How to Manage Pests
UC Pest Management Guidelines
Scientific name: Amyelois transitella
(Reviewed 3/09, updated 11/12, corrected 6/13)
DESCRIPTION OF THE PEST
Navel orangeworm is a primary pest of almonds in California and is found on several hosts. Adult moths have irregular, silver gray and black forewings and legs and a snoutlike projection at the front of the head. Females begin egg laying about 2 nights after emergence. Eggs are laid on mummy nuts in the trees or on new crop nuts after the initiation of hullsplit, and hatch within 4 to 23 days, depending on temperature. Eggs are white in color when first laid, later turning orange in color just before hatching. Newly hatched larvae are reddish orange and later vary from milky white to pink in color. Larvae have reddish brown head capsules and a pair of crescent-shaped 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. 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. Usually 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 and destruction of mummy nuts in fall or winter and rapid, early harvest—provide the most effective control of navel orangeworm. Insecticide treatments are needed when these practices are not carried out properly, when infested alternate host trees (fig, pomegranate, or pistachio) are nearby, or to achieve very low levels of damage. When infested trees of these 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 densities are low to moderate in a given area. Sprays are timed using egg traps, monitoring of hullsplit, and/or degree-days. Two parasitic wasps may be found in orchards, but they cannot be relied on to provide effective control alone without other cultural or compatible chemical practices also being used.
Unless the orchard is close to overwintering sites, navel orangeworm pressure tends to become increasingly greater as one moves further south in the San Joaquin Valley.
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 for good control of navel orangeworm.
Removal of mummy nuts
Remove mummy nuts from trees before bud swell by mechanically shaking the tree or by hand poling. Trees should be cleaned to less than two mummies per tree by February 1 in the Sacramento Valley, and fewer in the San Joaquin Valley where winter storms or bird and squirrel activity are not adequate for natural removal of the mummies. Blow or sweep fallen mummy nuts to the row center and destroy them by discing or flail mowing by March 15 where ground cover is not present. 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.
Sample for mummy nuts
Sample your orchard for mummy nuts on or before January 15 to determine if further shaking is required. Examine and count the overwintering nuts on 20 trees per block to determine the average number of mummies. Refer to the previous section for guidelines for their removal and destruction.
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. If susceptible nuts can be harvested (i.e. removed from the tree) before third-generation eggs are laid, sprays will not be needed. Keep track of navel orangeworm development by using egg traps and degree-day calculations.
Organically Acceptable Methods
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.
Monitoring and Treatment Decisions
Calculate degree-days for navel orangeworm in your location.
Learn to use degree-days to time insecticide applications.
In orchards where a thorough sanitation program and early harvest are carried out, insecticide applications are not usually needed unless there is a source of moths from infested trees outside the orchard. If cultural controls are not properly implemented, an insecticide application may be necessary. A harvest sample will help to evaluate the effectiveness of your management program.
Treatment is generally required in orchards that have more than two mummies (fewer in the central and southern San Joaquin Valley) remaining per tree after bud swell. Usually only one treatment is necessary to keep kernel damage below 2% at harvest and when nuts are removed before the start of the third flight. Depending on the product used, applications can be made either in spring or at hullsplit. Dormant sprays do not control navel orangeworm. If you also need to control peach twig borer and leafrollers, because a dormant or bloom spray was not applied, you may be able to apply one spray in spring (May) for all three insects, if degree-day calculations indicate that egg hatch for peach twig borer, leafrollers, and navel orangeworm will occur within a few days of each other. Only apply pyrethroids during hullsplit, because they can cause mite numbers to increase and will have a long-term negative impact on pollinators and natural enemies. In general, pyrethroids are more appropriate for protecting late almond varieties.
Apply a spring spray for navel orangeworm just after the first eggs of the spring brood hatch using reduced-risk products, the insect growth regulator methoxyfenozide, diamide chlorantraniliprole or flubedimide, or spinosyns such as spinosad and spinetoram. Spinetoram and spinosad are toxic to bees, so only apply when bees are not foraging. Apply at night to avoid bee activity and because spinetoram works best when moths are active, which occurs at night. Under moderate pest pressure Bt could be used as well.
The time of brood hatch will vary according to year and location, so use degree-day accumulations to predict egg hatch (lower threshold of 55°F; upper threshold of 94°F).
The biofix for the start of degree-day accumulation is the beginning of a consistent increase in egg laying on egg traps. When at least 50% of the egg traps in a given location show increases in the number of eggs on two consecutive monitoring dates, the biofix point is the first of those two dates. (Be sure to remove eggs from the trap after it is examined.) Egg hatch is expected when 100 DD have accumulated.
Navel orangeworm egg traps
Back up degree-day accumulations with observations of eggs deposited on traps to determine when egg hatch begins. Black egg traps baited with almond presscake and 3–10% almond oil are more effective than white egg traps with presscake alone. Fill traps half- to three-quarters-full. Keep presscake dry; if it gets wet, lumpy or moldy, replace it. Remember that egg-trap counts will not tell you if a spray is needed, only that moths are present and are laying eggs. Egg traps will have few eggs following hullsplit, once new crop nuts have cracked, but eggs will again be laid on the traps following harvest. Evaluate your orchard considering the criteria mentioned above to determine if you need to spray. More details on monitoring procedures and a form for keeping track of egg-trap counts are included in the online version of this guideline.
If you choose to treat at hullsplit, time the spray to the beginning of hullsplit if eggs are being laid on egg traps; otherwise time it to an increase in egg-laying on traps or the predicted initiation of egg-laying following hullsplit. Hullsplit is determined to begin when sound fruit in the tops of the trees begin to split. At this time, the nuts at eye level will be less mature than those at the top and have only a deep furrow in the hulls. Nuts in the top southwest quadrant of the tree split first. Blank nuts (usually 3 to 5%) will split 1 to 2 weeks ahead of sound nuts. Use a long-extension pole pruner to cut small branches from this top portion of five or six trees in the orchard to check whether hullsplit nuts are blank or sound.
Check for eggs on egg traps. If hullsplit has begun, but eggs are not being laid, wait until egg-laying starts. After hullsplit begins, egg-laying on traps may decrease due to competition of the traps with the new crop nuts. Therefore, if you do not see eggs on traps, use degree-days and apply a treatment at 1200 degree-days from spring biofix.
If the crop was exposed to a significant third flight of navel orangeworm or peach twig borer before harvest, a postharvest fumigation may be warranted.
UC IPM Pest Management Guidelines:
Insects and Mites