Pest profile

Pacific Flatheaded Borer

Chrysobothris mali · Coleoptera: Buprestidae[3]

Pacific flatheaded borers lay eggs on stressed, sunburned or wounded trees, so good care and trunk protection are the best defense.[1]

Pacific flatheaded borer, a bullet-shaped dark bronze beetle with pale mottling, seen from above
Pacific flatheaded borer, a bullet-shaped dark bronze beetle with pale mottling, seen from above. (c) Michele Jones, some rights reserved (CC BY) — via iNaturalist, CC BY 4.0

How to identify it

  • Adults are bullet-shaped, 1/2 to 3/4 inch, coppery, dark bronze or gray with pale mottling.[1]
  • Larvae are pale and legless with a greatly enlarged segment behind the head.[1]
  • Early signs are wet spots and cracking bark, then winding rough bark and round to D-shaped exit holes.[1]

Look-alikes

Look-alikeHow to tell them apart
American plum borerShares hosts with flatheaded borers; check which borer is present before acting.[2]

Where it lives

It's the most common of at least 69 Chrysobothris species in California and attacks dozens of broadleaf, fruit and nut trees and ornamentals.[1]

US map: public records from 6 states since 1990 (GBIF).

Biology and life cycle

Each female lays about 100 eggs singly on bark, and larvae tunnel in the cambium.[1]

Mature larvae overwinter under bark and pupate in spring; adults fly mostly April through August.[1]

There's usually one generation a year.[1]

Damage and signs

  • Larvae tunnel under bark in winding paths, injuring the cambium of stressed trees.[1]
  • Heavy adult feeding can partly defoliate young trees.[1]

Health risk

It feeds on trees only; UC IPM mentions no risk to people.[1]

Control

Prevention

Plant well-adapted species, avoid trunk and root injuries, and water properly; drought stress invites borers.[1]

Whitewash young trunks with indoor white latex diluted with equal water, or wrap them, to prevent sunburn.[1]

Sanitation

Prune out limited infestations, remove dying trees, and don't stack fresh-cut wood near trees; solarize it under clear plastic.[1]

Biological control

The wasp Trigonura californica and a predatory mite attack the larvae.[1]

Common questions

Why does my young tree have wet spots and cracked bark?
They're common first signs of flatheaded borer larvae feeding under the bark.[1]
How do I prevent it?
Keep trees healthy and protect trunks from sunburn and wounds.[1]

Sources

  1. University of California Statewide IPM Program, “Pacific Flatheaded Borer (Pests in Gardens and Landscapes).” ipm.ucanr.edu/PMG/GARDEN/FRUIT/PESTS/pacfltheadbor.html (accessed 2026-10-06)
  2. University of California Statewide IPM Program, “American Plum Borer (Pests in Gardens and Landscapes).” ipm.ucanr.edu/PMG/GARDEN/FRUIT/PESTS/amerplumbor.html (accessed 2026-10-06)
  3. iNaturalist, “Chrysobothris mali taxon page (order and family).” www.inaturalist.org/taxa/455233 (accessed 2026-10-06)

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