Craig Kallsen

UCCE Farm Advisor, Kern County

Citrus, Subtropical Horticulture, Pistachios

January 7, 2002

Navel Orangeworm Seen as Increasing Threat to Pistachio Production

Navel orangeworm, Amyelois transitella, played havoc with the economic returns of many growers in the San Joaquin Valley this season. This pest is particularly insidious in that not only does it directly reduce yield by reducing the number of harvested nuts and by increasing the number of nuts culled at the huller, but infested nuts that are missed during processing end up in consumer packaging. Navel orangeworms that survive processing usually hatch in bags of pistachios that are not immediately eaten by consumers. Nuts which have been damaged by navel orangeworm but which no longer house a living navel orangeworm, are composed of decayed nutmeat, frass and secondary fungal invaders that produce potentially poisonous aflotoxins. Products containing moths and decayed pistachio nuts do not usually encourage consumers to make further purchases. During the past season, it was common for growers to find 2 percent or more of their nuts arriving at the huller infested with navel orangeworm. Levels of 5 percent or more of infested nuts will probably result in the entire load of nuts being processed as shelling stock or lesser products instead of being packaged as the more valuable in-shell nuts that consumers associate with pistachios.

Typically in the San Joaquin Valley the navel orangeworm has 4 to 5 generations per year. Early-season infestations in an orchard can be determined based on the use of egg traps baited with mixtures of almond press cake and almond oil. The first generation of moth egg-laying activity usually peaks in late April and early May and the second generation in late June or early July. Generally, however, only the third generation is treated with chemicals. If populations are high early in the season, appropriate insecticides are applied approximately 300-400 degree-days after third-generation egg laying begins, usually in early August. After about mid-August pistachio nuts are so attractive to the moths that the egg traps no longer work. If third-generation egg-traps lose their effectiveness, treating approximately 1000 degree-days after the onset of second-generation egg laying will approximate the appropriate time for treating third-generation navel orangeworms. More information on monitoring navel orangeworm populations, which insecticides to use, and when to treat using degree-day calculations is available in the University of California Integrated Pest Management (IPM) Guidelines. These guidelines are available on line at www.ipm.ucdavis.edu/PMG/r605300111.html and also from most University of California Cooperative Extension Offices in the San Joaquin Valley. Make sure that the length of the preharvest interval (PHI) of any applied insecticide will not interfere with the scheduled harvest date.

Frequently, navel orangeworm populations do not reach damaging levels until late in the season. In early to mid-August, if the orchard has not yet been treated, nuts can be collected from the field and examined with a magnifying glass. The greater the number of early split nuts the more likely it will be that navel orangeworm will be a problem. Usually a sample of 100 to 200 randomly collected nuts from the orchard are inspected and if 3 to 4 percent of the nuts have eggs, the orchards will be treated with an appropriate, registered insecticide. In an untreated field, the percent infestation of the nuts can climb by 1 percent a week. Generally the later in the year that pistachios are harvested, the greater the number of infested nuts. As many growers discovered this year, chemical control may not be adequate to reduce infestations sufficiently.

Navel orangeworm does not over winter in the egg, so is dependent for survival as a larva in unharvested nuts left on the tree or on the ground during the winter in the San Joaquin Valley. Many crops such as fig and nut crops such as almonds, walnuts and pistachios host this pest. Navel orangeworm has the ability to fly inter-orchard distances so effective control is dependent on measures conducted on an area-wide basis. Adequate control in pistachio usually involves both insecticide applications and winter sanitation.

Orchard sanitation is necessary to control navel orangeworm. In an isolated orchard, the primary source of infestation is from last year's nuts. Most growers are shaking and or polling the old unharvested nuts (referred to as mummies) from the tree sometime during the period from November through February. Shaking one or two days after a rain or foggy morning, which helps loosen the nut's attachment to the tree and adds weight to the nut, increases the shaker's effectiveness in removing mummies. The sooner the old nuts are on the ground and blown off the berm into the middles between tree rows for destruction or burial by shredding or disking, the sooner navel orangeworms will lose overwintering sites. Some data suggests that nuts left on the berm provide a more favorable habitat for survival than those left in the tree, so destruction of the nut is necessary for control. Most evidence suggests that Goniozus legneri, a parasite of navel orangeworm, does not provide adequate control of overwintering navel orangeworm by itself. Goniozus is most active in the spring and summer, with much reduced activity in the winter. Those releasing them should continue to remove as many mummy nuts as possible, leaving Goniozus the task of seeking out the few remaining larva extant in the field after sanitation.

The harvest results are in and the message is clear. The navel orangeworm is a pest with significant potential to seriously damage the pistachio, almond, walnut and fig industries. Beating the navel orangeworm will require vigilance at home, an unusual degree of cooperation among neighbors and a multifaceted control strategy involving monitoring, targeted insecticidal treatment, orchard sanitation and further research.