Know your enemy. It’s a piece of advice as important to those fighting pests in a walnut orchard as to those on any battlefield.
It takes on added importance when the pests at hand are the codling moth or Navel orangeworm (NOW). Both can render walnuts unmarketable.
That’s why Charles Burks, research entomologist with the U.S. Department of Agriculture’s Agricultural Research Service in Parlier, Calif., tackled the topic at a Walnut Day gathering in Visalia in a talk titled “Codling Moth and Navel Orange Worm: Keeping the Worms Out.”
Burks discussed monitoring tools and pest management tactics for each.
He pointed out the differences between the two in appearance. Codling moths have a dark, coppery brown band at the tip of the wings. NOW larvae are distinguishable by the crescent-shaped markings on the second segment behind the orangeworm head and by the excess webbing left in the nut.
Codling moth larvae are light pink “worms” with a dark brown head.
The orangeworm has about 40 hosts, Burks said. The codling moth has few hosts, mostly apples, quince, and pears, plus walnuts.
NOW lays more eggs – about 200 in a lifetime. The codling moth lays less than 100.
But NOW is not as dispersed as the codling moth, half of which may be dispersed to a distance greater than three miles. Less than 15 percent of NOW’s are dispersed that far.
“With NOW, there is high reproductive capacity but low survival,” Burks said. “Survival is probably under 10 percent under most field circumstances. But a small increase in survival can result in a large increase in abundance.”
Burks said the codling moth is the easier of the two to manage.
NOW is “much more messy, and it stays around . . . there’s a lot of frass (insect excrement).”
It’s generally easier to detect codling moth damage. “At harvest, NOW makes its presence known,” Burks said. “Codling moth damage is more subtle.”
He characterized NOW as “an opportunist” that can take advantage of a weakness, including husk shrivel stemming from hot days.
University of California (UC) researchers say Navel orangeworms do not damage sound walnuts until the husks begin to split. Nuts infested only by NOW may show no external signs of damage. By comparison, larvae feeding on the kernels leave holes that are discernible.
“With codling moths, you can see the damage,” Burks said. “You get surprised more by the Navel orangeworm.”
Small nuts damaged by codling moths early in the season will drop off trees soon after damage occurs. Nuts damaged later in the season will remain on the trees, yet the kernels are inedible.
NOW and the codling moth are lepidopteran pests, and damage can be limited through sanitation and prompt harvest.
Pheromone traps are used for both pests for monitoring. Nut damage and nut fall can be an indicator of codling moth infestation. Egg traps are used to monitor NOW, Burks said. And a chemical substance called Kairomone is used as a lure in monitoring for codling moth and NOW.
Treatments include insecticides and mating disruption.
To best determine whether you are dealing with NOW damage or codling moth damage, nuts must be cracked open, said Emily Symmes, UC Cooperative Extension area integrated pest management advisor.
“If worms are present, NOW is distinguished from codling moth by the presence of a dark, crescent-shaped mark behind the head,” Symmes said.
You can also take note of the size or stage of the worm to gain insight into the timing of initial infestation and help guide pest management timing in the future.
She said multiple NOW will infest a single nut, whereas codling moths typically infest nuts singly.
“What can be difficult, and at this point, we don’t have a good solution, is distinguishing nuts that had codling moth first, and the NOW later if the worms are not present,” she said.
“NOW makes quite a mess of the nut, which can mask whether codling moth had previously damaged the nut.”
Other UC researchers also suggest observing nut damage at harvest to identify pests that escaped your management program. Look for the presence or evidence of Navel orangeworm, walnut husk fly, codling moth, and ants. Knowledge of the current year’s problems will help plan for next year.
Here’s a link to information including photographs of the varying damage each pest causes - http://www.ipm.ucdavis.edu/PMG/C881/m881hppests.html.
Burks has conducted research on whether more NOW males are likely to appear in an orchard neighboring other nut orchards in Tulare County. He has found this is not the case during pheromone trapping, indicating “a significant resident population.”
He said the codling moth is “important and relatively predictable” while NOW is “potentially important, and often less predictable.” He said distinguishing the damage and the source is important to management, and management tools continue to evolve.
UC researchers say selecting varieties that are less susceptible to codling moth damage is wise, and those can include late-leafing walnuts. They say it’s easier to keep moth numbers low from the start than to suppress a well-established population.