Navel orangeworm—Amyelois transitella
Navel orangeworm (family Pyralidae) larvae feed in the nutmeats of almond, pistachio, and walnut and the fruit of fig and pomegranate.
Identification
Young larvae are reddish orange and later instars are generally cream-colored but can take on the color of their food. Larvae have a distinguishing, blackish, crescent shape on each side of the second thoracic segment behind the head. Young larvae have a brown head that becomes reddish brown as they mature. Last instars are up to 3/4 inch long.
Adults (moths) are a variable, mottled mix of black and dark and light gray. They are 2/3 inch long and with wings spread are about 1 inch wide. Paired appendages that project forward give the head a snoutlike appearance.
The flattened, oval eggs are reddish orange, yellowish, or white and are laid on old nuts (mummies), new crop nuts after hull split, and injured fruit or nuts. The brown pupae occur within silk inside the fruit or nuts where they fed as larvae.
Life cycle
Navel orangeworm develops through 4 life stages: egg, larva, pupa, and adult. They overwinter as larvae in nut mummies or fruit left on the tree or dropped on the ground. The larvae do not have a resting phase (diapause) and during warm winter days will continue to feed. Overwintered larvae pupate in spring and emerge as the first generation of adults in March or April. Because not all adults emerge at the same time, adults of each generation can be present and laying eggs for about 2 months.
Early in the growing season, eggs are laid on old fruit or nuts from the previous season. Green fruit and nuts that have been damaged by other causes, such as other caterpillar species, can also become infested. Later in the growing season most egg laying is after nut hull split or on overly ripe or injured fruit.
Navel orangeworm has at least three generations per year. During growing seasons that are warmer than average, and especially in the central and southern portions of the San Joaquin Valley, navel orangeworm can have a fourth generation.
Damage
The navel orangeworm feeds in almond, fig, pistachio, pomegranate, and walnut. The first signs of an infestation are small, pinhole-size entrances in the fruit or nutmeat. As the larvae grow the entire fruit or nut is fed upon and extensive amounts of webbing and frass (excrement) accumulate. The presence of navel orangeworm or other fruit- and nut-damaging pests commonly results in fruit and nut molds.
Solutions
Sanitation is the most important control method. Because mummies are the main source of adults in the spring, the most effective control is to knock down old fruit and nuts from trees. Pick up the dropped fruit and nuts and dispose of them before the overwintering larvae pupate in late winter and mature into adults. Additional management methods include good cultural practices to reduce the likelihood of injured fruit and nuts, early harvest while the pest's abundance is lower and before fruit or nuts have become infested, and conserving natural enemies, especially parasitoid (parasitic) wasps.
Small true bugs including Phytocoris species (e.g., P. californicus and P. relativus) feed on navel orangeworm eggs. These bugs can be very abundant on nuts in the spring.
Important parasitoid wasps that can reduce damage from navel orangeworm larvae include Copidosomopsis plethorica, Goniozus legneri, a Habrobracon species, and Trichogramma species wasps that parasitize the eggs. Goniozus legneri is commercially available for purchase and release and this is done for organically managed trees. If G. legneri may be released, see Natural Enemy Releases for Biological Control of Crop Pests. To conserve resident and any released natural enemies, control ants, minimize dust, and avoid the application of broad-spectrum, persistent insecticides for all pests on the tree. See Protecting Natural Enemies and Pollinators for more information.
Provide trees with appropriate cultural care and a good growing environment. Especially avoid drought stress during the dry season because this can cause early hull split or cracked fruit, creating entry sites for navel orangeworm.
Generally, no insecticides are recommended for controlling navel orangeworm on residential fruit and nut trees. Effective timing is complex, and it is difficult for home orchardists to get spray penetration into injured fruit and split hulls where the larvae of navel orangeworm feed. However if this pest's damage has previously been unacceptable despite following the recommended cultural practices, nuts can be sprayed at the time of hull split with Bacillus thuringiensis (Bt) or spinosad to provide some control.
Adapted from Integrated Pest Management for Almonds Second Edition, Integrated Pest Management for Walnuts Third Edition, Pest Management Guidelines: Pistachio, and Pest Management Guidelines: Walnut, University of California Statewide Integrated Pest Management Program (UC IPM). |
Walnut damaged and fouled by navel orangeworm.
Feeding damage, frass (excrement), larva (right), pupa (lower left), and webbing of navel orangeworm.
Navel orangeworm larvae have a dark, crescent-shape on each side of the second thoracic segment behind the head.
Adult navel orangeworm.
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