Beetle and Fungus Have 'Gone Rogue,' Threaten Walnut Trees

The walnut twig beetle was once nothing but a nuisance for black walnut trees across the western United States. Now, however, it appears that the beetle has gained a partner in crime - a mutated fungus that infects anywhere the beetles go. This has allowed the once-pests to "go rogue," now inflicting near-irreversible and sometimes fatal damage to these trees and trees like them.

The beetle in question, Pityophthorus juglandis, has long been a predator of the black walnut tree, chewing on twigs and small branches. The beetles also often converge in unusual meeting alcoves etched between the tree core and its bark know as "galleries" to mate and reproduce.

However, by 2004, this pesky predation exploded into utter slaughter, with extensive forest degradation along the Front Range of Colorado linked to the presence of the beetle and the fungus it carries, Geosmithia morbida, according to the US Department of Agriculture's Forest Service.

Now a new study conducted by researchers from the Forest Service, Colorado State University, and Perdue University has revealed that the impact of these beetles and their fungus may have spiked thanks to a surprising amount of mutation. (Scroll to read on...)

The study, recently published in the journal PLOS One details how Geosmithia morbida has been hopping from the beetle to infect galleries, attacking black walnut trees at their core. This results in a tree illness called thousand canker disease that is changing faster than the black walnut's defenses and conservationists can hope to catch.

"The high mutability of this fungus means we can expect the unexpected," Perdue scientist Keith Woeste explained in a release. "We can't count on the fungus' genes to be the same year after year, which certainly makes it harder to control. It will also be harder to breed trees resistant to this disease."

Interestingly, the beetles appear to have co-evolved with their fungal partner-in-crime, suddenly focusing not just black walnut trees, but any related to the walnut variety. Worse still, they appear to have adapted to attack the vulnerable trucks of trees, not their branches, far more often.

"It sort of went wild," Woeste said in a statement. "That was very surprising."

"It's too early to panic," he added. "There's still a lot of walnut in the US, and there's going to be for a long time. However, people should stay abreast of changes as we continue to learn more about this disease and how landowners and companies can protect their resources."