A second-year-in-a-row blowup of navel orangeworm in California almond and pistachios could be coming.

Conditions similar to those that contributed to last year’s navel orangeworm (NOW) population explosions — a dry winter and warm spring — are being repeated in 2013.

“This season navel orangeworm levels look as potentially damaging as 2012,” says entomologist Joel Siegel, with the USDA’s San Joaquin Valley Agricultural Sciences Center, Parlier, Calif.

Last year NOW damage varied across the state. Tulare County pistachio orchards were hit particularly hard. Populations skyrocketed early in the harvest and stayed until the end.

“On the very first day, loads were coming in with as much as 4 percent navel orangeworm damage,” Siegel says. “In a normal year, you may get a fluke load like that early in the season. But, then it subsides. Last year, insect damage in Tulare County remained high throughout the harvest.”

In other areas, like Kern County, the low levels of NOW damage in pistachio orchards at the beginning of harvest rose as the harvest progressed.

As much as 15 percent of Nonpareil almond loads in Madera county orchards suffered damage from the feeding worms, Siegel notes.

Like 2012, almond and pistachio orchards started this season with already-high populations of overwintering NOW. In some instances, the numbers were very high.

Brad Higbee, research entomologist with Paramount Farms, reports egg counts totaling 8,000 per trap in Kern County, Siegel says.

“Up to then, the most he had seen in a trap was around 1,400,” Siegel says. “Many growers have never seen more than 300 eggs in a single trap.”

One reason for the high early-season numbers is limited winter rainfall both this year and last season.

“The key period for navel orangeworm mortality is from mid-December through mid-February, which usually coincides with winter rains,” he says. “But, we haven’t had those rains the last two winters. That has resulted in the much higher survival rates.”

Mounting threat

This year’s warmer weather is also adding to the increased NOW threat. Many areas of the San Joaquin Valley have accumulated more degree-days so far this year compared to last season.

“Heat units are the fuel which drives development of navel orangeworm,” Siegel says. “With more heat units, you get a larger population of the insect earlier in the season.”

Two other factors favor more NOW numbers in the spring. In almonds, winter drought has caused insect-harboring mummies to stick tighter to trees. Stick-tights are more difficult to shake loose and some growers may have relaxed their winter sanitation practices, Siegel suggests.

For pistachios, there’s the trend to more two-shake harvests as growers glean as many nuts as possible to make the most of current high prices. “About 70 percent of the industry has gone to double-shaking pistachios,” Siegel explains. “Consequently, pistachios are staying on the trees longer into October for a second shake, and that’s when navel orangeworm populations are high already. So, even in years of normal navel orangeworm levels, it’s becoming inherently more of challenge to protect the nuts.”

In the meantime, Siegel is recommending that almond growers facing more pressure from the pest than usual this year, consider moving up their peach twig borer (PTB) insecticide spray using an insecticide with activity against both peach twig borer and NOW. 

Growers facing high pressure also might benefit from an extra NOW treatment after hull split treatment (hull split and post hull split spray), he notes.

“I like to treat for navel orangeworm at 1 percent hull split to see if I can knock down the population earlier,” he says. “Regardless of your management practices, the high navel orangeworm populations this year call for doing more than you’re used to in order to control this pest.”

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