WCN August 2020

Continued from Page 9
traps baited with a combination of CM
pheromone and pear ester plant volatile,
as mating disruption will reduce trap
counts in pheromone-only traps.
Traps should be checked to determine
first flight biofix, the date where
moths are consistently caught in traps
and sunset temperatures are 62 degrees
F or above. After that first flight biofix
day is determined, begin tracking
degree-day accumulations to schedule
spray applications and predict the onset
of subsequent flights. Symmes said the
degree-day models for CM pair the
known developmental requirements –
heat units for a specific pest in a specific
crop with the actual heat units.
The degree-day calculator for CM is
on the UC IPM web site at ipm.ucanr.
edu, in Identify and Manage Pests at
weather and degree days. Temperatures
can be obtained from the nearest
CIMIS station. There is a range for
each flight prediction and it should be
confirmed with trap activity.
“
Determining the need for a spray application is
based on orchard history, in-season trap catches,
and for the second and third flights, damage
evaluation including dropped nuts and
canopy counts.
Treatment Decisions
Determining the need for a spray
application is based on orchard history,
in-season trap catches, and for the
second and third flights, damage
evaluation including dropped nuts and
canopy counts.
High crop damage due to CM in
recent years is puzzling, according to
Symmes, even with well-timed spray
applications. Warmer, drier weather
may be one factor in the increased CM
damage seen. Another factor may be
the loss of chlorpyrifos as a critical tool.
Use of chlorpyrifos has dramatically
decreased in recent years, she said, but
it was a reliable tool that could be used
”
in the case of a severe infestation. With
chlorpyrifos applications, coverage
of the tree canopy and precise timing
to target the egg or early larval stage,
while important, was not as critical as
it is with more selective pesticides now
in use.
There is one walnut growing region
of California that does not have high
CM pressure. The Lake County region,
where about 4,200 acres of walnuts are
farmed and the majority are in organic
production, has some environmental
advantages, according to UCCE Lake
County pomology farm advisor Rachel
Elkins. Late spring rains and frost
events are common and suppress the
resident CM populations, she said. In
addition, the varieties grown in that
region help lessen the susceptibility to
CM damage. Finally, Elkins said, pears
grown in the area are a preferred host
for CM.
When growers did plant the earlier
maturing varieties, they did have issues
with CM damage, Elkins said. Once
those were replaced with later varieties,
the CM infestation lessened. An added
benefit, she noted, was that without CM
pressure in the orchards, their NOW
damage disappeared.
Lower insect pressure in Lake
County is one of the reasons growers
there are able to use organic production
practices, Elkins said.
The main insect pest in the area is
the walnut husk fly that prefers later
blooming and maturing walnut varieties,
and can be managed with organic
practices.
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10 West Coast Nut August 2020