How to Manage Pests
UC Pest Management Guidelines
Walnut kernels are mature and of lightest color and highest quality when the packing tissue between the kernel halves turns brown. To maximize kernel quality and minimize insect and mold damage, harvest as close as possible to the time when the most nuts have reached the packing tissue brown stage. The problem often encountered is that hull dehiscence (separation of the hull from the nut) occurs later than kernel maturity, and hot weather can further delay this process. To speed up hull dehiscence, accelerate maturity, and promote fruit abscission, ethephon is often used. The use of ethephon on the earlier-maturing varieties avoids the late season walnut husk fly and navel orangeworm flights.
Walnuts are either harvested with one or two shakes. When two shakes are planned, ethephon advances hullsplit and allows harvest to be conducted closer to the time when most nuts are at the packing tissue brown stage and of highest quality.
For a two-shake harvest, ethephon is applied when 95 to 100% of the nuts have reached packing tissue brown. Harvest can usually begin 7 to 10 days earlier than the normal harvest date, followed by a second shake about two weeks later.
For a one-shake harvest, ethephon is applied 10 to 14 days before the normal harvest date. This timing is when 100% of the nuts have reached packing tissue brown. With this application timing, nut removal is increased and a second shake is not needed.
Notes on the use of ethephon:
UC IPM Pest Management Guidelines: Walnut