Navel Orangeworm Pressure On The Rise In Walnuts

Navel orangeworm damage in walnut. Photo: UC Statewide IPM Project
Navel orangeworm damage in walnut.
Photo: UC Statewide IPM Project

Navel orangeworm is notorious for attacking almonds, but it’s now becoming increasingly prevalent in walnut orchards.

Emily Symmes, University of California area IPM advisor, says that reports of higher NOW damage in walnuts have become more common over the past few years. “This makes sense given our milder, drier winters and early, warm springs,” she says. “Because NOW has only more recently been high on the pest management radar for walnuts, there is still a lot that we need to learn about how to accurately measure the threat of damage.”

Symmes adds that research has begun to look at NOW monitoring and trapping in walnuts but, at this early stage, there is not a lot of concrete data on how to best track and assess the damage threat of NOW populations in walnuts.

While NOW may have been attacking walnuts for some time, Symmes says that the type of insect or worm damage may not have been specified at harvest and instead simply listed as “insect” or “worm” damage. “So it is possible that some NOW damage in the past has been attributed to codling moth damage,” she says. “This is why it is extremely important to take additional harvest samples from orchards and go through the process of cracking out a few hundred nuts per orchard or block to identify the exact sources of the damage.”

While time consuming, this attention to detail will be worth it in the long run, as it will allow growers to know which pests caused the damage and thus be better able to evaluate the effectiveness of pest control practices. “The life cycles and key pest management timings for codling moth and NOW are different, so it’s important to know which is posing the greater threat in a given orchard in order to time practices accordingly,” Symmes says.

Why NOW?
Symmes attributes increased NOW pressure in various crops to a number of factors. First, milder, drier winters mean reduced natural mortality of overwintering pests. NOW overwinters in mummy nuts and in harvest waste materials. “In years where we get more rainfall, these overwintering sources become more moldy and rotten, contributing to die-off of some of the overwintering population,” she explains.

In addition, earlier springs the past few years have contributed to earlier biofix timings for various pests, including NOW. “The combination of early biofix and high temperatures during the growing season leads to the likelihood of additional generations being able to complete development,” Symmes adds. “This is what we’ve observed for NOW in the past few years – a partial or full fourth generation in late summer through fall. Historically, the worst years for NOW damage are those with at least a partial fourth generation. This is particularly important for walnut production because, unless previously damaged, walnuts are most susceptible to NOW damage when husks split, which can coincide with third or fourth generation moth activity in depending on cultivar and the source of the moths.”

NOW is also tricky because it’s a mobile pest, and how far adults can travel between hosts is unknown, Symmes says. Although some infestations can come from outside sources, that’s not always the case. “If proper management practices (especially dormant season mummy nut sanitation) are not done within the orchard environment, NOW populations can easily build up and survive in walnut orchards throughout the season and into next season,” she says.

Detecting NOW Damage
To determine whether you’re dealing with NOW damage or codling moth damage, nuts must be cracked open. “If worms are present, NOW is distinguished from codling moth by the presence of a dark, crescent-shaped mark behind the head,” Symmes says.

You can also take note of the size or stage of the worm to gain insight into the timing of the initial infestation and help guide pest management timing in the future, she notes.

If worms are not present, NOW damage can be detected by looking for insect excrement and webbing. “Multiple NOW will infest a single nut, whereas codling moth typically infests nuts singly,” Symmes adds. Codling moth-infested nuts, on the other hand, will exhibit excrement at the point of entry but have very little, if any, webbing. She also notes that NOW-damaged nut shells may have a slightly dark and oily appearance.

“What can be difficult, and at this point we don’t have a good solution, is distinguishing nuts that had codling moth first, and then NOW if the worms are not present,” she says. “NOW makes quite a mess of the nut, which can mask whether codling moth had previously damaged the nut.”

Reducing Risk Of NOW Damage In Walnuts
Many of the same practices almond growers use to control NOW can be employed by walnut growers, too. Symmes offers a few tips:

  • Sanitize after harvest. Remove mummy nuts from trees. Flailing or disking of mummy nuts on the ground before March of the following season will limit NOW overwintering sites and reduce the availability of developmental sites for early generations the following season.
  • Clean up waste materials in the orchard and around hullers and other harvest and processing equipment.
  • Hang traps in the orchard and monitor them throughout the season. Visual monitoring for eggs or worms in early split, damaged, or dropped nuts may also indicate NOW presence.
  • Maintain historical pest-tracking records on a site-specific basis to help with future pest management decision-making.
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