Agriculture: Walnut Pest Management Guidelines

Navel Orangeworm

  • Amyelois transitella
  • Description of the Pest

    Navel orangeworm overwinters as larvae inside mummy nuts on the tree and in trash nuts left on the ground and around hullers. Pupation begins in March and may continue through early May. Moths of the overwintered brood start emerging in April, and peak emergence usually occurs from late April to mid-May, depending on season and locality. Females of the overwintered generation lay their eggs singly on mummy nuts, codling moth-infested nuts, or blighted nuts. The first generation, and most of the second, is completed in these nuts. In late summer, some of the second-generation larvae infest the new crop as the husks begin to split. Females emerging at this time prefer to lay eggs on the opened husk or on the exposed nutshell. In later-harvested varieties, nuts may also be exposed to infestation by third generation larvae.

    The moth is silver gray with irregular black patches on the forewings. The snoutlike palps in front of the head help distinguish this moth from the codling moth.

    Eggs of the navel orangeworm are opaque white when first laid. After about a day, they turn pink, then reddish orange.

    The navel orangeworm larva has a pair of brown, crescent-shaped marks on the second segment behind the head. These marks are absent on codling moth larvae. After hatching, the tiny caterpillars enter nuts through the soft tissue at the stem end and do not emerge until they are adults. Several larvae may infest one nut and produce substantial webbing. In contrast, only a single codling moth is found in each nut, and codling moth larvae produce little webbing.


    Nuts infested with navel orangeworm are unmarketable because the larvae feed on the nutmeats and produce webbing and frass. Navel orangeworm do not damage sound walnuts until the husks begin to split. Nuts infested only by the navel orangeworm may show no external signs of webbing or frass, but shells of heavily infested nuts will have an oily appearance.


    Sanitation, reducing damaged nuts, and prompt harvest are the most reliable approaches to avoid navel orangeworm damage. Use sanitation practices to reduce overwintering and development sites. Good control of codling moth, walnut blight, and sunburn is also essential because navel orangeworm attacks only damaged walnuts prior to husk split. Harvest as soon as nuts are ready to limit egg laying and infestation by larvae.

    Biological Control

    Two wasps that parasitize navel orangeworm, Copidosoma (=Pentalitomastix) plethorica and Goniozus legneri, are established in many walnut and almond growing areas and account for some larval mortality. A ground cover maintained during wet winters aids in decomposing trash nuts by molds and other microorganisms.

    Cultural Control

    A good sanitation program is essential for navel orangeworm management.

    1. Reduce the number of overwintering navel orangeworm by removing remaining nuts from trees and flailing them and destroying all crop waste containing nuts before mid-March. This includes removing all mummy nuts found in the trees during the dormant period, all windfall and huller waste materials found in the field, and all waste materials cleaned up from bins, hulling and drying equipment, and buildings after harvest and dehydration.
    2. Reduce damaged nuts that allow entry of naval orangeworm and increase in their numbers during the season by controlling walnut blight, sunburn, and codling moth, especially the second generation.
    3. Harvest as early as possible. Use of ethephon to advance husk splitting is advantageous, particularly during heavy worm populations or prolonged dry falls. (For more information, see USING ETHEPHON.)
    4. Dry nuts immediately and either fumigate on the farm, if stored, or ship immediately to a facility for fumigation.

    Organically Acceptable Methods

    Use biological and cultural controls in an organically certified crop.

    Monitoring and Treatment Decisions

    Insecticide treatments may not be necessary in most orchards where a good cultural program has been carried out, depending on proximity to external sources of navel orangeworm.

    Monitor once husk split has begun. Examine split nuts and nuts on the ground for egg laying from the second and third generations or from moths that may be immigrating into the orchard. If egg laying is occurring at husk split, consider an insecticide treatment and harvest promptly to avoid damage.

    At harvest, collect and crack out 1,000 nuts to assess damage, properly identify the pest responsible for the damage and to plan for next year. In harvest samples, it is easy to identify codling moth damage from navel orangeworm damage when the worms are present. Navel orangeworm has a brown crescent-shaped marking behind the head capsule on both sides of the first thoracic segment; this mark is absent in codling moth larvae. There can be multiple navel orangeworm larvae but only one codling moth larva per nut. If the worm is not present, look at the damage: navel orangeworm leaves behind more webbing and frass. However, navel orangeworm frequently infest nuts that were previously infested by codling moth, so if navel orangeworm is present, it doesn't mean codling moth wasn't previously there.

    Common name Amount to use** REI‡ PHI‡
    (Example trade name) (conc.) (dilute) (hours) (days)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.
    (Brigade WSB) 8–32 oz 12 21
    COMMENTS: Provides approximately 21 to 28 days of residual protection at the high label rate.
    (Warrior II with Zeon) 2.56 fl oz 24 14
    COMMENTS: Larvicide.

    (Intrepid Edge) 10–18 fl oz 4 7
    COMMENTS: Apply at the beginning of egg hatch, which is earlier than organophosphate or carbamate insecticide timings.
    (Intrepid 2F) 8–16 fl oz 4 7
    COMMENTS: An insect growth regulator that provides 10 to 18 days of residual protection depending on the rate of application and nut expansion. Kills young larvae but does not kill adult moths. It is a reduced risk insecticide that has little or no effect on beneficial insects and mites. Spray coverage is extremely important. Do not apply to large trees unless adequate spray coverage can be verified. Use no less than 100 gal water/acre for ground applications. Sprayer speed should not exceed 1.5 mph. The use of Latron B-1956, CS-7, or similar spreader-sticker is highly recommended. Apply at the beginning of egg hatch, which is earlier than organophosphate or carbamate insecticide timings.
    (Altacor) 3–4.5 oz 4 10
    COMMENTS: Larvicide. The best timing is to apply before egg hatch. Do not make more than four applications per year. To reduce the development of resistance do not make more than three consecutive applications of any group 28 insecticides (anthranilic diamide) per generation per season.
    (Delegate WG) 6–7 oz 1.5–1.75 oz 4 1
    (Imidan 70W) 5 lb 1–2 lb 168 (7 days) 28
    COMMENTS: Do not apply after husk split. Buffer to a pH of 5.5 to 6.0.
    (Asana XL) 9.6–19.2 fl oz 4 fl oz 12 21
    COMMENTS: This is a broad-spectrum pesticide that is harmful to natural enemies at higher rates and can cause outbreaks of aphids and mites.
    (Sevin) Label rates 12 14
    COMMENTS: Carbaryl causes mites to reproduce more rapidly, so monitor for the mites if this is used. This insecticide is best used later in the season. Highly toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
    1 Rotate chemicals with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; chemicals with a 1B group number should be alternated with chemicals that have a group number other than 1B. Mode-of-action group numbers are assigned by IRAC (Insecticide Resistance Action Committee).
    ** For concentrate application, use the amount given in 80 to 100 gal water/acre, or lower if the label allows; for dilute application, amount is per 100 gal water to be applied in 300 to 500 gal water/acre, according to label.
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of these two intervals is the minimum time that must elapse before harvest may occur.
    * Permit required from county agricultural commissioner for purchase or use.
    Not recommended or not on label.

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    Text Updated: 07/17
    Treatment Table Updated: 07/17