Halloween has come and gone, but almond growers should still be concerned about mummies: mummy nuts, that is. Navel orangeworm (NOW) damage is higher this year than it’s been in more than a decade, reducing almond quality and posing a health threat. Mummy nuts left in the 
orchard provide the overwintering link from one season to the next for NOW, and removing them from your trees and destroying the orangeworm within them is the objective of winter sanitation to combat this pest.
While winter sanitation is important every year, it is even more vital this year, with rejection from NOW damage estimated at more than 2% for the 2017 almond harvest. The goal is to keep this rejection below the 2% level and strive for 1% or less, according to Almond Board of California, so there is a serious concern about almond quality this year. The concern reaches beyond quality; NOW opens the door and promotes fungal infections and contamination. Research shows the Aspergillus mold and the aflatoxin contaminant it produces is associated with reject kernels, and there is a synergy between the NOW and this mold. Aflatoxin produced by Aspergillus mold is a known carcinogen and mutagen. A major concern for tree crop industries, including almonds, is stringent tolerances for aflatoxin contamination in key export markets.
NOW bores into the kernel meat and feeds on it after hull split, creating pores that can leave the nut vulnerable to Aspergillus molds once they are shaken to the ground. Aspergillus molds cause aflatoxins, which is a major food safety concern, because it is potentially carcinogenic to humans.
NOW Risk Factors
The potential for migrating moths could greatly increase your risk of NOW infestations. Adult moths are strong fliers and scavengers that can subsist on almost anything. Previously, Almond Board-funded research told us that if your orchards were within one-quarter of a mile of an infestation source, then your risk increased, but new research indicates that having almonds within even three miles of an infestation source increases your risk.
Pistachios are not the only neighboring nuts that can pass along migrating moths. NOW can pass from almond orchard to almond orchard if too many mummies have been left behind. Every almond grower should implement winter sanitation practices to prevent the spread of NOW to neighboring orchards.
Additional risk factors shared by Jhalendra Rijal, area IPM advisor, UCCooperative Extension, include keeping in mind the history of infestation of your orchard, the infestation rate of the mummy nuts as well as the number of mummies and considering if the winter weather is dry and favorable for NOW breeding.
Almond Board of California funded a NOW prediction calculator developed by Joel Siegel (USDA, ARS) and entomologist Brad Higbee that can help you determine your level of infestation risk.
NOW Management
Winter sanitation, which is the foundation of a NOW pest management program, puts you ahead of the NOW population and the resulting threat of aflatoxin. The two cultural practices that provide the most effective control of NOW include:
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Winter sanitation: Removal of mummy nuts by shaking and or poling – those nuts that remain on the tree after harvest and are harborage for NOW – is fundamental and the most effective control method. Shake the trees, again, after pick-up to remove remaining mummy nuts. The goal is to have fewer than two mummies per tree before bud swell, around February 1. In the San Joaquin Valley, where there is little winter rain, trees should be cleaned to fewer than one mummy per tree. This might require hand poling to achieve. |
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Wet and foggy weather conditions help nuts come off the trees more easily. Do not let overly wet weather stop you from completing sanitation. This was the case in 2016 and contributed to the high percentage of NOW damage this season. An added benefit is that wet orchard floors and cover vegetation can increase NOW overwintering mortality rates. |
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Mummies should be destroyed once they are on the orchard floor with flail mowing by March 15. Mid-March is when the NOW life cycle begins again. |
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Early harvest: While harvesting early is something that will need to be considered for next season, it is still important to plan for it. Almond Board-funded studies show that about 30 days after hullsplit of the first maturing varieties (typically Nonpareil), there is an explosive flight of NOW. This is due to the excellent new food sources they have available in the new nut crop once the hulls open. So, it is wise to harvest as soon as possible after nuts are mature – when 95% of nuts are at hullsplit at the 6- to 8-foot level of the tree canopy. |
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As a result of Almond Board and other research funding and efforts, there is a new biological approach to reducing aflatoxin potential in almonds that will be available in the 2018 season. The orchard is “seeded” by applying AF 36 branded as Prevail to the orchard floor. The AF 36 is a non-toxin forming strain of Aspergillus flavus and displaces the naturally occurring toxin forming strains of the fungus present in the orchard. This agent has shown a reduction in toxic Aspergillus strains in the soils of pistachio orchards. It is available to use for the first time during the late 2018 spring period and Almond Board advisories on its use will be forthcoming.
To learn more about winter sanitation and controlling navel orangeworm, visit almonds.com/pests and UC IPM website. Click on the Year-Round IPM Program for Almonds, then click on Navel Orangeworm.
