Can a beetle repellent help save Hawaii’s ohia lehua trees from a deadly fungal blight?

That is what Hilo-based entomologist Kylle Roy of the U.S. Department of Agriculture Forest Service is examining, with hopes of finding another way to save more of the native trees from rapid ohia death. An effective repellent is key since studies have shown invasive ambrosia beetles transmit the disease.

For one, ambrosia beetle frass or sawdust contain spores that can infect ohia by entering a tree’s wound.

This was the finding by Roy and a team of researchers from the USDA Forest Serv­ice, the University of Hawaii at Manoa, UH Hilo and others in a study published in September. This frass is made up of macerated wood parts the beetles kick out while tunneling into the trees to make their homes.

“That’s the major vehicle for rapid ohia death in the environment,” said Roy.

What’s alarming is that these resting spores can survive in frass for at least six months, possibly longer, she said. This is why it is so important for people hiking or hunting to clean their boots and gear with at least 70% rubbing alcohol before entering and after leaving a forest.

But also, ambrosia beetles carry the fungus on their bodies and can directly transmit the pathogens to trees.

In June the team published another study in Forest Pathology confirming the beetles can infect and kill ohia by directly vectoring fungal spores, particularly if the tree is wounded or stressed.

Theoretically, said Roy, a single beetle with C. lukuohia spores on its body could attack and kill a stressed ohia tree.

What it all points to is that these invasive beetles are “bad news” for Hawaii’s ohia forests, she said, because they can vector directly and inoculate the environment, too.

“It’s pretty scary,” she said. “Because they’re so small, there’s a lot of them, and it’s hard to control them.”

The ohia lehua, or Metrosideros polymorpha, is the most abundant native tree in Hawaii, vital to the state’s watershed and native birds, and central to Hawaiian culture.

The state has recognized the keystone species as Hawaii’s state endemic tree and designated April 25 as Ohia Lehua Day.

But the ongoing battle against rapid ohia death, a disease caused by the Ceratocystis fungus, continues unabated as it spreads into more territory.

Since its discovery in 2014, rapid ohia death has decimated Hawaii’s forests, killing more than 1 million ohia trees on Hawaii island alone.

“We’re still losing ground,” said J.B. Friday, extension forester at UH Manoa’s College of Tropical Agriculture and Human Resources. “We’re losing unsustainable amounts of trees, and losing trees faster than they’re growing.”

For many years the fungal blight had been detected only on Hawaii island, but has in recent years also been detected on Kauai, Oahu and Maui.

Friday said tree mortality has also been seen higher up in Hawaii island’s forests than in the past decade, while invasive weeds are taking over the lower elevation that have lost ohia trees to the blight.

But scientists also know much more about rapid ohia death today than before, with tools being used and developed to combat it.

In 2018, researchers determined there were actually two different species of Ceratocystis attacking trees in Hawaii. In consultation with the Edith Kanaka‘ole Foundation, researchers named one Ceratocystis huliohia, which roughly translates to “overturns ohia,” and the other Ceratocystis lukuohia, or “destroyer of ohia.”

C. huliohia causes a canker disease beneath the bark and spreads slowly, eventually killing off the tree’s water-conducting tissue. The other, C. lukuohia, is more lethal and quickly chokes off the tree’s water supply, causing the entire crown to go brown.

Both, however, can kill.

So far, C. lukuohia has been found only on Hawaii island and Kauai, not on Oahu or Maui.

Roy’s team has tried to determine which of four beetle species are more likely to carry the fungi on their bodies, and found that Xyleborus affinis — one of the most widespread ambrosia beetles in the world — to be a top contender.

“We think that might just be due to their behavioral characteristics,” she said. “Maybe they’re not cleaning themselves as much, or they’re a little bit more hairy, so we think the spores are more likely to stick.”

The Xyleborus ferrugineus beetle, meanwhile, produces the most frass.

Both of these beetles have been implicated as vectors in other Ceratocystis- disease systems, she said.

Roy and her team are testing two different beetle repellents — SPLAT Verb and SPLAT Beetle Guard — containing the active ingredient of verbenone in the Waiakea Forest Reserve on Hawaii island.

The repellents were initially placed in traps to see whether they would deter the beetles, and seemed to work, she said.

One is now being tested on ohia trees afflicted with rapid ohia death as well as in healthy trees to see how effective it is in keeping away the beetles, and so far the results are promising. One study was conducted last summer, followed by another this summer to obtain multiple sets of data.

“There’s promise this will be a really great management tool,” she said.

If proved effective, she said, the beetle repellent could be used in addition to other tools, such as fences, to keep hoofed animals from wounding trees and spreading the disease. Fences help but will not be able to keep beetles out of the forests.

The repellent would potentially be another line of defense, according to Roy, especially when new outbreaks occur, and both infected and healthy trees could be protected.

Friday welcomes a repellent as another tool and remains hopeful that ohia forests will prevail.

“The forest is resilient,” he said. “It has withstood volcanoes and earthquakes for millions of years.”