There are a few pests that have reached what seems like celebrity status in the tree nut industry, with codling moth and navel orangeworm being household names for walnut growers. With research is continuing on how to best eliminate these pests, there are still methods and products that can help with the complications of infestation.
Navel orangeworm and codling moth are featured here in this first part of a two-part series focusing on pests in walnut orchards.
Navel orangeworm has garnered a lot of attention, as growers and industry personnel continue to battle this pest. When mummy nuts are left on trees, the navel orangeworm identifies these as ideal places to lay eggs. Those eggs—which are a solid white when they are first laid, then turn pink, then reddish orange and overwinter as larvae. This moth starts emerging in April, and peaks in late-April to mid-May.
The cycle continues in the second generation as the females of the overwintered generation find other mummies, or damaged, sunburned, blighted, or codling moth-infested nuts in which to lay their eggs. As summer moves on, there is potential for a third generation to infest nuts at late harvest.
Navel orangeworm begins to damage walnuts once the husk splits. Larvae feed on the nut meat and produce frass and webbing, and while this may not be seen externally, the nut will appear oily. Ultimately, the nuts become completely unmarketable, making navel orangeworm infestations costly to growers.
Various methods can help limit the impact of infestation. Even though they’re more difficult to reach in tall walnut trees, mummies and overwintering nuts must be removed and destroyed. Mummy nuts are perfect for egg laying because there will be no pesticide follow ups, and the insect will feed on its own frass.
The most reliable methods for minimizing infestation are prompt or early harvest and orchard sanitation. As of now, the most effective way of destroying these nuts is by shredding, and doing so before mid-March. This includes not only the waste from the orchard, but also from around hullers, bins, other equipment and buildings.
One solution that has seen the best results is applying insecticides to the trees at husk split. Other solutions include applying ethephon to advance husk split, and harvesting 10-14 days earlier than usual (or as early as possible). This may require two shakes. While this may be more expensive to do, it saves money in the long run and better protects the crop from significant infestation. Studies conducted in various regions of California have shown a dramatic decrease in infestation using this method. On the contrary, if growers harvest just 10-14 days later than usual, than infestation skyrockets and magnifies complications by exposing the harvest to a third generation of larvae.
University of California Agriculture and Natural Resources’ Statewide Integrated Pest Management Program (UC IPM) features a list of pesticides that are both the most effective and the least harmful to the environment, honey bees, and natural enemies of the navel orangeworm.
Similar to the navel orangeworm, codling moth overwinters as a larva. The females lay eggs on leaves near nuts or on nuts. These overwintered females begin the first flight and can have two peaks in this flight. These moths emerge between mid March to early April, typically coinciding with the leafing out of early walnut cultivars, and their eggs begin the first generation.
Once the eggs of the first generation are grown, it results in a second flight, and the eggs of this generation produce the third generation. This cycle can continue for as many as four flights in a season in warmer areas and regions as far south as Tulare.
When the disk-shaped, opaque white eggs of the first generation hatch, the larvae bore into the nutlet through the blossom end. Later generations bore anywhere on the surface of the walnut. Just as the boring habits are different between the first two generations, the damage caused by codling moth also differs among generations. The first generation reduces yield from the very beginning by causing nutlets to fall from the tree. Later generations infest nuts, remain on the tree, and create a breeding ground for their equally damaging counterpart, the navel orangeworm.
Protecting walnuts from codling moth is a battle. An immediate defense is more frequent pesticide use, but timing for chemical applications can be a challenge, and pesticide usage is dependent on knowing when the various generations emerge. There is also the concern of pesticide residue. Though research is always evolving on how best to deal with this, UC IPM has more details, covering the timing of spraying for various flights, generations, and pest populations.
Another promising tool for use against codling moth is pheromones. After improving the stability of synthetic codling moth pheromones, these pheromones were used to trap male moths. With male codling moths coming in at a ratio of 1:1 with females, this helped to give a more accurate count to pest populations.
By using pheromone disruptants available as a sprayable liquid, a hand-applied dispenser, or an aerosol, the male insect cannot locate the female for mating. The female has a finite number of eggs, and if it doesn’t mate, those eggs are absorbed instead of laid, and biological potential drops.
Mating disruption has been received with mixed reviews. If growers enter into the season with high populations of codling moth, they can still curb infestation with insecticides in conjunction with mating disruption. Ultimately, pesticide applications will be needed as mating disruption alone won’t achieve necessary control. However, with lower counts of the moth, mating disruption works very well with limited insecticide applications, especially in square, flat orchards.
Whether growers suspect one or both insects have infested their orchards, nut sampling is key to assessing damage. In both cases of navel orangeworm and codling moth, it’s recommended to collect and crack out a large number of nuts at harvest time in order to assess this season’s damage and to determine which pest is doing the damage. To easily tell the difference between the pests, navel orangeworm is identified by its signature C-shaped markings on both sides of its thoracic segment, and can have multiple larvae in one nut. Codling moth doesn’t have these markings, and there is only one larva per nut. Since navel orangeworm often infests where codling moth once was, finding nuts with frass or webbing could mean having one or both pests.
The research into methods for how to suppress or eradicate these pests is constant and evolving. Experiments are ongoing and span a variety of methods—from dropping sterile navel orangeworm via drone, to planting cover crops that increase beneficial predatory insects that target pests. But, every orchard is different. Growers need to know their specific orchard conditions, and find the methods that work best given current research and industry best practices.