Tips for Nut Growers Living with Navel Orangeworm

Tips for Nut Growers Living with Navel Orangeworm

An infested nut from photo taken in January. There are no larvae visible, but navel orangeworm frass is evident. (Photo: Franz Niederholzer)

Navel orangeworm (NOW) damage in the 2017 almond crop was painfully bad in many blocks in California’s Sacramento Valley.

It cost some growers hundreds of dollars per acre, slowed crop processing (more money lost), and may be an issue in some international markets due to aflatoxin concerns. Ugly, ugly, ugly. While bloom is a “quiet” time for NOW management — other than destroying mummies — it’s a good time to review the pest and your options as the season begins.

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Management Options
The following is a chronological menu of management options for NOW management and/or control for growers to consider from now until pollinizer harvest, though not all activities are necessary in every orchard every year.

  • Hang NOW traps immediately: Use pheromone traps for adult males, egg traps for female oviposition, and kairomone (bait-bag) traps for adult females.
  • Hang mating disruptants: Manufacturer specifications indicate that these should be up before the first female moths emerge from mummies in late-March or early/mid-April — depending on the year.
  • May spray: A May spray should be considered only where there is evidence of very high in-orchard populations during the first generation, based on populations in last year’s mummies, mummy density, and monitoring. The goal with May sprays for NOW is to minimize early-season population build-up, reducing the number of crop-damaging moths at or near hull split. If the orchard faces high NOW pressure from outside sources, growers may get better value from an additional hull split spray instead of a May spray. Research by Dr. Frank Zalom at University of California (UC), Davis states that certain insecticides applied within 100 degree days (DD) of spring egg biofix can help reduce spring mummy infestation. Spring egg biofix is theoretically marked by two consecutive detections on the majority of egg traps when checking at least twice a week. However, in the Sacramento Valley, history has shown that often a single sampling date with eggs present can represent biofix — potentially due to low NOW populations and/or high competition from orchard mummy nuts. Remember that pyrethroids in May are a great way to flare spider mite populations. Altacor (FMC Corporation) or Intrepid (Dow AgroSciences) provided the longest control — three to four weeks — with minimal mite flaring in Zalom’s research.
  • Edge spray as blanks start to split: If sound nuts in trees on the edges of your orchard split first, treating the edge trees ahead of the bulk of the orchard may help keep NOW from infesting the edges of the orchard — before the first hull split spray is applied across the orchard — and becoming a source of moths, eggs, and damage through the rest of the season.
  • Early ‘Nonpareil’ (NP) hull split spray: Spray the NP as the first sound nuts within the orchard just begin splitting. Too early is better than too late. No pollinizers should be split at this time — check to make sure — so there’s no need to spray the pollinizers.
  • Second NP hull split spray: Come back and spray the NP a second time two to three weeks after the first hull split spray. If pollinizers are starting to split, make it a solid (every row) spray.
  • Harvest NP as soon as possible. A timely harvest at 100% hull split may leave a few nuts in the lower interior of the tree, but this yield loss is less than the additional NOW damage the crop — and you will suffer if you wait.
  • Pollinizers’ hull split spray: Using a short postharvest interval (PHI) material, spray the pollinizers as soon as the NP nuts are picked up if the NP harvest samples are showing damage.
  • Harvest pollinizers as soon as possible.

Navel orangeworm larvae is shown from a mummy cracked open in January. (Photo: Franz Niederholzer)

Know Your Enemy
Remember that not all of these activities are needed in every orchard every year. Work closely with your Pest Control Advisor(s) to evaluate the pressure in your particular orchard blocks and adjust your management plan accordingly. The timing and number of sprays needed to achieve successful — or at least tolerable — control of NOW will depend on multiple factors, and can be informed by a number of risk assessment methods. These methods include monitoring of both pest densities and crop phenology, historical/previous season damage, location of external sources of NOW, etc. Know your enemy.

  • NOW is a strong flier — at least a quarter of a mile, maybe more — and has dozens of hosts, including walnuts, pistachios, and pomegranates. Remember that mummies in your orchard, even if not infested heading into the season, can become infested from outside of your orchard during the early flights. Therefore, your nearby neighbors’ problems can become yours if you leave mummies in your trees this winter, ultimately damaging your new crop as it begins to split. Growers with big, remote blocks and good sanitation are much better off than growers with small block(s) and abandoned or non-sanitized almonds — or other hosts — nearby.
  • NOW has evolved to tolerate very poor living conditions (aflatoxin mold, etc.) and can rapidly adjust/ develop resistance to pesticides. It is rapidly developing resistance to (or at least tolerance of) pyrethroid pesticides such as Asana (Valent U.S.A), Warrior (Sygenta), Brigade (FMC Corporation), etc. Tank mixing with more effective and expensive pesticides such as Altacor, Intrepid, or Intrepid Edge (Dow AgroSciences) is a short-term and risky solution with no effective pesticides in development.

Mission Impossible
Given the pest, dense orchards, and the tools available, it is not possible to eliminate the pest. The aim is to minimize its damage by using all of the tried-and-true methods available: keeping your orchard clean using winter sanitation until hull split, reducing NOW pest levels with hull split sprays, and harvesting and picking up the crop as soon as 100% hull split arrives. Additional things you can do to reduce NOW damage in tough years:

  • Manage irrigation so hull split is less variable across the orchard, allowing treatments to be timed more consistently and provide more effective control.
  • Shake nuts as soon as 100% hull split has occurred and get them out of the orchard as soon as they are dry.
  • Fumigate stockpiles if you have to wait for hulling.
  • Calibrate sprayers to deliver the best coverage possible. Buy/lease/borrow enough sprayers to get across your orchards in less than a week. You can’t spray your way out of the trouble that arises from a lack of sanitation — a spray application provides about 50% to 60% control of the population at best — but you can keep damage lower with good coverage and timing.
  • Consider mating disruption if NOW pressure is high due to poor/no winter sanitation, dirty neighbors, etc. But do not consider mating disruption (or sprays for that matter) a replacement for winter sanitation.