Tiny Invader Killing Black Walnut Trees Causes Concern for Iowa Foresters
Posted: 02/01/2011

The most economically valuable tree in Iowa is under a possible threat from a beetle that is about the size of the period at the end of this sentence.

Thousand cankers disease (TCD) has been killing black walnut trees in the Western U.S. since the 1990s. It is not currently in Iowa, but is as close as Rocky Ford, Colo., to the west and Knoxville, Tenn., to the south.

"The introduction of TCD into Iowa would have disastrous effects economically and environmentally to the wood industry in the state," said Tivon Feeley, with the forest health program at the Iowa Department of Natural Resources.

Iowa's estimated 979 million board feet of sawlog-sized black walnut standing in the woodlands is the third largest state volume in the nation and valued at nearly $1.5 billion. The economic loss in Iowa to woodland owners and associated businesses that annually harvest more than 9.3 million board feet would be an estimated $62 million per year.

"Some experts believe that TCD has the potential to decimate black walnut in the same way Dutch elm disease, emerald ash borer and chestnut blight have destroyed their respective hosts," Feeley said.

The disease is caused by a walnut twig beetle that carries a fungus which is spread as it tunnels through tree tissue. Beetles can reach high populations and numerous cankers can develop. Instead of one large girdling canker, tree decline and death appears to result from the high number of cankers.

"We placed two logs about eight inches in diameter and two feet long in a box and eventually recovered around 23,000 beetles," said Ned Tisserat, professor with Department of Bioagricultural Sciences and Pest Management at Colorado State University.

Rapid detection and removal of infected trees is currently the primary means of managing thousand cankers disease. Stopping or slowing its spread from infested areas relies on quarantines of wood products and on public education.

In addition to Colorado and Knoxville, Tenn., the beetle has been killing black walnuts in New Mexico, Arizona, Utah, Idaho, California and New Mexico.

"It comes back to do not move wood," Feeley said. "The reason TCD is in Knoxville is because it was brought there in wood from a contaminated area from the west. Don't move firewood, wood logs or other untreated wood products."

Very little is known by the public about thousand cankers disease of black walnut, Feeley said.

"Scientists are investigating how serious of a threat this disease will be in the eastern U.S., and until more facts are learned, woodland owners are encouraged not to harvest their trees, but to continue working with their local forester to keep their woodland healthy," he said.

A more intensive survey of declining black walnut trees has been proposed in a grant to the U.S. Forest Service.