Walnut Twig Beetle

About the Walnut Twig Beetle

The walnut twig beetle (Pityophthorus juglandis) is a tiny, but formidable threat to North America’s walnut industry. Native to the southwestern United States, it has expanded its range in recent decades and poses a particular risk to Eastern Black Walnut (Juglans nigra). Although the beetle has not been detected in Iowa, it represents a serious environmental and economic concern.

The walnut twig beetle is best known for its role as the primary vector of the fungal pathogen Geosmithia morbida, which causes Thousand Cankers Disease. The beetle burrows into the bark of walnut trees to feed and lay eggs, carrying the fungus with it.

The walnut twig beetle has not been detected in Iowa.

Life Stages & Identification 

The walnut twig beetle is a small, about 1/16 inch, reddish-brown bark beetle. It is characterized by four to six concentric ridges on the upper surface of the pronotum (the shield-like cover behind and over the head).

Walnut twig beetle larvae are white, legless, C-shaped, and found in the phloem. Egg galleries created by adults are generally horizontal (across the grain) and the larval galleries tend to be vertical (along the grain). Male beetles bore into the bark first, creating a push-pin-sized hole in the tiny cracks and corky furrows of the otherwise smooth bark surface of English walnut. Several females then join the male and create the gallery system.

There are two generations of walnut twig beetles per year. Adults overwinter under the bark, emerging in April to lay eggs for the first generation. Larvae take four to six weeks to develop under the bark, emerging as adults to lay eggs for a second generation. Adults emerge through tiny holes that they chew in the bark.

Impacts

As walnut twig beetles build galleries beneath the bark to feed and reproduce, they introduce spores of Geosmithia morbida, which causes Thousand Cankers Disease, into the tree.

• The fungus infects the phloem, creating small dead areas known as cankers.
• Tree mortality occurs only after thousands of beetle attacks, as cankers merge and disrupt nutrient flow.
• While Arizona walnut shows relative tolerance, Eastern Black Walnut is highly susceptible and often dies once symptoms appear.

Current Location

The walnut twig beetle is established in parts of the western and southwestern United States and has expanded into additional states over time. Its spread is closely linked to human movement of infested walnut materials.

• Natural spread: The beetle typically flies only short distances to nearby trees.
• Human-assisted spread: Long-distance movement occurs through transport of infested walnut logs, firewood, wood packaging, and nursery stock.
• Walnut nuts: The disease is not seed-borne; walnut nuts do not spread the beetle or pathogen.

The walnut twig beetle has not been detected in Iowa.

Signs and Symptoms of Infestation

Walnut twig beetle damage is often difficult to detect in the early stages because activity occurs beneath the bark. Symptoms typically become noticeable after the fungus associated with the beetle disrupts nutrient flow. Early detection is critical to slow spread and reduce tree mortality. Advanced infestations may result in branch mortality and eventual tree death.

What to look for:

• Yellowing, wilting or thinning leaves in the upper canopy.
• Dieback of twigs and branches, especially in the upper canopy.
• Gradual canopy decline, starting in the crown and moving downward over time.
• Numerous small entry holes (about the size of a pinhead) on branches and the main stem.
• Fine sawdust-like frass in bark crevices or at the base of the tree.
• Circular to oblong cankers form where the beetle enters and exits the bark. They are visible only when you peel the bark back. They appear as small patches on branches at first, but as the tree becomes more infested, the cankers begin to merge into larger cankers that may be visible on the trunk. These large cankers can girdle tree branches and trunks.
• Epicormic shoots (water sprouts) forming along the trunk or major branches as the tree declines.
• Increased woodpecker activity on infested trees.

What You Can Do

If you have walnut trees on your property, monitor them regularly for signs of decline:

• Look for leaf yellowing or flagging in mid-summer
• Check for thinning or dead branches in the upper canopy
• Inspect sunny (often west-facing) portions of the trunk and branches, especially near leaf scars

If you suspect an infestation:

• Contact the Iowa DNR Forest Health Program or the ISU Plant and Insect Diagnostic Clinic before collecting or submitting samples.
• If instructed to submit samples, freeze them for 48 hours and double-bag prior to transport to prevent accidental spread.

Prevention Tips

Don’t Move Firewood

• Buy firewood locally and burn it where you buy it
• Never transport leftover firewood from camping or hunting trips
• Remember that Walnut Twig Beetle can survive for long periods in cut wood

Manage Walnut Wood Responsibly

• Avoid moving raw walnut logs, stumps, or green lumber from infested areas
• Purchase nursery stock and scion wood only from reputable, certified sources
• If walnut lumber must be moved, ensure it is kiln-dried or fully debarked

Tree Health and Maintenance

• Reduce tree stress through proper watering and mulching, especially during drought
• Dispose of pruning debris on-site whenever possible
• Monitor trees annually, as early detection is critical to slowing localized infestations

Helpful Resources

• ISU Plant and Insect Diagnostic Clinic

Learn More

• Official Thousand Cankers Disease Information
• Walnut Twig Beetle and Thousand Cankers Disease of Black Walnut (Colorado State University)