Traps create an early-warning system for a nearly invisible pest

By Tom Hallman

Insect trapAt less than 2 millimeters long, the walnut twig beetle might seem a meager opponent to a black walnut tree. Yet the petite pest can topple a 130-foot hardwood.

Native to the American West, the tiny bug originally fed on Arizona walnut, but developed a taste for black walnut as it spread east.

And that’s why Olivia Souther has spent her summer planting and regularly checking traps amid the branches of black walnut trees across South Carolina.

Setting an insect trap“Walnut twig beetles have never been detected here, but recently have been found in North Carolina, so we believe they are on their way,” said Souther, a senior biological sciences major who worked as an intern this summer for the Department of Plant Industry, a regulatory arm of Clemson that helps protect the state from plant pests and diseases.

“This was the first year we trapped for walnut twig beetle,” she said. “We identified other insects of the same genus in these traps, but I’m surprised we didn’t find it.”

Detecting an invasive pest is the first step in controlling it; once it appears, it is both an environmental and an economic issue.

The black walnut trees the beetle attacks produce a nut that is prized in certain ice creams and confections. The quality of its wood also makes it valuable for furniture and other products.

Although the beetle is potentially devastating, it isn’t easy to locate.

“They are very tiny,” said Sarah Morrison, who coordinated the walnut twig beetle trapping program. “They burrow into the bark in the small twigs of the tree, which is why the beetles are so hard to find. Unless you are climbing into the top of the tree and cutting limbs, it’s hard to find them. That’s why we try to trap them.”

The walnut twig beetle carries with it a secret weapon: a fungus, Geosmithia morbida, which causes a fatal disease in the tree.

“You wouldn’t think something this small would hurt a big tree, but they’re a vector for disease,” Morrison said. “As they bore through the bark they carry the disease with them. The cankers disrupt the movement of nutrients in the tree. It actually chokes off the tree.”

Aptly named “thousand cankers disease,” it is usually first noticed in the tree’s crown, where the beetles tend to burrow. Leaves will start to die in a tell-tale sign that the uppermost twigs and limbs of the black walnut tree aren’t getting the nutrition they need.

Souther checked 29 traps in a dozen counties across the state, from Pickens to Dorchester, every week for six weeks. The traps included a scent to lure the bugs and then capture them in a liquid solution. Souther extracted the captured insects and sent likely suspects back to the lab for identification.

“We found close relatives, but not our pest of concern,” said Sherry Aultman, Clemson’s Cooperative Agricultural Pest Survey coordinator.

Learn more about invasive species: www.clemson.edu/invasives.