How to Manage Pests

UC Pest Management Guidelines

Walnut

Navel Orangeworm

Scientific Name: Amyelois transitella

(Reviewed 12/07, updated 3/11)

In this Guideline:


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.

The adult 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. 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.

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 the codling moth produces little webbing.

DAMAGE

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.

MANAGEMENT

Management of navel orangeworm relies on using good orchard sanitation to eliminate overwintering and feeding sites and on harvesting the new crop before the worms can enter the nuts. Good control of codling moth, walnut blight, and sunburn is also essential because navel orangeworm attacks only walnuts with damaged or split husks. Insecticides currently registered for the control of navel orangeworm in walnuts are not very effective, so preventing infestations is the most reliable approach. Harvest as soon as nuts are ready; do not allow them to remain on the tree into the fourth generation.

Biological Control
Two wasps that parasitize the 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 winter aids in decomposing trash nuts by molds and other microorganisms.

Cultural Control
A good sanitation program is essential for navel orangeworm management. There are three phases to the program:

  • Reduce overwintering populations by removing remaining nuts from trees and flailing or burning 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.
  • Reduce damaged nuts that allow entry of naval orangeworm and population increase during the season by controlling both walnut blight and codling moth, especially second generation.
  • 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.)
  • Dry nuts immediately and either fumigate on the farm, if stored, or ship immediately to a facility where fumigation will be performed.

Organically Acceptable Methods
Biological and cultural control are acceptable in an organically certified crop.

Monitoring and Treatment Decisions
Insecticide treatments should not be necessary in most orchards where a good cultural program has been carried out; in any event, chemical treatments are only partially effective (about 50% control) against navel orangeworm infestations.

Monitor split nuts and nuts on the ground for egg laying from the third generation. If egg laying is occurring at husk split, harvest promptly to avoid damage. At harvest, collect and crack out 1,000 nuts to assess damage and to plan for next year.

Common name Amount to Use** R.E.I.+ P.H.I.+
(trade name) (conc.) (dilute) (hours) (days)

 
The following materials are listed in order of usefulness in an IPM program, taking into account efficacy and impact on natural enemies and honey bees. When choosing a pesticide, also consider information relating to environmental impact.
 
THIRD FLIGHT and THIRD GENERATION LARVAE
A. METHOXYFENOZIDE
  (Intrepid 2F) 16–24 fl oz 4 14
  MODE OF ACTION GROUP NUMBER1: 18A
  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 sticker/spreader is highly recommended. Apply at the beginning of egg hatch, which is earlier than organophosphate or carbamate insecticide timings.
 
B. SPINETORAM
  (Delegate 25WG) 6–7 oz 1.5–1.75 oz 4 14
  MODE OF ACTION GROUP NUMBER1: 5
  COMMENTS: Do not exceed 28 oz/acre/season or 4 applications.
 
C. BIFENTHRIN*
  (Brigade WSB) 8–32 oz 2–8 oz 12 7
  MODE OF ACTION GROUP NUMBER1: 3A
  COMMENTS: Provides approximately 21 to 28 days of residual protection at the high label rate. Do not exceed 0.5 lb a.i./acre/season.
 
D. PHOSMET
  (Imidan 70WP) 5 lb 1–2 lb 7 days 28
  MODE OF ACTION GROUP NUMBER1: 1B
  COMMENTS: Do not apply after husk split. Do not apply more than 8.5 lb/acre/application or more than 5 times per season. Buffer to a pH of 5.5-6.0.
 
E. METHIDATHION*
  (Supracide 25WP) 4–12 lb 2 lb see comments 7
  MODE OF ACTION GROUP NUMBER1: 1B
  COMMENTS: Do not graze livestock in treated orchard. Make no more than one applications during the dormant period or more than one cover spray each season. REI is 48 hours when applied at rates less than or equal to 8 lb/acre and 14 days when applied at rates greater than 8 lb/acre.
 
F. ESFENVALERATE*
  (Asana XL) 9.6–19.2 fl oz 4 fl oz 12 21
  MODE OF ACTION GROUP NUMBER1: 3
  COMMENTS: This is a broad-spectrum pesticide that is harmful to beneficials at higher rates and can cause outbreaks of aphids and mites.
 
G. CARBARYL*
  (Sevin) Label rates 12 14
  MODE OF ACTION GROUP NUMBER1: 1A
  COMMENTS: Carbaryl causes mites to reproduce more rapidly, so monitor for the mites if this material is used. This material is best used later in the season.
 
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 additional information, see their Web site at http://www.irac-online.org/.
** For concentrate application, use the amount given in 80–100 gal water/acre, or lower if the label allows; for dilute application, amount is per 100 gal water to be applied in 300–500 gal water/acre, according to label.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) 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.

IMPORTANT LINKS

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Walnut
UC ANR Publication 3471

Insects and Mites

  • C. Pickel, UC IPM Program/UC Cooperative Extension, Sutter/Yuba counties
  • J. A. Grant, UC Cooperative Extension, San Joaquin County
  • W. J. Bentley, UC IPM Program/Kearney Agricultural Center, Parlier
  • J. K. Hasey, UC Cooperative Extension, Sutter/Yuba counties
  • W. W. Coates, UC Cooperative Extension, San Benito County
  • R. A. Van Steenwyk, Insect Biology, UC Berkeley
Acknowledgment for contributions to Insects and Mites:
  • W. H. Olson, UC Cooperative Extension, Butte County
  • L. C. Hendricks, UC Cooperative Extension, Merced County
  • G. S. Sibbett, UC Cooperative Extension, Tulare County

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