College students investigate invasive, beetle-carried disease killing walnut trees

Students from different universities attending a summer program at UT's National Institute for Mathematical and Biological Synthesis are investigating the spread of thousand cankers disease, which has killed off black walnut trees in the western United States and is rapidly moving east. 

An established outbreak of the disease occurred in Knoxville in 2010, and it has been detected in five more eastern states. 

The disease is spread by a seed-sized beetle called the walnut twig beetle, which is native to the southwestern United States. Scientists believe the beetles made their way east in black walnut lumber bark and sought out new hosts from there. 

Heating up

Three undergraduates used Geographic Information Science to create models that show how the distribution of black walnut trees and walnut twig beetles could change with the climate, and how their two habitats could overlap. 

"Our models all predicted a general northward shift, which you would expect for these trees as climates warm over time," Ben Reber, who came to UT from Houghton College in New York. 

The models are based off different levels of carbon emissions over the next thirty years.  The worst case scenario shows the southeastern-native trees spreading from southeast Texas to Nova Scotia by the 2050's.

As they move, however, the beetles - native to the southwestern United States -would find more suitable habitats northward.

While the trees spring up northward, rising temperatures would stress established black walnut trees in the southeast, making them more vulnerable to diseases like thousand cankers disease. 

So expect black walnut products to get pretty expensive. 

Death by a thousand cuts

The disease gets its name from a method of torture and execution used in China from the 10th century until the early 20th century, Lingchi, or more commonly, "Death by a thousand cuts."

The disease occurs when a tiny beetle burrows into a tree, inoculating it with a fungus, which begins to form a canker, or a small area of dead tissue. Each time a beetle tunnels into the tree another canker begins forming and the trees begin to die. 

Compounding the disease, the dying trees attract more of the fungus-carrying beetles, which speed up the process by forming even more cankers, until the cankers overwhelm the tree. 

"Cankers restrict nutrient flow from the tree's roots to its canopy," Reber said. It only takes about three to four years for an infected tree to die. 

The students hope the models will help scientists and forest managers to create informed strategies for managing future forests.