Iowa walnut trees face new threat from migrating insect
April 21st, 2014 by Ric Hanson
While Iowans are seeing the first ash trees cut down to slow the spread of the emerald ash borer, yet another invading insect is causing a stir as it could threaten our walnut trees. Robin Pruisner, an entomologist with the Iowa Department of Agriculture, says the pest called the walnut twig beetle is being found in nearby states, but it hasn’t been found in Iowa — yet. “Research is ongoing on how to protect walnut trees,” Pruisner says. “We just don’t have a lot of answers. This is even newer than the emerald ash borer at this point in time.” The walnut twig beetle carries what’s known as “thousand canker disease,” which is deadly to black walnut trees.
There’s been no way found to reverse the disease or to kill the beetle without also killing the trees. “The geosmithia pathogen is actually very common in our environment and this is just kind of a new cousin of that,” Pruisner says. “The walnut twig beetle is native to the southwest United States and down into Mexico.” For many years, the beetle was only found in states like Arizona, California and New Mexico. Now, the rice grain-sized pest is being found well beyond the southwest, in states as far away as Virginia and Pennsylvania — and closer to home in Ohio, Colorado and Tennessee.
Pruisner suspects the insects are moving such great distances because people are enabling them to hitch long rides. “Aunt Sally out in Colorado has a walnut tree that dies in her backyard but Cousin Ed here in Iowa would like to make a coffee table out of it,” Pruisner says. “This is the kind of thing that people throw in the back of their truck and they drive to Iowa and they could be inadvertently bringing along with it thousand canker of walnut.”
One way to stop the spread is to only use local firewood in campfires. Iowa ranks sixth in the nation for the production of black walnut, prized for its grain and color and it’s exported all over the world.