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Walnut Twig Beetle and Thousand Cankers Disease in NC

Steven Frank & Stephen Bambara, Extension Entomologists

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


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

General Information

The walnut twig beetle is not native to NC. 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 tunnelling 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.

Biology

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

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 NC and a flight peak is expected to occurr in mid-late summer.

Identification

The first signs tend to be yellowing of leaves and thinning foliage. Dieback or flagging of branch tips may follow. Eventually, larger branches may die and foliage may wilt. Bark may die and cankers should be evident beneath. 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. For a screening aid to identification, link HERE.

walnut tree dieback

Advanced walnut tree dieback.

walnut twig beetle

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

cankers with bark removed

Cankers and entrance holes exposed with bark removed.

Control

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 new.threat@ncdenr.gov or your county Extension agent.

See also:
Colorado State Pest Alert
(pdf)
Questions and Answers About Thousand Cankers Disease (doc)
Links to Lots of Educational Information from Colorado State University
NCDA & CS External Quarantine on Walnut (pdf)
NCDA & CS Pest Alert (doc)
USDA APHIS Pathway assessment of Geosmithia sp. and Pityophthorus juglandis

Recommendations for the use of chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by the North Carolina Cooperative Extension Service nor discrimination against similar products or services not mentioned. Individuals who use chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage and examine a current product label before applying any chemical. For assistance, contact your county North Carolina Cooperative Extension Service agent.© 2001 NC Cooperative Extension Service

Other Resources

For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service personnel.

By Steven Frank & Stephen Bambara, Extension Specialists. North Carolina Cooperative Extension Service. Heavily based on information provided by Whitney Cranshaw and Ned Tisserat of Colorado State University.

ENT/ort-158. Prepared October, 2010. Photos Curtis Utley, Whitney Cranshaw, Ned Tisserat, forestryimages.org. Inset beetle image by James LaBonte, Oregon Dept. Agr.

Web page last reviewed January, 2011 by the webperson.