees in the Pandanus genus. Populations have been recorded on multiple species within the Pandanus genus, while other species are resistant to infestations. During a 2024 trip to Singapore and Indonesia, six Pandanus spp. were found to have infestations of T. pandani, along with two resistant varieties. On Maui, T. pandani did not infest a Pandanus vandersmeeschii tree native to Mauritius in a botanical garden. Thysanococcus pandani has both females and males and is not a parthenogenic scale insect, potentially decreasing its ability to spread quickly to new localities. The crawler stage of T. pandani can spread via wind and through the movement of infested material.
Hala scale adults are around 0.5mm in size and appear to the naked eye as a black dot within the vein lines of a Pandanus leaf, with or without a white waxy fringe (Figure 1). Both adult males and females are present, with females having three instars and males having five instar stages. Both stages are flattened ventrally and are dorsally convex, with crawlers emerging from the hard structure that the females form on the leaves. As these are found in tropical areas, there is potential for multiple generations a year, though they have low fecundity. In Hawai‘i, infestation usually begins with a few adults that are visible with the naked eye on the underside of the leaf. This causes minor discoloration and is noticeable at a close distance. When there are high densities within the canopy, yellowing is visible, and leaves can have aggregations of white waxy fringe that form around T. pandani adults. The fringe is usually on the underside of the leaf, and populations of T. pandani can number in the thousands on leaves of adult trees.
Figure 1: Images of T. pandani at no magnification with top and bottom of the leaf on the left and right side, respectively (top image) and a photo at 10x magnification (bottom image).
Hala forests are indigenous across many Pacific Islands, as their seeds can be dispersed by both animals (zoochory) and flotation, facilitating their arrival before Polynesian contact. These seeds are viable for months and are able to eventually form coastal forests after establishment on new islands. These coastal forests were present across the Hawaiian Islands prior to Polynesian arrival, but their range has been reduced due to development and invasive species. The hala tree is a Hawaiian canoe plant that spread across the Pacific Islands where Polynesian voyagers traveled. In the Hawaiian Islands and the Hawaiian language, lauhala refers to the leaf of hala trees. The practice of ulana, or weaving, was integral to Hawaiian culture and exploration, as the sails on their boats were made of woven hala. For many Native Hawaiians, the practice of ulana lauhala is integral to their survival, folklore, continuation of culture, and part of their identity. Outside of weaving, the roots and seeds were used for traditional medicine, food, and cordage. A lauhala lei, or necklace, was given to a person going through a transition in their life. Polynesian voyagers were able to smell the scent of hala fruit as they approached the shorelines.
Hala forests are being encroached on by invasive ironwood (Casuarina equisetifolia), which has inhibited the growth of native forests on other Pacific Islands. The establishment of T. pandani across the extensive hala forests in Hana, Maui, is a cause for concern, as severe infestations can kill young seedlings and prevent forest regrowth or expansion. There are many deleterious impacts ranging from discoloration, stunting, twisting, yellowing, leaf deformities, shorter leaf length, crown drop, fruit deformities, and potentially early tree death. Figure 2 displays the visual differences in appearance between a healthy hala forest, which appears greener, and a forest infested with T. pandani that has yellow discoloration associated with a high infestation level. As a frequently planted landscape tree, T. pandani infestations potentially decrease the aesthetic appeal of hala trees. There is limited knowledge regarding the dispersal capability of this pest or the extent of its distribution in Hawai‘i. Therefore, the purpose of this survey was to evaluate the current distribution of T. pandani across the Hawaiian Islands.
Figure 2: A healthy hala forest (left) compared with a forest with an established T. pandani infestation (right).
Methods: In 2023, we conducted an extensive survey of hala trees throughout urban, residential, forested areas, and rural landscapes on O‘ahu, Kaua‘i, Moloka‘i, and Hawai‘i Island. These surveys searched plant nurseries, airports, universities, hotels, resorts, and ornamental hala trees near potential pathways of entry. Areas of cultural significance, botanical gardens, popular hiking trails, and places frequented with tourists were included. In some cases, the surveyor visited private residences to inspect hala trees and conducted public outreach about T. pandani. We did not visit Lāna‘i, due to permit issues. They conducted their own survey and reported that there are no current infestations.
The surveys were relatively straightforward, as healthy trees will have a green, or variegated appearance, and heavily infested ones will have discoloration such as yellowing leaves (Figure 2). A closer inspection of the underside of the leaves will reveal a pattern of black dots, with a characteristic white waxy fringe. The waxy fringe may be absent from the top of the leaf, but adult T. pandani can be present. Trees that exhibited symptoms were observed by hand or with binoculars, while other accessible trees were surveyed randomly to determine early signs of infestations. If a tree was too tall to survey, the fallen fruit was inspected as T. pandani infests ripe fruit. There can be residual white substances from Pseudococcidae and other insects on leaves that look similar to the waxy fringe left by T. pandani. The waxy fringe surrounding the hala scale can be wiped off, leaving the entrenched scale insect on the leaf, while residue from other insects will leave a leaf that is clean when wiped off. If a plant is recently infested with a few adults, a closer inspection by hand may be necessary to confirm this, but it is possible to miss adults if they are in areas with new growth where they cannot be seen yet. After the survey was completed for each tree or cluster of trees, a pin was dropped using BaseMap™ App. Upon completion of the survey, these pins were downloaded as a KML file and uploaded to Google Earth.
Results:
Kaua‘i. In 2023, these locations across Kauai from a previous survey were revisited, and other gaps from the previous survey, such as the Limahuli Garden, Kalalau trail, nurseries, and landscape halas, were visited (Figure 3). No hala scale was found on Kaua‘i.
Figure 3: Kauai surveys conducted in 2023. Yellow pins indicate no infestation of T. pandani at available survey sites.
O‘ahu. The O‘ahu survey (Figures 4 and 5) re-confirmed the known hala scale populations on the Chaminade University Campus, University of Hawai‘i at Mānoa campus (but see below), and at various resorts in Waikiki. In 2022, there were successful efforts to eradicate the University of Hawai‘i at Mānoa population. Around 15 infected trees were cut down, placed in a shipping container, and incinerated. Healthy trees on the University of Hawai‘i at Mānoa campus received preventative imidacloprid treatments during this timeframe. In May 2024, a population was detected at Punahou High School by its staff and was brought to our attention. In November 2024, a population was detected in the Kakaako area by HDOA staff and was brought to our attention.
Figure 4: O‘ahu surveys conducted in 2023, with 2024 detections provided by Punahoe High School and HDOA. Yellow pins indicate no infestation of T. pandani at available survey sites, while a red pin indicates infested plant clusters.
Figure 5: O‘ahu survey conducted in 2023 in Waikiki. Yellow pins indicate no infestation of T. pandani at available survey sites, while an orange pin indicates infested plant clusters.
Moloka‘i. With an established population known on Moloka‘i, the 2023 survey aimed to evaluate the extent of the population from a previous survey and fill gaps (Figure 6). We visited the coastal hala forests located in the northeast areas of Molokai via helicopter and confirmed that the hala scale is widespread across the area from Kilkipua and Hakaano peninsula, to Kaho’olaiki Bay to Waiehu peninsula. The hala forests near Kalaupapa National Historic Park had yellow discoloration visible from the helicopter, but we were not permitted to land there.
Figure 6: Moloka‘i surveys conducted in 2023. Yellow pins indicate no infestation of T. pandani at available survey sites, while a red pin indicates infested plant clusters.
Hawai‘i Island. Multiple surveys of Hawai‘i Island were conducted in 2023 (Figure 7). Survey sites included airports, nurseries, resorts, schools, gardens, beach parks, and ornamental trees. We confirmed eradication at a previous location on the Hamakua coast and surveyed Waimanu Valley and Puna Hala forests. No hala scale was found on the Hawaii Island.
Figure 7: Hawai‘i Island survey conducted 2023. Yellow pins indicate no infestation of T. pandani at available survey sites.
Other Hawaiian Islands. We did not attempt to survey Ni‘ihau. Kaho‘olawe had its last P. tectorius plant die in early 2021. Lāna‘I has no population as of 2023. Maui has widespread populations.
Discussion: This was the most recent and comprehensive survey for T. pandani across the Hawaiian Islands. Although complete eradication of this pest is not possible statewide, it is important to monitor the distribution of pests to determine the likelihood of statewide dispersal. The eradicated populations of T. pandani on Lāna‘i and Hawai‘i Island indicate that crawlers probably cannot survive long-distance dispersal across ocean channels via wind. Currently, the most effective eradication tactics are to cut down, treat, and bury or incinerate the infected trees to ensure complete removal of populations. Preventative measures with the interisland movement of potentially infested material can help mitigate or stop the arrival of T. pandani for years to come. The results of the surveys showed that populations of T. pandani are not yet widespread across all major Hawaiian Islands.
Acknowledgements: This project was funded by Dr. Zhiqiang Cheng’s Hatch and Smith-Lever projects, SEED IDEAS Grant, and GSO 23-02-18. We thank Christina Martiney, Kenneth Choi, Dr. Roshan Manandhar, Nina Ronsted and Mike Demotta from NTBG, Marty Fernandes from Na Aina Kai Botanical Garden, Dr. Roxana Meyers, Glenn Teves, Ane Bakutis, Kamran Fujimoto, Jess Russo, and many others for their help with various aspects of the project.
Zhiqiang Cheng is a Professor and Extension Specialist working on turfgrass and landscape pest management at UH Manoa; Department of Plant and Environmental Protection Sciences, CTAHR, University of Hawai‘i at Mānoa
Mason Russo was a graduate student in Dr. Cheng’s Lab when this survey work was conducted. He is now working at HISC.
0 Comments