California Walnuts: Staying On Point Through August

    Walnut orchard.


    • If walnut husk fly traps are not set out by now, you’re late! Yellow sticky traps with an ammonium carbonate lure work best and should be checked 2-3 times per week. Depending on treatment approach, treat based on first detection of husk fly, detection of eggs or trends in trap catch numbers. For more details on treatment decision-making, see:
    • We’re heading towards the second codling moth biofix. This biofix is set when trap numbers increase, approximately 800 to 1300 DD after the first biofix. Use the following decision table to determine whether treatment is necessary:
    • If Bot pressure is low, mid-June to early July is the best timing for a single fungicide treatment for Bot canker. More info at:
    • Monitor spider mites weekly through August. Examine 10 leaflets from 10 trees (5 should be from higher branches). If more than half the leaflets with spider mites don’t also have predators (predaceous mites and/or sixspotted thrips), this is a cause for concern. Treatment guidelines based on spider mite and predator presence can be found at:


    • Codling moth third flight occurs in late July to early August (on average, 1,100 DD after the second biofix). Check traps to pinpoint the third flight biofix. Treatment decisions are based on a combination of factors including previous treatments, number of nuts infested in the previous generation, trap catches, and the ability to harvest early.
    • If sprays are going to be applied for eggs from the second generation, apply at 300 Degree Days (DD) after the third biofix (200-250 DD if insect growth regulators are used). More details:
    • Continue weekly monitoring for husk fly and spider mites. See trapping and monitoring details above.
    • Take July leaf samples to assess nitrogen, potassium, and zinc, as well as boron toxicity, depending on your circumstances. Sample 1-2 terminal leaflets from at least 29 trees, each at least 100 feet apart, on the same rootstock scattered throughout the orchard.


    • Monitor for navel orangeworm. Healthy, intact walnuts are only susceptible to NOW damage at and after hull split. When considering treatment options, bear in mind pre-harvest intervals and duration of residual activity. More information at:
    • Be careful when timing ethephon applications for advancing walnut harvest this year. A late bloom and leafout may delay application timing compared to the last couple years. The most reliable way to avoid damage is to examine nuts for the packing tissue brown (PTB) stage. Do not treat earlier than 100% PTB. For more on sampling for PTB and safely applying ethephon, see:

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