Countering Thousand Cankers Disease

Male walnut twig beetles are the first to enter trees. They initiate colonization by producing a pheromone that attracts both male and female beetles. Photo by Whitney Cranshaw, courtesy of Bugwood.

In recent decades, thousand cankers disease has become a concern for walnut growers and hardwood forest managers in the United States. A variety of measures have been investigated or developed to counter the disease.

A study led by USDA Forest Service research entomologist Albert Mayfield and former University of Tennessee graduate student Jackson Audley looked at one measure: quarantine treatments.

The complex disease is caused by the interaction of an insect and a fungus. As they bore into trees, walnut twig beetles carry spores of the fungus, which can kill patches of bark. This results in dead branches and stems. The disease affects a variety of trees, including some walnut, butternut, and wingnut species. Eastern black walnut (Juglans nigra L.) is of particular concern — it’s highly valued for lumber, veneer, nuts, and other commercial products.

In an effort to stop the spread of the walnut twig beetle, certain states have prohibited the movement of regulated articles, including kiln-dried walnut lumber that still has its bark. The scientists investigated the risk of that type of lumber becoming colonized during movement to evaluate whether regulation of that product is necessary. Their paper was published in the Journal of Economic Entomology.

In the experiment, eight eastern black walnut tree slabs with bark – four kiln-dried, four not – were placed at each of 14 eastern black walnut trees in Knox County, Tennessee. All trees showed signs of thousand cankers disease and were sources of walnut twig beetles. At each tree, they evaluated the effects of wood treatment (kiln-dried vs. fresh wood), a beetle pheromone lure (present vs. absent), and wood position (on the ground vs. in the crown).

After two weeks in the field, the slabs were placed into insect rearing containers, which are designed to allow the scientists to capture emerging walnut twig beetles. The team counted them at regular intervals for six months. Slabs were considered colonized if adult emergence was detected.

The study found that fresh slabs were colonized on the ground only when they were baited with the pheromone lure, and that kiln-dried slabs placed on the ground were not colonized with or without pheromones. In the crown, fresh slabs were colonized with or without pheromones, and kiln-dried slabs were only colonized when baited with pheromones. This suggests that the risk of kiln-dried walnut bark becoming colonized is very low, since it only occurred in an unrealistic situation: wood baited with pheromones and placed in the crown of an infected tree.

A second study led by Mayfield, along with Northern Research Station scientist Jennifer Juzwik and University of California at Riverside entomology doctoral candidate Jamison Scholer looked at a second possibility of controlling the spread of thousand cankers disease. The team investigated the efficacy of Beauveria bassiana, an entomopathogenic (or insect-killing) fungus as an alternative to synthetic insecticides to combat walnut twig beetles. Their findings were also published in the Journal of Economic Entomology.

walnut twig beetle
The walnut twig beetle and thousand cankers fungus are native to the extreme Southwest and Mexico and were observed originally in the western U.S. Photo by Curtis Utley, courtesy of Bugwood.

While synthetic insecticides often work for killing insect pests, they aren’t always the best option. “Since black walnut trees produce an edible nut, the number of options you have for treating those trees with a synthetic insecticide or fungicide is limited,” explains Mayfield. “As an alternative, we were interested in trying a biologically-based treatment.”

To test whether B. bassiana actually reduced the number of attacks by walnut twig beetles, the scientists used a similar setup to the first experiment on kiln-dried wood. Bolts of black walnut wood – essentially short logs – were divided into three groups. One group was treated with B. bassiana, one with synthetic permethrin insecticide, and the other with water as a control. Walnut twig beetle pheromone lures were then applied to all bolts. Finally, the bolts were placed in black walnut trees infested with walnut twig beetles.

After two weeks, the bolts were collected for analysis. Half of the bolts were placed in rearing containers to count emerging beetles, and the other half were dissected to count the number of egg galleries – tunnels in the bark made by the beetles. Finally, recovered beetles were examined for infection with the insect-killing fungus, B. bassiana.

“The synthetic insecticide permethrin kept the beetles from boring into the bolts,” says Mayfield. “And although the B. bassiana treatment did not prevent beetles from boring into or emerging from the bolts, it increased the percentage of beetles that were infected with the insect-killing fungus. In other words, the treatment was augmenting the natural rate of infection. The fungus grows into the beetles and is typically fatal.”

The B. bassiana fungus infection in adult walnut twig beetles occurs naturally at around 25 percent. The B. bassiana fungus treatment increased the infection rate to 62 percent.

Because of this promising result, Mayfield and colleagues conducted a follow-up study, led by Cornell University scientist Louela Castrillo, to further evaluate the effectiveness of B. bassiana and another insect-killing fungus, Metarhizium brunneum. Their findings were published in Biological Control.

In that study, both fungi dramatically reduced the number of walnut twig beetles that emerged from walnut bolts. Together, the two studies suggest that insect-killing fungi could be used to reduce the impact of thousand cankers disease — if an efficient way to apply these products to full-sized trees can be developed.

Fortunately, thousand cankers disease has yet to have widespread impacts in eastern forests. Nonetheless, this research provides helpful information for the regulation of walnut wood movement and possible management strategies for eastern black walnut should the disease flare up in the eastern U.S. in the future.

Read the full text of the quarantine article in the Journal of Economic Entomology.

Read the full text of the fungus article in the Journal of Economic Entomology.

Read the full text of the article in Biological Control.

For more information, email Albert Mayfield at albert.e.mayfield@usda.gov.

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