Adult coconut rhinoceros beetles, a highly destructive invasive pest that has devastated Oahu palm trees, have been detected on the Big Island for the first time.
The University of Hawaii’s Coconut Rhinoceros Beetle Response on April 15 detected a single adult beetle in a camera trap at the West Hawaii landfill. Then, on Monday, the Big Island Invasive Species Committee reported that it had detected two more beetles in traps in the Waikoloa Dry Forest Reserve.
While beetle larvae were discovered on the island last October — in a decaying palm tree by Waikoloa Village — these are the first adults of the species detected on the island. And they likely won’t be the last.
“This means they’re out there,” said BIISC manager Franny Brewer. “It’s just such a … well, a big island. We have 150 traps out there, and that’s just a drop in the bucket.”
The beetles have a typical lifespan of about three to four months, said Keith Weiser, deputy incident commander of UH’s CRB response. While their adult phase is relatively short, that is when they cause the greatest damage.
Adult beetles burrow into spears of growing palm trees to feed, but it can take months before their presence is easily detectable, Brewer said. Only when the trees’ growing leaves unfurl, revealing distinctive cuts caused by the beetles, does the presence of the pest become obvious. No palm damage was observed in Waikoloa Village in February and March.
In addition to palms, the pests also feed upon and damage taro, banana, hala and sugar cane plants.
The insect was first detected on Oahu in 2013, and has become rampant across that island — Weiser said the beetle is “pretty much everywhere.”
Brewer said that determining how the insects came to the Big Island might be impossible, but speculated that eggs or larvae likely hitched a ride in local mulch transported from Oahu. The beetle lays its eggs in decaying plant matter — often mulch — and larvae have been detected in mulch piles on Oahu in the past.
She added that temporary regulations controlling the interisland transportation of mulch — specifically to prevent the spread of the beetle — lapsed in mid-2023 and were only reinstated in January.
“I’m not thinking an adult beetle snuck into a package of mulch. They’re too big for that,” Brewer said. The adult beetles measure about 2 inches long.
With the presence of adult beetles confirmed on the Big Island, state Department of Agriculture teams are planning extensive surveying and mitigation efforts to limit the spread of the insect.
Brewer said surveys of the Waikoloa area will be conducted within the next week — including the deployment of beetle-detecting dogs — and traps will be set out in sensitive areas.
Brewer said it is possible, but highly unlikely, that these measures will eradicate the beetle on the island, calling the bug a challenging one to mitigate.
Weiser said there are two main approaches for managing the beetle: by getting rid of its breeding areas — e.g., mulch and other decaying biomass — and through the use of pesticides and netting.
West Hawaii residents are advised to check their mulch and green waste for signs of the pest. Should a property owner detect a beetle or its larvae, Weiser and Brewer urge them to contact the state’s toll-free pest hotline at (808) 643-PEST (7378). If possible, specimens should be caught and kept for further analysis — dead or alive, Weiser said, although he added dead larvae tend to decay quickly and should be frozen.
Weiser also advised that live adult beetles should be kept in glass, as they are capable of chewing through plastic bags.
The larvae also can be confused for grubs of a different invasive beetle species. The oriental flower beetle is another pest species that can be a problem for Big Island gardeners, but is not nearly as destructive.
Although the oriental flower beetle grubs can be confused for coconut rhinoceros beetle grubs, Brewer said they can be distinguished by the presence of a line on its rear end — the latter larvae lack this “buttcrack,” Brewer said.
However, Brewer also said that anyone still uncertain if a grub is a coconut rhinoceros beetle can just record a video of the grub’s movements and send them to BIISC, where experts will be able discern the species.
Email Michael Brestovansky at mbrestovansky at mbrestovansky@hawaiitribune-herald.com.
