Agriculture: Walnut Pest Management Guidelines

Pacific Flatheaded Borer

  • Chrysobothris mali
  • Description of the Pest

    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.

    Related flatheaded borers with similar habits have also been found to infest walnut in California.  They include Chrysobothris analis (a relatively rare invasive species); Chrysobothris wintu (medium abundance); and Dicerca horni (abundant). The latter species is the largest and has two extended horn-like tips on the end of its wing covers that are described by its Latin name (Di cerca = two horns).


    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 and flatheaded borer invasions by painting them with interior, white latex paint by itself or mixed with water. Paint the tree trunk above and 1 inch below the soil line. 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.
    • Do not apply pesticides for this insect.
    Text Updated: 07/17