Researchers design more effective aerial device to collect Rapid ‘Ōhi‘a Death samples

Study: A new aerial device, used to collect larger ‘ōhi‘a branches to test for Rapid ‘Ōhi‘a Death in Hawai‘i’s forests, has led to a higher rate of successful diagnostic testing.

By Susan Enright.

Ryan Perroy pictured
Ryan Perroy (Kirsten Aoyagi/UH Hilo Stories)

A geographer at the University of Hawai‘i at Hilo has published a paper on a new device to aerially sample tree branches for the diagnostic testing of forest fungal pathogens, including those responsible for Rapid ‘Ōhi‘a Death.

The device, named Kūkūau, was developed by geographer Ryan Perroy and his research team in collaboration with researchers at ETH Zürich, a public research university based in the city of Zürich, Switzerland, and R&R Machining/Welding in Hilo, Hawai‘i. Kūkūau consists of a small rotating chainsaw with a robotic gripper claw mounted beneath a drone, and can cut and retrieve branches up to seven centimeters in diameter.

The name Kūkūau reflects its place of origin and design. Kūkūau is the name of an ahupua’a (land subdivision) in the Hilo area, and is also a term for a type of crab, Metopograpsus thukuhar, or ‘alamihi in Hawaiian.

The researchers found that the earlier samples of twigs were often too small to detect the fungal pathogens, or the twigs had dried out because of their small size, so they developed the new drone attachment to cut larger branches, especially useful where sampling is needed in remote or rugged environments.

“There have been times when we detected an ‘ōhi‘a tree suspected of infection with the pathogens responsible for Rapid ‘Ōhi‘a Death, but because of the location it is too dangerous or problematic to send field crews out to sample it for confirmation,” says Perroy. “Kūkūau has the potential to help in those types of situations.”

Three photos: Drone with large branch dangling below, cut samples of the branch, and a person holding the collected sample branch.
Aerial branch sampling illustration from the study. (A) The new Kūkūau branch sampler in the air following a successful cut; (B) Samples from a single branch, U.S. Quarter dollar (25 cents coin) for scale; (C) Two different collected branches, branch on the right contains healthy green leaves from a tree exhibiting partial C. lukuohia canopy symptoms; (D) >2 m tall branch collected by the Kūkūau branch sampler. (Courtesy images)

Since 2014 when it was first discovered, Rapid ‘Ōhi‘a Death has killed hundreds of thousands of mature ‘ōhi‘a trees (Metrosideros polymorpha) throughout the Hawaiian Islands. ROD is caused by two invasive fungi, Ceratocystis huliohia and Ceratocystis lukuohia, and has the potential to irreversibly change some native Hawaiian ecosystems.

“We successfully detected the target fungal pathogen from the collected branches and found that branch diameter, leaf presence and condition, as well as wood moisture content are important factors in pathogen detection in sampled branches,” write the researchers in the published paper.

None of the smallest branch samples tested positive for C. lukuohia, while 77 percent of the largest diameter branch samples produced positive results.

The research shows that the new branch sampler, capable of retrieving the larger branches, provides the right size for a higher rate of successful diagnostic testing.

Aerial image of a single dead tree in a forest.
From the study: A high-resolution aerial image from east Hawai‘i Island showing an ‘ōhi‘a tree with the characteristic reddish canopy coloration suggesting likely infection with C. lukuohia, the fungal pathogen responsible for Ceratocystis wilt of ‘ōhi‘a. (Courtesy photo)

Perroy is principal investigator at the UH Hilo Spatial Data Analysis and Visualization (SDAV) Laboratory, a research unit applying geospatial tools to local environmental problems in Hawai‘i and the Pacific region. His group has been working on the detection of ROD and invasive species populations over forests across Hawai‘i using high-resolution cameras and other sensors carried by drones and helicopters. The collected images and data provide managers precious time to respond to outbreaks, and gives scientists better information on how diseases and invasive species spread.

In 2019, Perroy won $70,000 in a competition sponsored by the National Park Service for his innovative use of drones and remote sensing devices to detect Rapid ‘Ōhi‘a Death. Over the last three years, Prof. Perroy and his team have continued to hone and refine the equipment needed to conduct aerial sampling using a small unoccupied aerial system or sUAS.

By Susan Enright, a public information specialist for the Office of the Chancellor and editor of UH Hilo Stories.

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