Submitted to: University of California Division of Agriculture and Natural Resources
Publication Type: Book / chapter
Publication Acceptance Date: 4/1/2010
Publication Date: N/A
Citation: Interpretive Summary: The purpose of this chapter is to provide a broad overview of the life history and the current recommendations for controlling important arthropod pests of almond. The navel orangeworm, Amyelois transitella (Walker), is the most important lepidopteran (moth) pest of almond and originally was a native of the southwestern United States and Mexico. It was first reported in California in 1942 and by 1949 it had spread throughout the Central Valley. In California it is also the primary pest of pistachios and an important pest of walnuts, although other hosts for this insect include the rotting or mummified fruit of pomegranate, stone fruits, pear, apple, fig, and grapefruit. Adult moths vary in size from 16 to 27 millimeters and are silver gray with irregular black patches of scales on the forewings. Females lay eggs that typically hatch after 4 - 40 days, depending on temperature. The caterpillars that emerge from the eggs range in color from reddish orange or pink to milky white, are typically 16-23 millimeters long at maturity, and are the damaging stage. Feeding damages the kernel and the presence of insects or insect fragments in nuts is a problem to consumers. There are several generations of this insect each year, depending on temperature, but it is impossible to identify discrete generations because there is substantial overlap. The rate of development is dependent on both the quality of the host and temperature, and this rate typically speeds up in late July and August. Adult emergence from unharvested nuts and fruit begins in late February and the peak of the spring flight occurs from late April through mid May; there are subsequent peaks in July, August and September. Adults continue their life cycle when they lay eggs on unharvested nuts from the previous year or lay eggs on new crop nuts that become available from late June through October. The single most important method used to control navel orangeworm is the practice of sanitation. Unharvested almonds are removed from the trees by shaking or knocking them down between December and early February. This procedure can be quite costly, as much as $300 dollars per acre, but is an essential practice. Sanitation is supplemented with the use of insecticides in many counties. Insecticides are timed based on population monitoring using egg traps, which consist of tubes containing almond meal, that in turn attract females seeking a host on which to lay eggs. Insecticide applications may be made at several times during the season, but a typical timing is the use of sprays at hull split in late June – early July, followed up with an additional spray in mid – late July. There are several classes of insecticides available today that do not cause as many problems to beneficial and nontarget arthropods as older materials such as organophophate insecticides that are being phased out. These new chemicals include insect growth regulators and calcium channel activators that target skeletal muscle. The goal of insecticide treatment is to suppress the navel orangeworm but preserve beneficial arthropods.
Technical Abstract: The navel orangeworm, Amyelois transitella (Walker), is the most important insect pest of almond in California and can cost as much as $500 dollars per acre to control when the costs of insecticides and sanitation are included. It is a native of the southwestern United States and Mexico and was first collected in Southern California in 1942, and by 1949 it was distributed throughout the Central Valley. Almond, walnut, and pistachio are the hosts this insect attacks most severely, but other hosts include the rotting or mummified fruit of pomegranate, stone fruits, pear, apple, fig, and grapefruit. Adults are silver gray with irregular black patches of scales on the forewings and range in size from 16 to 27 millimeters. Female moths usually begin oviposition on the second day after emergence and will lay an average of 85 eggs per female over a 1-to 3-week period. Eggs are laid within or on the surface of host fruit or close to fruit on twigs or fruit stems and hatch in 4 to 40 days, depending upon temperature. Larvae, containing a characteristic horseshoe-shaped sclerite on the side of the thorax, are reddish orange when first hatched. Larval development is dependent on the quality of the host and can be completed in as little as 18 days in midsummer but may extend to over 7 months when eggs are laid in the fall. This insect does not diapause and as a consequence the emergence of adults from different generations overlaps. Adults from the overwintering population typically emerge from February through June. Emergence begins at different times in the Sacramento and San Joaquin Valleys because it is dependent on degree-day accumulation, but is well underway by April and the majority of the adults have emerged by late May. After mating, female moths return to mummy nuts in the tree and on the ground and lay eggs on these nuts; these host fruits are the only known nutritional source for the larvae. When adult traps are used to monitor populations there may be no discrete break in capture and adult flight can be continuous from March through October. Management of this pest includes orchard sanitation, peach twig borer control, early and rapid harvest, insecticide application and postharvest fumigation. Sanitation is accomplished by removing as many mummy nuts as possible from the trees from December through February, followed by disking or shredding any nuts that are not buried in wet weeds or grass. In some counties, mummies should be removed from the trees to a maximum of two mummies per tree prior to bloom while in the San Joaquin Valley a more stringent standard of 0.5 mummies per tree is recommended. Growers treat navel orangeworm at one or three points in the season. The first is in May, when the overwintering adults infest mummies; the second is in late June – early July, when the new crop nonpareil nuts begin to split; the third is in mid July when hull split is well underway and a substantial portion of the kernel is exposed. The choice of insecticide will affect the timing of the late June-early July and post hullsplit treatments. Ovicides/larvicides such as insect growth regulators and calcium channel modulators work best when the eggs are laid on treated surfaces, necessitating an earlier application than for organophosphates or pyrethroids that have adult activity. An early, rapid harvest helps minimize exposure of nuts to oviposition by the later flights. Nuts dry faster on the ground and, when grounded, are less susceptible to oviposition by navel orangeworm. In some situations, on-farm fumigation of harvested nuts may also be advisable to arrest feeding and reduce damage, particularly if deliveries to hullers are delayed after nut pickup in the field.