by Keith Weiser, Ph.D.
Something has been munching on Oahu’s palm trees and UH researchers are testing some innovative approaches to stop it. The coconut rhinoceros beetle (Oryctes rhinoceros) AKA CRB was found on Oahu for the first time in December of 2013 and a multi-agency response has been combating the invasive insect to eradicate it from Hawai’i. This native of Southeast Asia lays eggs in decaying plant material like mulch or compost and the eggs hatch into larvae that feed on the decaying material. After growing and feeding for several months, the ~3 inch long white larvae go through metamorphosis to become adult beetles. The adults are ~2 inch black beetles with a distinctive horn on their head. The adults emerge at night and fly to palm trees to feed. Coconut trees (Cocos nucifera) are their favorites but they will feed on a variety of palms and other plants including date palms (Phoenix sp.), native Hawaiian palms (Pritchardia sp.), sugar cane (Saccharum sp.), and many common landscaping palms. When they feed on palms, the beetles move to the top of the tree to the spear where the fronds have not yet unfurled and burrow into the softer tissues to feed on the plant juices. Throughout their adult life they will feed and mate moving between tree crowns for feeding and mulch piles for mating and egg laying. The feeding damage leads to unsightly v-shaped notches in the palm fronds and can kill the tree. Although the coconut farming industry is small in Hawai’i, coconut trees and other palms are an important part of the aesthetic of the island and many worry that damaged or dead trees may affect tourism. They are also a historical and culturally important species that is popular in landscaping.
University of Hawai’i researchers work in association with the Hawai’i Department of Agriculture to overcome challenges in eradicating this pest. The primary challenges are finding the beetle’s breeding sites, effective and efficient methods of killing beetles in mulch and trees, improved trapping, detecting beetles quickly and easily, and protecting Hawai’i from accidental reintroduction. The beetles are primarily found between Sand Island and Ewa, North to Pearl City Peninsula. This range has been determined by catching beetles in the large black lantern-like panel traps hanging from trees across Oahu. Beetles are attracted to these traps by a pheromone lure and a light. As beetles fly at night, they collide with the side of the trap and fall into the cup at the base of the trap.
Improvement to the current beetle traps has been undertaken by Dr. Daniel Jenkins. Like moths crowding around lights at night, many other insects are attracted to light. Each type of insect may respond differently to different wavelengths and intensities of light. Dr. Jenkins has developed a solar-powered microcontroller-driven lighting system that can emit light in 6 different wavelengths from red to ultraviolet in a variety of patterns and times of day to find what attracts the beetles best. These units are currently being tested in the field and may improve the efficiency of the CRB traps.
Dr. Jenkins is also collaborating with other UH researchers to use sound to detect and destroy beetles and larvae. There are a few sounds that CRB make that can be distinguished from other species found in Hawai’i. As beetles fly, their wingbeats make a humming sound. Beetles in breeding sites make a grunting or honking sound called stridulation by rubbing their bodies against their wing covers. Both of these sounds can be detected with microphones and recognized using computer algorithms. Both of these sounds could be used to locate beetles to quickly find breeding sites and areas of high beetle activity. UH scientists aren’t just using sound to find the beetles; they’re also using it to kill them. They are testing high-power ultrasound to destroy the larvae. Hopefully, this approach can be used to kill the larvae in breeding sites without using pesticides.
Sound isn’t the only way to find a beetle, sometimes you can use poop. Yes, poop. Dr. Mike Melzer’s group is doing a number of studies on the coconut rhinoceros beetle including detecting the presence of beetles in an area by identifying it’s DNA in the feces of mongoose that have eaten them. This procedure can also be used on larvae, eggs, and beetle fragments that might be hard to identify otherwise. Dr. Melzer’s group also maintains the quarantine lab for studying CRB at UH and conducts studies on the life cycle, breeding habits, and mulch preference to be able to identify high risk mulch and compost material. They also investigate ways to make plant waste unsuitable for breeding including addition of salt and use of fungi that infect and kill CRB larvae.
More traditional control methods, such as pesticides, are also being investigated. Dr. Zhiqiang Cheng and his team are discovering which pesticides are effective against CRB. Based on results of multiple lab assays, two promising systemic pesticides have been injected into trees to kill beetles that feed on them reducing off target effects and environmental contamination. Dr. Cheng’s lab is also working on greener methods of biocontrol that use small beneficial worms called nematodes and entomopathogenic fungi that can infect and kill the larvae.
Even if many of these control methods lead to better treatments and eradication of the beetle on Oahu, there is also the chance of reintroduction from countries that have CRB infestations including the Palau, Guam, and Southern Asia. It’s important to understand how the beetle got here to defend against reintroduction. Genetic studies have been done by Dr. Dan Rubinoff’s lab to identify where the Oahu beetles came from. Identification of the source can be followed up by increased screening and changes in transportation procedures to reduce the likelihood that CRB and similar pests can be introduced to Hawai’i again.
Keith Weiser is the Research Liaison for the Coconut Rhinoceros Beetle response administered by HDOA in Hawai’i.