How to Manage Pests

UC Pest Management Guidelines


Pacific Flatheaded Borer

Scientific Name: Chrysobothris mali

(Reviewed 12/07, updated 12/07)

In this Guideline:


Pacific flatheaded borer adults are generally present in May and June. When spring months are warm, borers may be seen as early as March or early April. The adult beetle has a dark bronze body with coppery spots on the wing covers and is about 0.4 inch (10 mm) long. A full-grown larva is light colored, with a prominent, flat enlargement of the body just behind the head. There is one generation each year.


The Pacific flatheaded borer is attracted to diseased (e.g., Phytophthora, Armillaria, etc.) or injured limbs, such as those affected by sunburn, scale insects, or major pruning cuts, where it lays eggs. When larvae hatch they excavate large caverns just beneath the bark; just before pupating they bore tunnels deep into the wood. Excavations are usually filled with finely powdered sawdust. Feeding by Pacific flatheaded borers may cause a portion of the bark to die, and may girdle and kill young trees. Infested branches on older trees often die. Dead, brown leaves remain on these branches during summer and fall.


Flatheaded borers often invade sunburned areas on the trunk of newly planted first-year trees. At planting time protect the trunks of newly planted trees from sunburn by painting them with interior, white latex paint by itself or mixed with water. Painting the tree trunk above and 1 inch below the soil line will protect the trunk from sunburn and flatheaded borer invasions. Repaint if soil settling occurs. In older trees the best way to avoid infestations is to keep your trees sound and vigorous. Prune out all badly infested wood, and burn or remove it from the orchard before the growing season starts. Spraying for this insect is not recommended.


[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Walnut
UC ANR Publication 3471

Insects and Mites

  • C. Pickel, UC IPM Program/UC Cooperative Extension, Sutter/Yuba counties
  • J. A. Grant, UC Cooperative Extension, San Joaquin County
  • W. J. Bentley, UC IPM Program/Kearney Agricultural Center, Parlier
  • J. K. Hasey, UC Cooperative Extension, Sutter/Yuba counties
  • W. W. Coates, UC Cooperative Extension, San Benito County
  • R. A. Van Steenwyk, Insect Biology, UC Berkeley
Acknowledgment for contributions to Insects and Mites:
  • W. H. Olson, UC Cooperative Extension, Butte County
  • L. C. Hendricks, UC Cooperative Extension, Merced County
  • G. S. Sibbett, UC Cooperative Extension, Tulare County

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