Joel Siegel: Beware of Navel Orangeworm Over Next Few Weeks

By Patrick Cavanaugh, Associate Editor

Joel Siegel, research entomologist with USDA Agricultural Research Service (ARS) in the Parlier office in Fresno County, is worried about Navel Orangeworm (NOW) pressure on almonds, pistachios and walnuts this season, “because we have this pattern of hotter winters, warmer springs. And, there is more than enough degree-day accumulation for an extra generation of NOW, compared to what people were dealing with four years ago. And with that, there’s the potential—if you are not on top of things—for it get out of hand.”

Those higher temperatures, he says, are what the worms desire, “Temperature—you can think of it as fuel—fuel for the fire. So the faster the generation time, the more they can start overlapping and possibly resulting in an extra generation, prolonged pressure, and at the tail-end, more NOW going into the next season as well. So you have this cycle that keeps on increasing,” says Siegel.

In describing the different monitoring and spray strategies for the each nut crop, Siegel says, “Well, with pistachios, hull split is not as predictable, so if you have hull integrity maintained, there is less NOW pressure because the nuts are not vulnerable. Navel Orangeworm seems to find pistachios once that hull begins to split. If hull break-down occurs earlier, you are dealing with more pressure.”

Joel Siegel, research entomologist with USDA ARS in the Parlier office, Fresno County

“On walnuts,” he explains, “people have been harvesting them later, going into September and October. So, if sun damage or anything else has damaged the hull in these late varieties, NOW will find these nuts as well. So, growers are experiencing higher pressure with late harvest walnuts.”

“NOW management timing is a bit more obvious for almonds,” Siegel explains. “That hull split spray is probably the most critical spray application, plus the new crop nuts are increasingly becoming more vulnerable to NOW. When that hull begins to open seems to be when this moth really notices the almonds.”

Siegel states, “One problem with almonds in particular, is that drought stress may cause prolonged hull-split that is not synchronous within an orchard. You’ll see NOW on the edges and the middle of the orchard, for example, just out of sync. So growers are having to apply an extra spray to treat all of their nuts the first time, and that is a relatively new phenomenon.”

“Second,” he says, “some people get burned in almonds, as they are used to the NOW pressure they encountered two to three years ago when they were not dealing with that extra generation. So they’ve only been applying this single spray; whereas, currently, many people need to do a hull-split spray followed by a post-hull-split spray.” And the way this season is progressing, growers may need to do this second spray over the next ten days.

“With  pistachios,” Siegel notes, “these NOW generations are building. And because of the high economic value of pistachios, people are doing a  second, or even a third shake. So if you have a scenario in which your crop is not synchronous in development, a lot of nuts become available late in the season, just when the NOW population is high as well. So that last 20% of the pistachio crop is where a great deal of damage is occurring.”

Navel Orangeworm does so much damage to the kernels that many processors are offering price premiums to growers for pistachios with  less than 1% damage. Siegel clarifies, “I’m assuming that is the goal of increased subsidies; to help offset either the cost of increased insecticide applications or to offset the cost of puffers for mating disruption.”

Siegel notes some unique NOW attributes, “They are very good at eating a lot of different things.  People don’t realize that although these different nut commodities—almonds, pistachios, and walnuts—have chemicals that help protect them from insects, this worm is very good at detoxifying or eliminating these protective chemicals. So, NOW is able to pressure many different crops and moldy fruit, so that any moldy mummies on the ground can serve as food for Navel Orangeworms. This is why sanitation is so critically important.”

Siegel says California tree nut growers are well-known for their high quality product, and this excellent reputation must be maintained. “Pistachios are a valuable crop,” Siegel says. “Growers must balance these advantages, talk with their processors, and look at how aggressive their pest management practices need to be.”

“The reality,” he continues, “is that a lot of people did quite well last year, and their damage was quite acceptable. So again, my advice is to stay the course; if you are happy with your results, continue to do the things that made you happy. If you got stung a little bit, consider adding sanitation or an additional spray.”

Finally, Siegel summarizes, “The California advantage is a quality nut crop that is high in demand. I assume that quality is never static; it always has to improve and respond to the market. As processors continue to pay premiums, they will expect nuts of a certain quality, and that will be the challenge for growers’ management strategies.”

(Featured Photo: Almond damaged by navel orangeworm larvae, UC ANR)