Researchers Track A Puzzling Pattern Of Tree-killing Insects

The red oak borer, a middle-sized, nocturnal brown "long-horned" beetle, lived in relative obscurity in red oak trees in the Ozark Mountains until 1999, when forestry professionals and researchers began noticing oaks dying in droves. When Fred Stephen, University Professor of entomology, and his students began examining the trees, they found them filled with borers.

The red oak borers have a two-year life cycle, spent mostly as larvae that bore into the heartwood of the host oaks. The larvae carve out galleries in the wood, chewing through layers of rings in the middle of the tree and creating small holes.

Most of the time natural controls on population growth, including the defenses that oaks mount, successfully combat the larvae. But an unexplained dramatic increase in larval density -- from an average three or four to a tree to 70 or 80 in a tree -- led to the deaths of tens of thousands of trees.

Then in 2005, the red oak borer population dropped 98 percent, returning to typical levels.

"We're tying to understand why all of this happened," Stephen said.

To do so, Stephen and geosciences professor Jason Tullis have taken their research into the air. Tullis is working with light detection and ranging (LIDAR), which uses pulses of laser light to accurately map a forested area of the Ozark National Forest near White Rock Mountain, where many trees succumbed to red oak borers during the infestation.

The LIDAR instrument, mounted in an airplane, shoots about 50,000 near-infrared laser pulses per second at the earth. Depending on the path of a given laser pulse, up to four return pulses are recorded by the instrument's receiver. In the case of a typical red oak tree, return pulses originate from leaves, branches, the trunk and the ground.

Using GPS, gyroscopes and other instrumentation, the location where each LIDAR return pulse originated is computed, allowing the researchers to study a three-dimensional "point cloud" representing forest structure and terrain. This information will be used in conjunction with field studies to look for patterns that might provide insight into the origins of the outbreak.

"If we could map the vulnerability of these ecosystems, we might be able to determine what areas need extra attention," Stephen said. Practices such as thinning the forest or changing species composition might help create forests less vulnerable to such infestations.

"GIS and remote sensing can help find a pattern, and this can help us understand the underlying science of the ecosystem," Tullis said.