How to Manage Pests
UC Pest Management Guidelines
Scientific Name: Amyelois transitella
(Reviewed 7/17, updated 7/17)
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. 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.
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.
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.
A good sanitation program is essential for navel orangeworm management.
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.
UC IPM Pest Management Guidelines: Walnut
Insects and Mites
J. A. Grant, UC Cooperative Extension, San Joaquin County
Acknowledgment for contributions to Insects and Mites:W. J. Bentley, UC IPM Program, Kearney Agricultural Research and Extension Center, Parlier (Emeritus)
L. C. Hendricks, UC Cooperative Extension, Merced County (Emeritus)
W. H. Olson, UC Cooperative Extension, Butte County (Emeritus)
C. Pickel, UC IPM Program, Sutter and Yuba counties (Emeritus)
G. S. Sibbett, UC Cooperative Extension, Tulare County (Emeritus)
D. Light, USDA, Albany, CA (Emeritus)(Codling Moth)