Navel Orangeworm

Young worms are reddish orange and later appear cream-colored, although their diet can influence coloration. They have a crescent-shaped sclerite on each side of the second body segment behind the head. As the worm matures, the head becomes reddish brown. Adult moths range from 0.5 to 1 inch (1.5-2.5 cm) long with a snoutlike projection at the front of the head. Most moths have gray forewings with black markings, though actual shades of gray vary from light gray to almost black, and the black markings (or wing scales) often rub off when moths get old or get caught in pheromone traps. Females begin egg laying about 2 nights after emergence. Eggs are laid on mummy nuts or on new crop nuts.

The navel orangeworm feeds on a variety of fruits and nuts and is the most damaging caterpillar in pistachio. Almonds, figs, pomegranates, and walnuts are also major hosts. The pistachio nut is susceptible to infestation as soon as hull split occurs. The first signs of an infestation are small, pinhole-size entrances into the nutmeat. As worms grow in size, the entire nut is fed upon and extensive amounts of webbing and frass (insect excrement) are present.

Navel orangeworm also damage pistachios by predisposing nuts to contamination by fungal organisms (see FRUIT MOLDS) that produce aflatoxins.

Navel orangeworm is managed by removing unharvested nuts in the fall and winter, the destruction of any nuts left on the soil surface, and protecting nuts with insecticides from the time they split through harvest. Early harvest is also an important component of a good management program.

There are several parasitoid species such as Goniozus legneri, Copidosomopsis plethorica and a Habrobracon species that can reduce damage from navel orangeworm. Goniozus legneri is commercially available for release and serves as an alternative control in organically managed orchards.

Small bugs in the genus Phytocoris (P. relativus and P. californicus) feed on navel orangeworm eggs. These bugs can be very abundant in pistachio clusters in the spring.

Orchard sanitation plays a key role at reducing overwintering survival of navel orangeworm. After harvest, remove and destroy unharvested nuts (mummies) from trees and the ground to reduce overwintering sites for navel orangeworm. Be sure to remove nuts from tree crotches, blow the berms, and destroy the nuts. Destruction can be accomplished by discing or flailing in combination with degradation of the kernels by fungal organisms when moisture is high. For that reason, it is advisable to remove mummies as soon as possible after harvest before rain. Grass or other cover crops between rows can also increase moisture on the soil surface that helps degrade mummies.

Avoid severe water stress in May during rapid shell growth to reduce the incidence of early shell split. In July and early August navel orangeworm develop in these early split nuts at a time when mummy nuts are both scarce and poor hosts, and the new crop is not yet susceptible. Navel orangeworm larva feeding in early split nuts can also introduce fruit molds and lead to aflatoxin contamination.

Harvest practices influence the amount of navel orangeworm in the current year as well as the next year. Poorly timed or poorly executed harvests can lead to an increase in the number of mummy nuts that stay on the tree as overwintering sites for navel orangeworm. Harvest date also influences the level of damage at harvest. For example, in the lower San Joaquin Valley where four flights of navel orangeworm occur, nuts harvested in late August through early September typically have low levels of damage compared to levels in nuts harvested after mid-September when the fourth flight has begun.

Some mating disruption products are approved for use in organic orchards. Biological and cultural controls are acceptable for use on organically certified crops, including predation of navel orangeworm eggs by Phytocoris sp., sprays of Bacillus thuringiensis and the Entrust formulations of spinosad, and releases of the parasite Goniozus legneri.

Degree-Days

Calculate degree-days for navel orangeworm in your location.

Learn how to use degree-days to time insecticide applications.

Monitoring the first and second navel orangeworm generations should be done through the use of egg traps, pheromone traps, or both, and degree-day calculations. Egg traps contain a mixture of pressed almond meal and almond oil (3 to 5%) that encourages egg laying by female moths. Traps should be placed in the orchard at the beginning of April. A density of at least 1 trap per 5 acres should be used. Check traps twice a week to note how often eggs are laid and to identify egg-laying peaks. Peak are typically observed in late April to early May and from late June to early July, signaling the start of the first and second generations.

Pheromone traps are used to monitor the flights of adult male moths. Pheromone lures should be placed into large delta or wing traps and hung in the orchard in mid-March. Count the number of moths in the trap at least once per week and track data to identify peaks in adult activity. Make sure not to confuse navel orangeworm with the meal moth (Pyralis farinalis) that is also attracted to the trap, but is a light brown color with dark brown bands on the wings.

Nearly all pistachio orchards should be sprayed for navel orangeworm approximately one month prior to harvest when the bulk of the new crop becomes susceptible to attack. Additional applications may be warranted in cases where navel orangeworm pressure is high, where large numbers of early split nuts are present, or where harvest is delayed until after the start of the fourth navel orangeworm flight.

Treatment timings are based on crop phenology and degree-days using a lower threshold of 55°F and an upper threshold of 94°F.

Second flight timing

In orchards with severe navel orangeworm pressure consider a treatment at the start of the second egg-laying period. This treatment should be made in late June to early July approximately 1050 degree-days after eggs are found on egg traps during the first egg-laying period in late April to early May. If using pheromone traps, the treatment should be made a few days after an increase in moth captures in late June to early July.

Early split timing

During the last two weeks of July monitor for damaged or otherwise compromised nuts that split early (pea splits). Consider making a treatment at this time if there are more than 2 early split nuts per 100 total nuts, and if navel orangeworm eggs are consistently found.

Hull split or hull slip timing

Nearly all pistachio orchards should be sprayed for navel orangeworm in August approximately one month prior to harvest. This is the period of time when the third flight of navel orangeworm is present as the bulk of the new crop of pistachios becomes susceptible to attack. This timing is often referred to as the hull split timing (a phrase adopted from almonds) and is when the pistachio hulls begin to slip free from the shell.

Treat at the start of the third egg-laying period. This starts approximately 2100 degree-days after the start of first egg-laying period in late April or May, or approximately 1050 degree-days after the start of the second egg-laying period in late June to early July.

Pheromone traps can also be used to predict treatment timing. Use pheromone traps to determine the start of the second flight of navel orangeworm in late June to early July. Adding 1050 degree-days to this date will estimate the start of the third navel orangeworm flight that typically occurs in August as the hulls start to slip. The hull split spray should be made at the start of the third egg-laying period about 4 to 7 days after pheromone traps indicate the start of the third moth flight.

Late timing

It may be necessary to make an additional insecticide application in orchards where harvest is delayed (or where a second shake will occur) and navel orangeworm pressure is high. When needed, this application should be made in early to mid-September approximately three weeks after the hull split spray or when insecticide residues have degraded.

Mating disruption is a relatively new technique for managing navel orangeworm. Dispensers should be hung from sturdy limbs mid-way up the tree in late March to early April according to manufacturer’s guidance. In areas where the wind blows from one predominant direction, dispensers should be placed such that there is a higher density of pheromone emitted on the edge of the field from which the wind originates. In orchards with mating disruption pheromone traps are not effective monitoring tools. For that reason, there is an increased reliance on egg traps and monitoring for eggs on early-split nuts to determine the need for and timing of insecticide treatments in orchards using mating disruption.