Navel orangeworm (NOW) is a significant pest in pistachios that requires an integrated and carefully planned strategy to keep damage levels below acceptable thresholds, according to experts at the UC’s Statewide Pistachio Day.
Private research entomologist Brad Higbee with Kern County, Calif.-based Paramount Farming Co., said NOW is one of the most difficult pests he works with in nut crops.
“This beast is every bit as nasty and hard to deal with as codling moth,” he said. “An aggressive approach to NOW in pistachios is going to include everything — sanitation, post-harvest treatments, biological control and insecticides during the growing season.”
Higbee has been conducting sanitation and insecticide trials on pistachios at Paramount since 2002, and said sanitation has the greatest single impact on NOW populations in both almonds and pistachios.
USDA entomologist Joel Siegel, with the Agricultural Research Station in Parlier, Calif., agreed that sanitation to minimize overwintering navel orangeworm is a critical component in an overall management program.
“For a lot of growers, sanitation is critical,” Siegel said, adding that he was disturbed by reports that suggested mummies overwintering on trees and orchard floors can be ignored.
“I can say unequivocally that nothing is further from the truth,” he said. “There are lots of high quality mummies going all the way into June that are infested with NOW. Without sanitation we saw blocks of trees with hundreds of these mummies per tree and I would say that is not a good thing.”
Navel orangeworm not only significantly affects grade sheets, but can lead to food safety issues for the pistachio industry if left unchecked.
Navel orangeworm overwinter in pistachio mummies and can re-emerge during the season and infest the new crop.
“When you are dealing with NOW you are dealing with constant pressure from prolonged emergence,” Siegel said.
Siegel has been working for six years on statewide trials looking at NOW sanitation, insecticides and factors associated with increased insect damage. He noted that different growing conditions between trials in various counties confirm that control should include some sort of insecticide timed according to degree-days rather than calendar month.
“One thing common to all counties is that time is expressed by degree days when we are talking about NOW control,” he said.
Siegel noted that several factors in his trial were associated with increases in NOW damage in the orchard. After looking at 25,000 grade sheets from Colusa to Tulare counties, Siegel concluded that percent split provides a good predictor for NOW damage. In most cases, as the percent split on loads increased on grade sheets, so did damage. Dark staining, percent blanks and shell defects were also important predictor factors in every county.
Siegel’s trials also showed that early harvest can help reduce NOW damage and factors associated with water stress may also be linked to insect damage.
In most cases, Siegel said insecticide applications are an integral part of minimizing NOW damage in pistachios.
“If you want to actually reduce the population, you need greater than 90 percent kill of eggs and larvae, so natural factors cannot do it alone. You need a spray to keep down that population,” he said.
ARS trials looked at a number of common and newer insecticides. Siegel noted that sprays that target the larvae, such as Intrepid, will not affect NOW that have already infested nuts, but will successfully knock down overall populations by providing a barrier so that new eggs and larvae are wiped out before they can damage the kernel.
Higbee also is looking at a number of standard and newer insecticides in his trials at Paramount. While he doesn’t yet have conclusions on specific insecticides, he did say that a spray targeting the first generation produced the lowest damage levels in all insecticide trials.
Higbee’s insecticide and sanitation trials also looked at the impact of mowing versus disking on NOW infested mummies and feeding damage. He said mowing is very effective at killing NOW in almond mummies, providing about 98-percent kill. A single disking pass only killed about 40 percent of the moth larvae in the mummies, however, adding a second pass raised that mortality rate to about 90 percent.
Higbee similarly compared disking to mowing on a 40-acre plot of pistachios starting in 2006 and measured both the number of mummies after both sanitation treatments and overall NOW damage in 2007 and 2008. His standard sanitation for both treatments included shaking, poling and blowing berms.
He then configured side-by-side comparisons in a commercial pistachio orchard of mowing or two passes with a disk. He said the number of mummies varied between years with both disking and mowing, but NOW damage was higher with disking compared to mowing in both 2007 and 2008.
“Right now it looks like maybe mowing is just as good as disking, but I’m not making any solid statements on that point just yet,” Higbee said.
One thing was certain, however.
“Sanitation does have an impact on NOW in pistachios,” he said. “We’ve got to get these populations down over the winter in order to deal with seasonal populations that occur.”