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Managing navel orangeworm takes all growers doing their part

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(Sponsored) A major threat to almonds and other nut crops, navel orangeworm (NOW) presents a primary source of direct crop damage when the females lay eggs on the nuts. But the secondary damage goes further than that, allowing for the growth of aflatoxin on the damaged crop. 

In order to minimize both the primary and secondary damage caused by NOW, Dr. Jhalendra Rijal, an area IPM Advisor with University of California Cooperative Extension, recommends a holistic approach, not just in utilizing integrated pest management (IPM) but from an industry-wide perspective as well. 

“The fundamental tactics of IPM are integrating different pest management methods including cultural, biological and chemical,” said Dr. Rijal. “Industry-wide and global customer demand for nut crops means that we need to continue to produce this nut crop in a sustainable way.” 

“What does a sustainable way mean? It means you care about your practices. Growers should be informed about what’s being practiced and used, and what their pros and cons are. You put it all together so it will be effective and economically beneficial but also not impact the environment or the beneficial organisms out there in the field.” 

Among the tools in IPM practices are certain chemicals that can target NOW, most notably during the larvae stage in late June and early July, when the hulls of the nuts start to split. 

“That’s when the NOW lay eggs on seasonal split nuts and cause damage,” Dr. Rijal said. “Based on multiple research studies, it shows if you use insecticide at that time, you kill the newly hatched larvae and in some cases eggs.” 

Insecticides are one of the important tools in the fight against NOW, but there are tools growers should be using in addition to insecticides. Dr. Rijal reiterated the importance of a year-round IPM strategy for all growers. 

“You need to incorporate all the science and options we know as NOW is not a trivial pest in terms of management,” he said. 

In the fall, if larvae are left feeding on the nutmeat and stay inside for overwintering, the leftover “mummy nuts” on the trees carry mature stage larvae throughout the winter. “We need to do cultural control to minimize the population going into the following season,” Dr. Rijal said. “If you remove most of the mummy nuts in the wintertime, you’re naturally removing NOW larvae from the orchard. So, when springtime comes, you have a smaller number of adults to start with.” 

In the spring before NOW adults emerge from the mummy nuts, using mating disruption — a behavioral control— to disrupt the mating behavior of the male moth, growers can further reduce the population. Recent studies from the University of California found that incorporating mating disruption combined with other practices can reduce overall damage in nonpareil variety by 35-65% (avg. 50%). If you have 4% damage, this can reduce that to 2%. Having said that, this can vary with individual orchard shape and size, pest pressure, strategy of deploying mating disruption units, etc. Growers should be informed on how mating disruption works and what are other considerations to take into account to achieve maximum benefits. A one-time application in the spring should cover multiple NOW generations throughout the season. 

Growers and crop consultants should be utilizing traps and lures to track the population and generations throughout the season, beginning in March. In late June and early July depending on the year and region the Central Valley, almond hull split occurs, which is where insecticides should come into play. 

“Of course, monitoring is key to actually finding out when to use the insecticide spray during hull split time. You want to do it at a time when the hulls are split and the NOW population is there. In order to coordinate those two factors, you have to do the monitoring for not only the pest but for the crop state,” Dr. Rijal said. “If you kill the larvae before they do damage on the nuts, that’s the damage you can prevent.” 

Another critical consideration is that insecticide only kills if it reaches the target — split nuts. If the application does not meet that goal, you cannot achieve desirable control, no matter how efficacious the product is. So, other factors such as tree height, sprayer type, calibration status, tractor speed are important considerations. Several studies indicate that 2 miles/hour tractor speed is optimal for effective NOW control. Higher than that, efficacy is reduced significantly, especially above the 15-foot height where higher NOW activity occurs. 

The final component in a year-round approach to managing NOW is at harvest. 

“As the NOW generation advances, particularly from second generation to third, the population abundance increases exponentially,” Dr. Rijal said. “So, the idea for the timely harvest is to harvest the nonpareil almonds before that third-generation egg laying happens to avoid that entire generation if possible.” 

He added that a timely harvest means the nut maturity is at the right stage, where 100% of nuts at 6-8 feet height are at hull split, based on the UCIPM Guidelines. 

“Keep in mind, though, that harvesting not-ready nuts can create quality issues when you send those nuts to the processor,” he said. 

As Dr. Rijal mentioned, it’s key that all growers utilize these tools and best practices for IPM in their orchard: from mummy nut sanitation and mating disruption to trapping and insecticides through to a timely harvest. NOW is an industry-wide threat that requires all growers to fight. 

“If every grower in the (Central) Valley does the winter sanitation, it means you are reducing the NOW population throughout the Valley,” he said. “It’s not just about your orchard. The more growers following this, the more effective it will be. We know NOW is a good flyer. They can fly two to three miles from one orchard to another if they need to. If everybody does their part, that’s the way to go for long-term management of NOW.” 

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