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How to Manage Pests
UC Pest Management Guidelines
Walnut
Pacific Flatheaded Borer
Scientific Name: Chrysobothris mali
(Reviewed 12/07,
updated 12/07)
In this Guideline:
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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.
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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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