A rapidly spreading plant and tree fungus on the Hawaiian island of Hilo is responsible for killing hundreds of thousands of precious ʻŌhiʻa trees annually.  Researchers at the University of Hawaii, led by geographer Prof. Ryan Perroy, have devised an ingenious method for collecting samples of the fungus to better understand why it spreads.  They’re equipping remotely-piloted aerial drones with a circular chainsaw to cut and collect diseased tree limbs for diagnostic testing in their labs.

The team began its drone development effort several years ago but soon learned that its original prototype was too small and lightweight to collect the large tree limbs that were m0st afflicted by the deadly fungus.  So they went back to the drawing board and devised a heavier-duty drone.  Support for the enhanced design and engineering work came from ETH Zürich, a public research university in Switzerland, the U.S. Department of Agriculture, and R&R Machining/Welding on Hilo.

The latest version of  the chainsaw drone hovers over ʻŌhiʻa trees previously identified through aerial drone surveying as heavily affected by the fungus – usually through discoloring and defoliation – and then lowers its chainsaw device on a long metal cable to chop off a large limb.  The drone also comes equipped with large automated grippers that hold the tree limb in place while it’s being cut, then helps pull it away from the tree as the drone returns to home base.  When deployed correctly, the drone’s grippers retain the cut limb sample, making additional ground-based collection completely unnecessary.

The team’s switch to a heavier-duty drone has clearly paid off.   While none of the smaller tree branch samples tested positive for the fungus, more than three-quarters of the large-diameter samples did.  The research shows that the enhanced version of the chainsaw drone, capable of retrieving tree limbs up to 3-inch in diameter, provides the right size for a higher rate of successful diagnostic testing.

Perroy’s team is not the first in the country to deploy unmanned aircraft for tree canopy sampling.  Other designs have been tested in Australia that allow for the sampling of tree branches beneath tree tops.  Some deploy smaller knife blades and come equipped with propeller guards to avoid collisions with trees.  Other drones deploy sophisticated sensor technology that can detect non-visual signs of tree distress and can target promising test samples based on wood moisture and other factors.

All current studies indicate that drones offer enormous advantages over more conventional sampling techniques that rely on trained arborists to climb trees, or that deploy ground-based cranes or pole-mounted saws that are limited to lower branches.  Most of the new drones can collect samples in a matter of minutes or even seconds,  saving time and labor costs without exposing human workers to injury.  Most sampling drones are still in the proof-of-concept phase for future commercial and scientific use.  Perroy’s team hopes to design a chainsaw drone that is fully autonomous and can be deployed across the entire Hawaiian archipelago on a range of tree species affected by fungi.