Walnut twig beetle

What is walnut twig beetle?

The walnut twig beetle is a very small bark beetle that carries the fungus Geosmithia morbida. The beetle brings the fungus in as it tunnels under walnut bark to feed. Dark cankers form under the bark, and as they grow together they disrupt the flow of water and nutrients. That is thousand cankers disease (TCD).

Black walnut is the main host. UW-Madison says the disease has been fatal to black walnut in all known cases, with yellowing at the top of the tree, then branch dieback, then death of the whole tree. PDA's own page is less grim: it says TCD has not been shown to kill the tree and that trunk death is now considered less likely, though repeated attacks can cause branch dieback.

Beetles spread the disease between nearby trees, but long jumps happen when people move walnut seedlings, firewood and wood products. The walnuts themselves have not been reported to spread it. That is why walnut logs, firewood and nursery stock were regulated in Pennsylvania from 2011 until PDA repealed its quarantine in 2022.

Signs and damage

• Leaves yellowing at the top of a black walnut, then lower in the crown
• Branch dieback; UW-Madison reports death of the whole tree in time, while PDA says trunk death is now considered less likely
• Flagging branches with yellowed leaves still attached, or collapsed, wilted leaves (a stronger sign, per UMD)
• Tiny holes, about the size of a pencil tip, on branches
• Many small dark brown to black cankers under the bark that grow together

Walnut twig beetle identification: what to look for

• Adult: Tiny reddish-brown bark beetle about 1.5 to 1.9 mm long (UMD). Its entrance holes are about pinhole size, so damage is easier to spot than the beetle.
• Damage: Small dark cankers under the bark around beetle tunnels, which merge; crown yellowing from the top down, branch dieback, tree death.
• Symptoms: Yellowing leaves starting at the top of the crown, then lower leaves, then dying branches.

Life cycle

Not stated in detail in the sources opened. The beetle tunnels under walnut bark to feed and carries the Geosmithia morbida fungus into its tunnels, where cankers form.

When to look: Crown yellowing and dieback show during the growing season. Check black walnuts that thin from the top down.

How to control walnut twig beetle

Cultural control

• Don't move walnut logs, branches, firewood, chips or nursery stock out of an area with TCD.
• Be careful with walnut transplants and wood products with bark from areas where TCD is reported.
• Buy local firewood and don't haul it; see our firewood rules guide.

Chemical control

UW-Madison says there are no formal management recommendations; researchers are still studying insecticides, fungicides and nutrition to extend the life of infected trees. There is no cure, so prevention and reporting come first. If any treatment is tried on a valuable tree, use only products labeled for this use; the label is the law, and the work belongs to an ISA Certified Arborist or licensed applicator. Muthler Landscaping does not treat trees or apply pesticides; see our pesticide applicator license guide.

Use only products labeled for this pest and this site; the label is the law. For trees, work with an ISA Certified Arborist. If you hire someone to apply pesticides, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Prevention

• Don't move walnut wood with bark, walnut nursery stock or hardwood firewood out of areas where TCD is known.
• Follow Pennsylvania's firewood rules; see our firewood rules guide.

When to call a pro

Call an ISA Certified Arborist and report the tree to PDA or DCNR if a black walnut is yellowing and dying back from the top down.

Related guides: Tree database: mature size and planting distance · Pennsylvania tree identification · Firewood rules by state · All pests and diseases

Questions about walnut twig beetle

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