Pest profile

Carpenterworm

Prionoxystus robiniae · Lepidoptera: Cossidae[3]

Carpenterworm caterpillars tunnel in the trunks and limbs of ash, oak, pear and other trees for two to four years, weakening branches that can break in wind.[1]

Carpenterworm moth, a large robust moth with mottled black-and-gray forewings, resting with wings folded
Carpenterworm moth, a large robust moth with mottled black-and-gray forewings, resting with wings folded. Photo: kcthetc1, public domain (CC0), via iNaturalist

How to identify it

  • Early signs are dark sap spots on the trunk, then frass and sawdust pushed out of gallery openings.[1]
  • Mature larvae are 2 to 3 inches long, greenish white with a dark brown head.[1]
  • Adults are robust moths with about a 3-inch wingspan and mottled black-and-gray forewings; males have orange hind wings.[1]
  • In older infestations, pupal cases stick about two-thirds out of the bark.[1]

Look-alikes

Look-alikeHow to tell them apart
Clearwing borersClearwing pupal cases are about 3/4 inch; carpenterworm cases are usually over 1-1/2 inches.[2]

Where it lives

Hosts include apricot, ash, birch, cottonwood, American elm, black locust, maple, oak, pear and willow; trees near streams and rivers are more susceptible.[1]

US map: public records from 45 states since 1990 (GBIF).

Biology and life cycle

Females lay 3 to 6 eggs in sticky masses in bark cracks or near existing galleries, often on the tree they came from.[1]

Larvae bore into sapwood and later heartwood; galleries reach about 1/2 inch wide and 6 to 10 inches long.[1]

Development takes 2 to 4 years in California, and moths emerge mostly May through July.[1]

Damage and signs

  • Tunneling weakens large branches that can break in high winds, a hazard in yards and streets.[1]
  • Girdling can cause branch dieback, and repeated attacks scar bark.[1]

Health risk

The caterpillars live inside trees; the hazard UC IPM describes is falling limbs, not the insect itself.[1]

Control

Prevention

Keep trees vigorous with proper watering and care, and protect them from wounds.[1]

Traps and physical control

Probe galleries with a long wire, then mark them and check weekly for fresh frass.[1]

Biological control

A single treatment with Steinernema feltiae or S. carpocapsae nematodes can be very effective.[1]

When to call a professional

Have an arborist inspect heavily infested trees; badly tunneled limbs may need removal.[1]

Common questions

What is the sawdust and sap coming from my ash tree?
Carpenterworm galleries push out frass and sawdust and start as dark sap spots.[1]
Do sprays kill carpenterworms?
Not inside the wood; apart from nematodes, insecticides only target adults, and control is hard.[1]

Sources

  1. University of California Statewide IPM Program, “Pest Notes: Carpenterworm (Publication 74105).” ipm.ucanr.edu/PMG/PESTNOTES/pn74105.html (accessed 2026-10-06)
  2. University of California Statewide IPM Program, “Pest Notes: Clearwing Moths (Publication 7477).” ipm.ucanr.edu/PMG/PESTNOTES/pn7477.html (accessed 2026-10-06)
  3. iNaturalist, “Prionoxystus robiniae taxon page (order and family).” www.inaturalist.org/taxa/126293 (accessed 2026-10-06)

Updated 2026-10-14. Built from sourced records. Spot an error? Tell us.