California pistachio blocks which experienced high rates of Navel orangeworm (NOW) damage in 2016 could be in for another round in 2017.

For many pistachio growers, last year was one of the worst on record with an average of 2 percent NOW damage. High numbers of NOW-infested nuts late in the season resulted in a larger spring flight more likely in spite of the wet winter.

Much attention is being directed at NOW control strategies to limit pistachio nut damage, says USDA-ARS Research Entomologist Joel Siegel based at Parlier, Calif. The ultimate goal is to reduce NOW populations and the percentage of NOW-damaged pistachios to ensure long-term expansion of the pistachio industry.

Calculating degree-days, monitoring and timing insecticide sprays, earlier harvests, and orchard sanitation are at the core of NOW control in pistachio production, yet the execution of each strategy determines the outcome.

Degree-days are the accumulated product of time and temperature between the developmental thresholds for each day. Calculations begin Jan. 1 using information from the California Irrigation Management Information System (CIMIS) weather stations closest to the orchard. Growers can use this calculation to time pesticide applications for maximum efficacy.

Growers are advised to begin monitoring for NOW as early as April and as degree day accumulations edge closer to 1,700 by late June to decide if a spray is necessary.

“With early heat this spring there is the potential to accelerate an extra generation,” Siegel said.

He says degree-day accumulations in the spring have increased since 2012, peaking in 2014 when 1,100 were recorded by May 24. This year’s total on May 24 was 886, down from last year’s 961.

“This is still elevated and there is the potential for the population to gallop along,” the entomologist noted.

This year appears to be more of a ‘normal’ year in terms of crop maturity compared to last year’s earlier start of harvest, he noted. Those extra days to harvest can increase chances of NOW infestation.

While egg and pheromone traps may indicate NOW activity levels in pistachios, the determination on whether to spray is more complicated. Sporadic hull split or the presence of ‘pea split’ nuts adds to the challenge.

The pea splits or small nuts develop ahead of the normal crop and are believed to be the insect development link between old mummies from last season and this year’s crop.

There are no established thresholds for NOW in pistachio. Nuts are susceptible to NOW infestation at hull split. The first visual signs are small pinhole-size entrances into the nutmeat. As the worms grow, they feed on the kernel, creating extensive webbing and frass.

Siegel says the 1,700 degree-day accumulation point marks a population peak or just prior to a population peak. When this level is reached, he says a pesticide application is worth the cost if there are pea splits present. Early splitting nuts and those with ripped or tattered hulls are egg-laying sites for NOW.

Environmental factors can cause early split nuts, but varieties which open earlier than the Kerman variety can be vulnerable. Monitoring for early splits can help growers estimate the NOW cycle and protect a major portion of the crop.

A fifth generation of NOW in the fall, not removed by orchard sanitation, sets the stage for early infestations the next growing season. At a minimum, growers should remove mummies from trees in the fall. Wet winters decrease the chances of larvae survival in the mummy nuts, but Siegel says growers should commit to a sanitation program for the long term.

Considering the logistics of pistachio harvest, Siegel says it’s important to communicate with processors about the need for quick and early harvest to avoid excessive NOW damage.

In Madera, Tulare and Kings counties, he says the average doubling time for NOW damage in pistachio is 14 days after the start of harvest. If counting starts at seven days, on Day 21 the damage is double from Day 7, on Day 35 the damage is double the amount from Day 21.