As he considers the potential for heavy navel orangeworm pressure in some of California’s almond and pistachio orchards this year due to early unseasonably warm weather combined with the ongoing drought, insect expert Joel Siegel gets uneasy.

“I’m on record as being nervous that the extra heat this season will product an extra generation of navel orangeworm as well as other insect pests,” says Siegel, an entomologist with the USDA-Agricultural Research Service’s San Joaquin Valley Agricultural Sciences Center at Parlier, Calif.

That’s why he’s advising growers of almonds as well as pistachios to be prepared.

The pistachio industry likes to limit NOW to 1½ percent or less of the crop. In 2013, some pistachio orchards suffered unusually high levels of navel orangeworm damage. The toll dropped considerably last year. Meanwhile, where the insect struck almond orchards, some growers were surprised to find as many as 20 percent, perhaps even more, of the nuts had been damaged in 2014, Siegel notes.

Why such a difference between the two crops? He suspects pistachio growers took the lesson of 2013 to heart and were better prepared to control the pest than were the almond growers.

Persistent high temperatures, like those California growers have been experiencing the past several years, have allowed NOW to mature and reproduce faster than usual. Siegel begins measuring the degree-days each year in his test plots on Jan. 1.

In 2012, he recorded a total of 207 degree-days from Jan. 1 to March 21, the first day of spring at his Fresno County test plots near Firebaugh, Calif. In 2013, that number rose to 337 during that same period, dropping to 295 degree-days last year.

That makes this past winter the second warmest he’s measured there in the past five years, Siegel notes.

As his research plots in Madera County, the number of degree-days between Jan. 1 and Oct 21 last year totaled 1,000 more than during the same period in 2011.

“In the past two years, we’ve had enough heat to produce one more generation each season than growers had been used to dealing with,” Siegel says.

By adding to the stress on trees, the drought has reduced the ability of almond trees to withstand attack by NOW in several ways.

One, of course, is by limiting the amount of irrigation water, which weakens the trees.

The lower quality of this water is another factor. To replace significantly-reduced deliveries of surface water, growers have increased use of ground water, which, typically, has a much higher salt content. This has led to higher soil salinity levels.

All this may have disrupted synchrony of the timing of hull split within an orchard. In past years, Nonpareil almonds split synchronously and the insecticide spray was easier to time, Siegel notes. However, in the new plantings that he as examined over the past five years, the split has been prolonged, making it more difficult to time a single hull split spray. This prolonged split may be worsened by water stress and high soil salt content associated with these drought years.

Still another factor is contributing to the possibility of unusually high navel orangeworm pressure this year. The warm, dry weather has enabled more worms to survive and mature this spring.

Cold temperatures, by themselves, don’t necessarily kill NOW, Siegel’s data shows. “The insect can survive sub-freezing temperatures,” he says. “But, when you combine cold temperatures with rain and wet nuts, navel orangeworm mortality is a lot higher.”

The prospect of high NOW pressure this year isn’t the same for every grower, he says. After all, a number of almond and pistachio orchards suffered little, if any, damage from the insect last year. Instead, he’s cautioning growers to manage their orchards this year in terms of the potential for unusually high NOW populations

“Increased numbers of moths in an orchard combined with a much more uneven hull split,” he says, “is never a recipe for happiness.”