Pest profile

Walnut Twig Beetle

Pityophthorus juglandis · Coleoptera: Curculionidae[3]

The walnut twig beetle and a fungus it carries cause thousand cankers disease, which kills black walnut.[1]

No licensed photo of this species is available yet; the identification notes below describe what to look for.

How to identify it

  • It's a minute bark beetle, 1.5 to 1.9 mm long, yellowish brown and about three times as long as wide.[1]
  • Despite its name, it tunnels in branches over about 2 cm thick, not twigs; look for tiny exit holes and shallow tunnels under the bark.[1]

Where it lives

Native to the Southwest, it spread with walnut die-offs across western states and was found in Knoxville, Tennessee, in July 2010.[1]

US map: public records from 5 states since 1990 (GBIF).

Biology and life cycle

Adult flight peaks from mid-July through late August, and beetles produce a second generation in early summer.[1]

Cankers from the Geosmithia fungus form around beetle tunnels and merge, depleting the tree's reserves.[1]

Damage and signs

  • Black walnut shows upper-crown yellowing and thinning, then branch dieback, and often dies within three years of first symptoms.[1]
  • Hickories and pecan are immune; English and other native walnuts show intermediate resistance.[1]

Health risk

It attacks trees; CSU Extension mentions no risk to people.[1]

Control

Prevention

Surveys work best in July and August, checking smooth-barked limbs 2 to 5 cm thick for beetle tunnels and exit holes.[1]

When to call a professional

When the pest alert was written, no effective controls were known, and trunk sprays hadn't stopped the disease in early-symptom trees.[1]

More from Utah State University Extension

Pesticides do little to protect trees from the beetle and its fungus; infested trees should be identified quickly, then removed and debarked.[2]

Common questions

Why is my black walnut dying from the top down?
Thousand cankers disease, spread by walnut twig beetle, causes crown yellowing and branch dieback.[1]
Does thousand cankers disease affect pecans?
No; CSU's pest alert says hickories and pecan are immune.[1]

Sources

  1. Colorado State University Extension, “Pest Alert: Walnut Twig Beetle and Thousand Cankers Disease of Black Walnut.” extension.colostate.edu/resource/pest-alert-walnut-twig-beetle/ (accessed 2026-10-06)
  2. Utah State University Extension, “Walnut Twig Beetle.” extension.usu.edu/planthealth/ipm/ornamental-pest-guide/arthopods/bark-beetles/walnut-twig-beetle.php (accessed 2026-10-15)
  3. iNaturalist, “Pityophthorus juglandis taxon page (order and family).” www.inaturalist.org/taxa/305405 (accessed 2026-10-06)

Updated 2026-10-15. Built from sourced records. Spot an error? Tell us.