CAUTION: This information was developed for North Carolina and may not apply to other areas.

Walnut Twig Beetle, Pityophthorus juglandis, Curculionidae, scolytinae, Coleoptera

The walnut twig beetle is not native to North Carolina. Its original range is thought to be the American Southwest and parts of Mexico where it lived on the native walnut species in that range. At some point, with the introduced widespread planting of black walnut (Juglans nigra) across the range, it moved to this new susceptible host. After being confined to the western US for many years, it finally made the jump to the east and first reported in Tennessee in 2010. It was probably spread by human-transported walnut wood or walnut wood products that were moved into the area. Black walnut trees across the eastern US are thought to be at risk of death by this beetle-fungus association called "Thousand Cankers Disease."

In addition to the tunneling damage by the beetles, more importantly is the introduction of at least one fungus, Geosmithia morbida, (name pending) which is carried by the beetles and infects the trees.

The walnut twig beetle is a tiny ¹⁄₁₀-inch (1.5-1.9 mm) yellowish-brown bark beetle (Figure 1).

Tunneling is almost always confined to branches ³⁄₄ inch diameter and larger, including the trunk. The beetles seem to overwinter as adults in cavities in the trunk bark. Activity resumes in spring, about April, where beetles move to the branches for mating and to excavate new tunnels. The beetles carry the fungus on the outside of their bodies and transfer it to the new tunnels where it grows into the tree tissue. Overwintering appears to be in the adult stage within in the trunk bark. Activity resumes in spring, perhaps about April. There are probably two generations in North Carolina and a flight peak is expected to occur in mid-late summer.

Figure 1. The adult walnut twig beetle is less than 2mm in size.

James LaBonte, Oregon Department of Agriculture

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Figure 1. The adult walnut twig beetle is less than 2mm in size.

James LaBonte, Oregon Department of Agriculture

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The first signs tend to be yellowing of leaves and thinning foliage. Dieback or flagging of branch tips may follow (Figure 2). Eventually, larger branches may die and foliage may wilt. Bark may die and cankers should be evident beneath (Figure 3). Tree death may takes several years, which is both good and bad. The bad part is that it allows the disease to go unnoticed and to spread.

Figure 2. Advanced walnut tree dieback.

Curtis Utley, Whitney Cranshaw, Ned Tisserat, forestryimages.org

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Figure 2. Advanced walnut tree dieback.

Curtis Utley, Whitney Cranshaw, Ned Tisserat, forestryimages.org

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Figure 3. Cankers and entrance holes exposed with bark removed.

Curtis Utley, Whitney Cranshaw, Ned Tisserat, forestryimages.org

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Figure 3. Cankers and entrance holes exposed with bark removed.

Curtis Utley, Whitney Cranshaw, Ned Tisserat, forestryimages.org

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There is no known control for this beetle or disease. Generally, by the time symptoms are noticed, even protective bark sprays would be of little value. Do not move walnut (or any type) wood / firewood away from your area or carry any into your area. NCDA&CS currently maintains a quarantine for bringing walnut wood products into the state. Coarse chipping of the wood is not entirely effective as these tiny beetles can live in chunks of cut wood or large chips for several months.

If you suspect thousand cankers disease in your black walnut, please contact your county Extension agent.

• Declaration of Exterior Quarantine for the Walnut Twig Beetle, Pityophthours juglandis, and the fungal pathogen, Geosmithia morbida sp. nov,. that coused Thousand Canker Disease in walnut trees, Juglans spp. Troxler, S. W., G. B. Cross, and S. P. Schmidt. 2010. NC Department of Agriculture and Consumer Services.
• Pathway Assessment: Geosmithia sp. and Pityophthorus juglandis Blackman movement from the western into the eastern United States. Newton, L. and G. Fowler. 2009. USDA, Animal and Plant Health Inspection Service.
• Questions and Answers about Thousand Cankers Disease of Walnut. Cranshaw, W. and N. Tisserat. 2009. Department of Bioagricultural Sciences and Pest Management, Colorado State University. (This publication is heavily based on this article.)
• Thousand Cankers Disease of Black Walnut. Anonymous. No Date. Plant Industry - Plant Protection Section, Entomological and Plant Disease Services, NCDA&CS.

For assistance with a specific problem, contact your local county Cooperative Extension center.