Navel Orangeworm

Navel orangeworm is a primary pest of almonds in California and is found on several agricultural and nonagricultural hosts.

Moths have irregular, silver gray and black forewings and legs and a snoutlike projection at the front of the head. Females typically begin laying eggs the second night after emergence. Eggs are laid on mummy nuts remaining in the orchard or on new crop nuts after the initiation of hullsplit. Eggs hatch within 4 to 23 days, depending on temperature. When first laid, eggs are white, later turning orange just before hatching. Newly hatched larvae are reddish orange and later instars vary from milky white to pink. Larvae have reddish brown head capsules and a pair of crescent-shaped dark marks on the second segment behind the head. Pupae are light to dark brown, encased in a woven cocoon, and found inside nuts or between hulls and shells.

There are three to four adult flight periods per year depending on emergence from the overwintering stage and temperatures throughout the season. The larvae overwinter in mummy nuts either in trees or on the ground.

First-instar larvae bore into the nutmeat, and later instars can consume most of the nut, producing large amounts of webbing and frass. Often, more than one larva can be found feeding in a nut. Navel orangeworm larval damage can also lead to fungal infections. Some cultivars are more susceptible to damage, especially later-maturing softshell almonds with a lengthy hullsplit period or a poor shell seal.

Two cultural practices—effective removal with destruction of mummy nuts in fall or winter (sanitation) and early harvest with rapid removal of nuts from the orchard floor— are essential components of an effective navel orangeworm control program. Insecticide treatments are often needed if these practices are not carried out, in situations with high navel orangeworm numbers, or when navel orangeworm may immigrate in from neighboring orchards. When infested trees of alternate hosts are harvested, navel orangeworm moths may migrate into almond orchards. Treating border rows (at least 10 rows) may be adequate to prevent the moths from infesting the almond crop when navel orangeworm numbers are low to moderate in a given area. Sprays are timed using egg traps or pheromone traps in conjunction with degree-days, and monitoring hullsplit. Two parasitic wasps may be found in orchards, but they cannot be relied on to provide effective control alone without using other cultural or compatible chemical practices.

In contrast to the mid- and southern San Joaquin Valley, navel orangeworms are typically less abundant in the Sacramento and northern San Joaquin valleys, so nut damage tends to be less severe. However, environmental conditions, proximity to sources of infestation, and effectiveness of sanitation practices impact the potential for damage by this pest, regardless of growing region.

Parasitic wasps that are known to parasitize navel orangeworm include Copidosoma (=Pentalitomastix) plethorica and Goniozus legneri. Goniozus legneri is now available from commercial insectaries and can be purchased and released. Releases must be supplemented with cultural and other management practices.

Remove Mummy Nuts (Sanitation)

Remove mummy nuts from trees before bud swell by mechanically shaking the tree or hand poling, or both. Blow or sweep fallen mummy nuts to the row centers and destroy them by discing or flail mowing by March 1, especially where ground cover is not present or in years with dry winters. Moist orchard floor conditions provided by winter-resident vegetation and rain will enhance mortality of navel orangeworms in mummy nuts that have fallen from trees in years with adequate rainfall. Conversely, mummy removal is even more important during periods of drought because survival of the overwintering larvae in the mummies tends to be greater than in wet years. Nevertheless, mummies remaining in the orchard, even if not infested with overwintering populations, may provide a development site for first flight prior to hullsplit.

Early Harvest

Harvest nuts as soon as good removal can be achieved; this is when 100% of nuts are at hullsplit at the 6- to 8-foot level of tree canopy. Risk of navel orangeworm infestation in early-harvested varieties such as Nonpareil can be greatly reduced if nuts are harvested before third-generation eggs are laid.

Varietal Selection

Hard-shelled varieties and those with a tight shell seal are resistant to navel orangeworm.

Some mating disruption products are approved for use in organic orchards. Cultural and biological controls and sprays of Entrust formulation of spinosad or Bacillus thuringiensis are organically acceptable methods. Sprays are not a substitute for cultural practices, which are necessary for acceptable control. Hard-shelled varieties can also be used to avoid navel orangeworm damage.

Management of navel orangeworm varies regionally within California.

• Central and southern San Joaquin Valleys: Provide excellent winter sanitation in combination with insecticide sprays when the new crop becomes susceptible. Base the number of applications and application timing on egg traps, pheromone traps, and crop phenology. When high numbers of navel orangeworm are present, a third spray may be needed, especially when late varieties are present that will be harvested after the start of the fourth flight.
• Northern San Joaquin and Sacramento valleys: Typically, natural winter mortality is higher, and fourth flights are less common. Provide good winter sanitation; and insecticides may or may not be needed depending on trap captures and environmental conditions.

Insecticides for navel orangeworm are primarily used at hullsplit and later when the new crop is susceptible. However, when high numbers of navel orangeworm are present in the San Joaquin Valley, an insecticide application might also be warranted in May. For information on spray programs, see the section on SPRING SPRAYS and HULLSPLIT SPRAYS below.

A harvest sample for nut damage will help assess the effectiveness of your management program.

Degree-days

Calculate degree-days for navel orangeworm in almond for your location using the navel orangeworm pest model. To learn more about using degree-days to time insecticide applications, watch the degree-days video.

Sample for Mummy Nuts

Sample your orchard for mummy nuts on or before January 15 to determine the density of mummy nuts in the orchard and assess sanitation needs. Examine and count the overwintering nuts on 20 trees per block to determine the average number of mummies. When sampling, be sure to evaluate trees from all varieties since mummy counts can be quite different from one variety to the next. In the southern and central San Joaquin valleys clean trees to no more than an average of 0.2 mummies per tree and eight per tree on the soil by February 1. In the northern San Joaquin and Sacramento valleys, clean trees to less than two mummies per tree. If possible, determine the percentage of infested mummies and the presence of live larvae or pupae to help estimate the carryover potential from one year to the next.

Navel Orangeworm Egg Traps

Place egg traps in the orchard by March 15 at a rate of 1 trap per 10 acres, or a minimum of 4 traps per orchard. However, more egg traps (as many as 10 per orchard) enable a more precise estimation of biofix. Traps consist of a black plastic tube with a snap top and mesh sides that is half- to three-quarters full of almond presscake containing 3 to 10% almond oil. Some growers grind almond or pistachio mummies to use as an alternative to presscake.

• Monitor traps once or twice per week until a biofix is established.
• The biofix to start degree-day accumulation is established when egg numbers and number of traps with eggs increase for at least two consecutive sampling periods (the biofix point is the first of those two dates) or when 50% or more of the traps have eggs. Note: Be sure to remove eggs from the trap after it is examined.
• Egg hatch of the first brood is expected when 100 DD have accumulated after biofix.

Egg traps are most effective during the first flight in April and May. Though egg traps can be used during the second and third flight, they provide a less accurate estimate of biofix since they are competing with the new crop nuts during that period. To keep track of egg-trap counts, use the monitoring form (PDF).

Navel Orangeworm Pheromone Traps

Pheromone traps are used to monitor the flights of male moths. Place pheromone lures into delta or wing traps and hang in the tree canopies at approximately six to eight feet in early March. Hang one trap per 50 acres and at least two traps per orchard. Count the number of moths in the trap at least once per week and track the data to identify adult flight. Make sure not to confuse navel orangeworm with the meal moth (Pyralis farinalis) that is also attracted to the lure. Meal moths are light brown with dark brown bands on the wings.

Spring Sprays

Only consider spring sprays for navel orangeworm in orchards with a history of high navel orangeworm damage and high trap captures in the central and southern San Joaquin Valley. In the northern San Joaquin and Sacramento valleys, spring sprays for peach twig borer often have a secondary benefit to suppress navel orangeworm, but a dedicated spray targeting navel orangeworm is most effective at hull split. If an insecticide is used in the spring (late April to mid-May), make the application just after the first eggs of the spring brood hatch or at the appropriate timing for peach twig borer. Use a reduced-risk product (non-pyrethroid) to prevent secondary outbreaks of pests such as spider mites.

Hullsplit Sprays

The initiation of hullsplit is the most effective timing for a single insecticide application. Time the spray to the beginning of hullsplit (no later than 1% hullsplit) if eggs are being laid on egg traps or if pheromone traps indicate that the second flight has begun.

Hullsplit is determined to begin when sound fruit in the top southwestern quadrant of the trees just begin to split. Note that this is before the suture is wide open. At that time, the nuts at eye level will be less mature than those at the top and have only a deep furrow in the hulls. Blank nuts (usually 3 to 5% of the crop) will split 1 to 2 weeks ahead of sound nuts, and these should not be confused with hullsplit of the new crop. Use a long-extension pole pruner to cut small branches from this top southwestern portion of five or six trees in the orchard to check whether hullsplit nuts are blank or sound.

Orchards with moderate to high numbers of navel orangeworms require a second insecticide application approximately 2 to 3 weeks after the initiation of hullsplit. At this time, the primary variety (e.g., Nonpareil) is fully split and pollinizers are beginning to split. To determine if this treatment is needed, consider

• the varieties in the orchard,
• anticipated harvest dates,
• pest pressure noted from egg and pheromone traps,
• overwintering mummy load, and
• proximity to an external navel orangeworm source.

Mating Disruption

Mating disruption is a relatively new technique for managing navel orangeworm in almonds. As a supplemental management tool, use mating disruption in conjunction with strategically applied insecticides, most notably in orchards with high numbers of navel orangeworm or in the first years of a mating disruption program. Mating disruption dispensers should be hung from sturdy limbs midway up the tree in late March or early April according to manufacturer’s guidance. In areas where the wind blows from one predominant direction, dispensers should be placed so there is a higher density of pheromone emitted on the windward edge (upwind) of the orchard.

In orchards with mating disruption, pheromone traps do not effectively monitor male flights.

• Use pheromone traps to assure that suppression of mating is achieved. Pheromone-only traps in mating disruption blocks should usually capture no adults, and no more than a few weekly.
• Use egg traps and monitoring of eggs on early-split nuts to determine the timing of insecticide treatments in orchards using mating disruption.

Postharvest Fumigation

If the crop was exposed to a significant third flight of navel orangeworm or peach twig borer before harvest, a postharvest fumigation of the crop may be warranted.

• Degree-day calculator
• Degree-day table
• Egg traps for navel orangeworm
• Harvest sample
• Photos of hull split
•  Using degree-days to time insecticide applications