Efforts to Slow the Spread of Invasive Species in Hawai‘i: A Focus on Integrated Pest Management Strategies for the Coconut Rhinoceros Beetle, Oryctes rhinoceros and the Hala Scale, Thysanococcus pandani

Abstract

The Hawaiian islands have been subject to a series of invasive plant, insect, vertebrate, and pathogen introductions. This started with the first Polynesian arrivals, increased with European contact, and substantially increased as globalization ensued. These introductions have resulted in the extinction of many endemic species and have altered many ecosystems from dryland forests to high elevation areas. This dissertation will focus on two specific invasive insects, the coconut rhinoceros beetle, Oryctes rhinoceros Linnaeus and the hala scale, Thysanococcus pandani Stickney. These insects impact native species and have caused ecological, cultural, and economic damage.Oryctes rhinoceros is an invasive insect that feeds primarily on coconut palms. It was previously controlled with the release of specimens infected with the virus, Oryctes rhinoceros nudivirus (OrNV, Alphanudivirus oryrhinocerotis). However, beetles resilient to OrNV were found in Guam, where they became widespread. Since then, this biotype of O. rhinoceros was found on the Hawaiian island of O‘ahu in 2013. This triggered an extensive eradication program using tools such as mass trapping, detector dogs, green waste sanitation, and insecticidal treatments. Populations have since spread across O‘ahu, and in 2023 highly localized O. rhinoceros populations were found on Maui, Kaua‘i, and Hawai‘i island. The insecticidal studies evaluated the efficacy of systemic insecticides in laboratory and field settings as a potential component of the long-term integrated pest management program for O. rhinoceros. The laboratory assays tested the efficacy of imidacloprid, dinotefuran, and abamectin at 25 and 250 parts per million (ppm) on adult O. rhinoceros beetles. Two concurrent field trials were conducted to evaluate the preventative and curative effects of injecting imidacloprid, dinotefuran, abamectin, and acephate into coconut palms trunks. Efficacy of treatments was determined by taking monthly visual measurements of O. rhinoceros feeding damage. Palm fronds were removed and tested on laboratory-starved beetles to see if feeding exposure caused negative impacts. The results of the laboratory trials indicated that all the chemicals were effective at both 25 and 250 ppm and that pesticide residue in palm fronds caused negative impacts as late as seven months after treatment. The field trial results indicated that imidacloprid injections reduced the total amount of new feeding damage, with chemical residue remaining seven months post treatment in palms. I conducted an areawide injection trial at this site that had inconclusive results. I conducted further trials to evaluate commercial and locally occurring entomopathogenic fungi that could be implemented to mitigate O. rhinoceros populations. I also conducted trials with locally collected entomopathogenic nematodes that did not yield significant mortality. Our initial laboratory trials in 2018 and 2019 narrowed down five locally collected fungal strains with greater than 60% mortality rates on first instar larvae. In 2021 these strains were tested in conjunction with the fungal insecticide BotaniGard ES, with decreased efficacy in some strains, but greater than 60% mortality with BotaniGard ES at the high label rate. In field trials there was greater than 60% mortality of first instar larvae using BotaniGard ES as a mulch amendment. This treatment is unfortunately difficult to implement in the field due to the shelf life of the product. The studies pertaining to T. pandani covered a variety of knowledge gaps surrounding this rarely studied insect. In Hawai‘i it was first detected in Hāna, Maui in 1995, and its primary host plant is the indigenous hala tree, Pandanus tectorius, which is found in coastal and riparian areas throughout the Hawaiian islands. It has been found on Moloka‘i, O‘ahu, Lāna‘i, and Hawai‘i island, but T. pandani is only widespread on the islands of Maui and Moloka‘i. The insect is a sedentary feeder on the leaves and fruit, and heavy infestations result in discoloration, frond, and possible tree death. Experimental field trials were set up to evaluate the efficacy of organic and chemical insecticides as potential treatments to mitigate the damage of T. pandani infestations. This was done on the Island of Maui where T. pandani is widespread. These trees were inoculated with T. pandani and evaluated throughout the trials for changes in plant height, canopy width, scale mortality, and visual infestation ratings. The results of two repeated field trials indicated that flupyradifurone and buprofezin were effective treatments in reducing the hala scale infestation and improving visual aspects related to plant health. In 2021 and 2023, statewide surveys were conducted to evaluate the distribution of T. pandani, and a dispersal study was set up on Maui in January 2023 to determine the ability of T. pandani to spread from infected to healthy hala trees over time. The visual surveys involved evaluating hala trees throughout plant nurseries, landscaping plantings, urban and residential areas, and forests. The dispersal study evaluated factors such as wind, spatial separation, population levels, and infestation ratings. The visual survey results indicated T. pandani is widespread on Moloka‘i and Maui, with established populations on O’ahu. The infestations on Lāna‘i and Hawai’i island were successfully eradicated. The dispersal study indicated that T. pandani was able to infest the entire field plot in the first 10 months. Available literature indicates that T. pandani infestations prevent seedling regrowth, suggesting that the hala forests on Maui and Moloka‘i may not regenerate. I carried out a statewide population structure study of coastal P. tectorius forests to test if presence of the hala scale was correlated with lower levels of recruitment. In 93 transects across four islands, this study documented population structure, hala scale presence, and biotic and abiotic factors that may also affect regeneration, including elevation, canopy openness, and understory invasive species cover. Heavy infestations of T. pandani infestation were present in all transects across forests on Maui and Moloka‘i, while Kaua‘i and Hawai‘i island did not have any infestations. Canopy cover varied across sites and relatively low levels of invasive plant cover were found in the understory. There was significantly lower sapling recruitment ratio (saplings (1-2m tall)/adult tree) within transects with the hala scale than those without. However, this result was driven by a single outlier. Immediate regeneration of P. tectorius may be inhibited by non-native plants and ungulate feeding. We conducted biological control surveys in Thailand, Indonesia, and Singapore to search for insects that could control T. pandani. The first trip found generalist coccinellidae predators and a botanical collection where it was found had many Pandanus spp. from Madagascar. In a survey to Madagascar, I found three undescribed scale species on various pandanus species, but no immediate evidence of T. pandani or biological control. The results of these studies helped fill knowledge gaps pertaining to integrated pest management of O. rhinoceros and T. pandani. I was able to conduct a variety of ecological and applied studies for T. pandani. These pests can severely alter the aesthetic landscape in urban areas of Hawai‘i that are characterized by palm and Pandanus plantings. They also have the ability to negatively impact natural areas with endemic Pritchardia spp. palms and wide ranging Pandanus forests found on coastal areas along the Hawaiian islands. It is possible that the spread of these pests within islands was facilitated by widespread urban planting of these species, thus creating a pathway for widespread dispersal of these pests throughout each island. In general, the accidental transport of these pests results in a range expansion within an island, or to a new area in the state. Public outreach and improved biosecurity protocols can mitigate the impacts of these pests. As these insects are not yet widespread across every Hawaiian island, further research and mitigative measures are necessary to slow the momentum behind the invasion of these pests.