Entomologists had been expecting higher navel orangeworm (NOW) numbers than normal this year in pistachios and almonds However, they’ve been surprised at how high these numbers have been.

Brad Higbee, research entomologist with Paramount Farming, Bakersfield, Calif., says a moderate trap count is 200 to 400 eggs in pistachio orchard pheromone traps in the spring. This year in Kern County he’s found as many as 8,000 eggs in a single trap. The numbers have been higher on the western area of the county than to the east where pistachio orchards are less concentrated, he notes.

However, the number of moths he’s trapped during this season’s first flight has been similar to years of lower NOW pressure. That reflects the physical size and sticky nature of these traps, which can reach the saturation point holding capacity even with just moderate population levels, Higbee explains.

The eggs that he and others have found this spring were laid by the moths that developed from worms overwintering in mummies left in the orchards after last year’s harvest. The worm population last fall was higher than it had been in a while. That meant more infested mummies than typical. And, more of them survived with dry, warm fall and winter weather.

This first flight of the season, which began in mid-March, ended in May. The next flight is expected to start sometime in mid-June. However, it’s the eggs laid by the third and fourth flights that are the most threatening to pistachios.

Unlike almonds, pistachios don’t have a regular and predictable splitting pattern. “Early splits” can be found in July with splitting continuing through harvest. It’s these split nuts that are attractive to egg-laying females. The worms that hatch damage the crop by feed on the pistachios.

The high NOW population this spring doesn’t necessarily mean that growers will sustain heavy crop damage. “That depends on how well growers can control the insect with their sprays and how many nuts are susceptible to the worms that emerge prior to harvest,” Higbee says.

“The best time to treat pistachios for NOW is when the third flight is starting to increase,” Higbee says. “Growers doing a two-shake harvest may come back after the first shake and make a post-hull split spray, too.”

Currently, researchers are looking for more effective ways of controlling NOW in pistachios. One promising possibility is mating disruption. Similar to the techniques walnut growers use to reduce codling moth populations, this approach disperses sex pheromones to confuse male moths, preventing them from finding females. It could be available for use by growers next season, Higbee reports.

Other researcher are studying how to improve on the types of mowers and shredders almond growers use to destroy mummies on the orchard floor “Because of the much smaller size and much harder shell of pistachios, using this equipment in a pistachio orchard is like trying to mow marbles” Higbee says. “Work is under way to develop different types of knives and blades that may do a better job.”

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