WCN April 2019

PACIFIC FLATHEADED BORER
Appears in Healthy Walnuts
By CRYSTAL NAY | Contributing Writer
Figure 1. Size of
Pacific flatheaded
borer larvae when
compared to a quarter.
All photos courtesy of
Jhalendra Rijal.
2018 SET A NEW PRECEDENT
for the Pacific flatheaded borer
(PFB), Chrysobothris mali, when
the pest was found in several healthy
and mature walnut orchards in Central
California.
This insect is no stranger to
California’s Central Valley, having been
around for about 100 years. But, the
borer was historically more isolated,
targeting only occasionally and appearing
in weaker trees, such as those with
sunburn, canker, or other injuries or
disease.
The discovery of last year’s increase in
walnut infestations has sparked intrigue
and concern for both growers and
researchers.
“2018 was a year we saw more
problems in walnuts,” said University
of California Cooperative Extension
(UCCE) area Integrated Pest
Management (IPM) advisor Jhalendra
Rijal. “Before, the Pacific flatheaded
borer would attack the weaker portion
of a tree, and their impression was
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small, but now they are attacking all the
other areas of the tree, and the problem
is growing.”
The Culprit
A beetle species of woodborers, the
Pacific flatheaded borer can measure
from 0.5 inches to .75 inches (12-19
mm) long as an adult, with a dark
bronze body and copper spots on the
wing covers, creating a metallic sheen.
A full-grown larva has a flattened body
whitish in color, with an amber-colored
head.
Usually found in riparian and forested
areas, the increase—and move—of
PFB infestation can be attributed to a
few factors, with researchers gearing
up to learn more. For one, infestation
increase can be simply correlated to the
increase in walnut acreage. It can also
be attributed to the uncharacteristic
dryness of previous years. The nearby
forest areas were very dry, which also
resulted in severe fire damage, and may
have encouraged the beetle’s stronger
push into the Central Valley.
Infestation
While this pest has caused problems
before, complications occurred
mostly in smaller trees overexposed
to sunlight, where split bark allowed
for easy entrance. Starting in spring or
early summer, the Pacific flatheaded
borer finds bark cracked from sunburn,
and enters the wood to lay eggs. This
can occur in either the trunk or the
branches. The larvae hatch and feed on
the cambium layer, which puts them
below the bark, but still not yet into the
wood of the tree.
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West Coast Nut April 2019