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Insect Threatens Black Walnut Trees

Jim LaBonte, Oregon Department of Agriculture

Black walnut trees can grow to 75 feet tall, yet they are at risk from an insect about the size of a pencil tip.

The Walnut Twig Beetle carries a fungus called Thousand Cankers Disease. When numerous beetles get into a walnut tree, they cause infection points that grow.

“(The infection points) all kind of coalesce and basically clog up the vascular tissue of the tree, effectively strangling it off, very much like Emerald Ash Borer does, except it’s the fungus itself that’s causing the damage versus the larva as with the Emerald Ash Borer,” said Scott Schirmer.

Schrimer is a Plant and Pesticide Specialist Supervisor and Emerald Ash Borer Program Manager for the Illinois Department of Agriculture, Bureau of Environmental Programs.

He said the insect has not been spotted in Illinois, but there is enough concern that the department has been setting traps and looking for it for about the past three years.  Schirmer said the bug comes from the west but has established itself in areas east of the Mississippi River, including Indiana. 

He said the ag department is not allowing anyone to bring to Illinois any firewood, logs, and other unprocessed black walnut wood from areas that are suspected of having walnut twig beetles. But there is no guarantee the department can prevent the bug from getting into the state.

“The probability that it ultimately does get here is probably quite high,” he said.

Credit Rich Egger
A black walnut tree

Schirmer said while black walnut trees are not common in the urban setting, they are grown commercially in Illinois and there are concerns about the potential impact to tree farms.

“It’s a very valuable crop as far as lumber and veneer goes,” Schirmer said. “There are people who grow it as a cash crop and they make long-term investments in walnut plantations.”

He said it can take 30 years for a tree to reach the point at which it can be harvested and sold.

Schirmer said black walnut trees can be found throughout Illinois and are native to the region, which also makes them a valuable part of the natural ecosystem because they feed wildlife and provide habitat.