Codling Moth

Codling moths overwinter as full-grown larvae in thick, silken cocoons under loose scales of bark or in trash on the ground near the trunk. Moth emergence usually coincides with the leafing out of early walnut cultivars. During the day, moths rest on branches and trunks. Codling moths can be distinguished from other small moths likely to occur in the orchard by the coppery markings on their wing tips.

The first flight of codling moth typically starts sometime from early March to early April and is from the overwintered generation. The flight of the overwintered generation may have two peaks (often referred to as 1A and 1B) and can last several months. These moths lay eggs that signal the beginning of the first generation. The second moth flight results when the larvae of the first generation complete their development. When the moths in the second flight lay their eggs, this starts the second generation. The following table outlines the life history of codling moth:

Each overwintered female deposits about 30 eggs singly on leaves near nuts. Later generations of females will lay an average of 60 eggs on leaves or nuts. Eggs are disk-shaped and opaque white. Eggs of the overwintered generation hatch after 5 to 20 days, depending on the temperature, and young larvae bore into nutlets through the blossom end. Most nuts with codling moth damage from the overwintered generation drop to the ground along with blighted nuts. However, if damage occurs from second flight peak (1B) of overwintered codling moths and the weather is cool, all damaged nuts do not drop. So only use nut drop thresholds when there is no second peak in the flight of the overwintered generation.

Codling moth egg hatch period is dependent on temperatures but typically the egg hatch period for the overwintered generation lasts 4 to 6 weeks and 4 weeks for later generations. The egg hatch period is important for timing sprays. In cool springs or cool locations, the flight of the overwintering generation lasts longer than subsequent flights and has two peaks.

The larvae leave the nut after completing their development and pupate under loose bark on the tree. Adults of the first generation begin to emerge from the end of May to as late as the last week of June in the Central Valley, depending on the season. In coastal areas, emergence begins in late June to early July. Because of the higher temperatures, eggs and larvae of the first generation develop faster than those laid by the overwintered generation.

Newly-hatched second-generation larvae bore into walnuts anywhere on their surfaces but prefer the spot where two nuts touch. If the nut has hardened, it may take them up to a week to enter the nut. The larvae develop into adults that begin to emerge by late July or the beginning of August. In most valley locations they produce a third generation; in warmer locations a partial fourth generation may be produced in September. These later generations can cause significant damage. Older larvae leave the nuts and move to tree trunks or debris to spin cocoons and overwinter. Occasionally some may be present in nuts if they are harvested before the larvae have matured. However, most larvae found in nuts at harvest are navel orangeworm.

It is important to distinguish between codling moth and navel orangeworm damage. In harvest samples, it is easy to tell codling moth damage from navel orangeworm damage when the worms are present. Navel orangeworm has a brown crescent-shaped marking behind the head capsule on both sides of the first thoracic segment; this mark is absent in codling moth larvae. There can be multiple navel orangeworm larvae but only one codling moth larva per nut. If the worm is not present, look at the damage: navel orangeworm leaves behind more webbing and frass. However, navel orangeworm frequently infest nuts that were previously infested by codling moth, so if navel orangeworm is present, it doesn't mean codling moth wasn't previously there.

The damage caused by the codling moth is different with each generation. First-generation larvae reduce yield directly by causing nutlets to drop from the tree. Codling moth-damaged nutlets have frass at the blossom end. Be careful not to confuse nuts damaged by codling moths with unpollinated nutlets or blight-infected nutlets, which have dark lesions but no frass and drop at the same time. Damage is generally most severe on early-season cultivars, although it has been increasing steadily over the years on late-season cultivars such as Chandler.

Nuts attacked by larvae from the last part of the first generation and from the second and third generations remain on the trees but are unmarketable because of the feeding damage to the kernel. These damaged nuts can also serve as a breeding site for the navel orangeworm. Feeding that is confined to the husk results in minor shell staining but no damage to the kernel.

You can often detect codling moth infestations by looking for frass produced by the larvae at the point of entry into the husk. Second-generation larvae often enter through the side of the husk where the two nuts touch. After the shell hardens, the larvae enter the nuts through the soft tissue at the stem end.

Management options for codling moth in walnut orchards include both pheromone mating disruption and insecticide sprays. The options that work best for a given orchard depends on the size of the orchard and the trees and the degree of codling moth infestation. In all cases, monitor with pheromone (codlemone), pheromone plus kairomone (CM-DA combo), or both and check for damage. Monitoring and checking for damage is necessary to follow codling moth generations, assess the degree of infestation, and assess the effectiveness of control actions. Programs that use mating disruption alone or in combination with sprays of least-toxic insecticides or parasite releases pose fewer water quality and environmental risks than programs that rely on organophosphate or pyrethroid insecticides.

Natural enemies alone do not keep codling moth numbers below economic levels. In orchards where mating disruptants are used, augmentative releases of the tiny, naturally-occurring parasitic wasp Trichogramma platneri, which attacks codling moth eggs, can be helpful to control eggs laid by mated female moths immigrating into the area from surrounding areas, but this may not be economically feasible. They are most effective when the orchard's codling moth population is low.

Organically approved insecticides and some pheromone mating disruption products are acceptable for use in organically certified crops. While certain oil products are organically certified and will supply 30–40% egg kill, there is a concern of phytotoxicity with oils, especially when weather is hot. Oils have also been shown to kill the walnut aphid parasite, Trioxys pallidus. Always check with your organic certifier to determine what products are approved for organic certification.

Degree-days (DD) are an important tool for managing many pests. Calculate degree-days for codling moth in walnut for you location using the codling moth pest model. To learn more about using degree-days to time insecticide applications, watch the degree-days video.

In early March, place traps in your orchards to determine first moth emergence.

• If using traps with standard 1 mg pheromone (1X) lures, put traps in the southeast quadrant of the tree about 6 to 7 feet high. Traps placed higher in the tree canopy catch more moths, which may be useful in orchards with low codling moth numbers.
• Traps with CM-DA lures should be hung mid-canopy and are most useful in orchards that are either using mating disruption or near other orchards using mating disruption.

Biofix is the first date that moths are consistently found in traps and sunset temperatures have reached 62°F. All moths caught in traps with standard 1 mg pheromone lures will be males. Traps with CM-DA combo lures, which contain codlemone pheromone (the male attractant used in 1 mg lures) and a kairomone made of pear volatiles, attract only males before females emerge and both males and females thereafter. The first sustained catch of female moths in these traps is referred to as "female biofix", but degree-day calculations and the treatment timings are all based on the biofix established using male trap captures, regardless of the lure used for monitoring. To predict egg hatch, begin accumulating degree-days from the biofix, using a lower threshold of 50°F and an upper threshold of 88°F.

Because biofix points vary from orchard to orchard, monitor each orchard separately to determine the biofix point for that orchard. See Table 1 for information on setting biofix points for subsequent generations.

Unless the orchard is isolated, mating disruption is most successful in large, uniform orchards on flat ground, with a square shape (as opposed to a narrow rectangular shape), and with relatively low to moderate numbers of codling moths. It is less effective in orchards with susceptible varieties (e.g., Ashley, Payne, Serr, Vina) or in orchards that have a history of high numbers of codling moths or economically significant codling moth damage. In these situations, make the transition to a mating disruption program using both mating disruption and chemical control the first year or two to reduce codling moth damage.

Air currents entering the windward (upwind) sides of orchards adjacent to open areas may reduce the effectiveness of mating disruption along orchard edges. In addition, the edges of orchards adjacent to other walnuts not under mating disruption may have immigration of mated females from those blocks. Monitor these situations closely, especially in puffer-treated orchards where the distance between dispensers is large. An insecticide spray applied 4 to 5 trees deep along the affected edge of the orchard may help reduce the risk of damage in these areas.

Traps using standard 1 mg pheromone lures catch few or no moths when mating disruption is present. Therefore, in mating disruption orchards, use codling moth traps with CM-DA combo lures to monitor development and moth numbers.

• Place CM-DA combo traps (1 trap per 25 acres) in the mid-canopy of trees. High counts of codling moths in these traps will help determine the need for supplementing mating disruption with insecticides.
• Also, hang a smaller number of standard 1 mg traps (1 trap per 50 acres or per block) to assess the effectiveness and longevity of the mating disruptant. Hang these traps at 6 to 8 feet in the trees. If moths are caught in these traps consistently for 2 consecutive weeks, the mating disruptant may have broken down or expired, and insecticides may be necessary.

Change trap lures and bottoms at the frequency recommended by the manufacturer.

There are three types of pheromone mating disruption products available for use in walnuts:

• Sprayable liquid formulations designed to be applied with standard orchard sprayers, which contain pheromones in tiny microcapsules that release pheromones into the air once they are deposited on leaves.
• Hand-applied dispensers of various sorts that are hung in the orchard at rates ranging from 20 to 200 units per acre. Pheromones are released into the orchard continuously over a prolonged period of time.
• Aerosol dispensers hung in the orchard at low densities, typically one unit per 1.5 to 2 acres. These mechanically dispense small amounts of pheromones into the orchard air at programmed intervals.

Aerosol or plastic dispensers: Hang in the upper quarter of the tree canopy before the historic date of first-flight biofix: typically mid-March in the central and southern San Joaquin Valley to early April further north.

Sprayable pheromone: Apply at or after biofix when leaves have started growing and are partially expanded. Sprayable formulations have short residual activity. They must be applied at 3- to 4-week intervals for sustained mating disruption, as applications at longer intervals have not been proven effective. Make additional applications shortly after the biofix of the second and third flights. When large moth numbers are present in an orchard, sprayable pheromones have been shown to reduce codling moth damage when added to a conventional spray program. As with hand-applied dispensers, standard 1 mg trap catches are helpful for deciding when sprayable pheromones need to be re-applied.

Check nuts for damage during each codling moth generation, particularly near the end of the generation when it is easier to see the frass (excrement). Examine 1,000 mid-canopy nuts in each block (20 nuts per tree on 50 trees per block) for signs of codling moth larval entry. Damaged nuts exceeding 1% after the first generation or 2% after the second indicate an infestation that may exceed 5% at harvest. In these cases supplement the mating disruption treatment with insecticide spray during the egg hatch of the next flight, which is 300 degree-days after the biofix.

At harvest, collect and crack out 1,000 nuts to assess damage and plan for next year.

When making the transition from managing codling moth with insecticides to mating disruption:

• High codling moth numbers (i.e., where damage from previous season's harvest sample was over 4%): supplement mating disruption with insecticide applications to reduce the codling moth numbers.
• Moderate numbers of moths (i.e., where the previous season's damage was 2 to 4% at harvest) or in the second year of transition: supplement mating disruption with sprays of insecticides that will not disrupt natural enemies.
• Low moth numbers (where the harvest damage was less than 1% the previous season): mating disruption alone can be used.

Where insecticide sprays are needed, use degree-day calculations (see below) to apply insecticides at the most effective time.

In orchards sprayed with contact or ingested insecticides (e.g., spinosad and oil, organophosphates, pyrethroids, and carbamates), time all insecticide applications to kill larvae as they emerge from eggs. If insect growth regulators are used, apply insecticides before egg laying (Dimilin) or egg hatch (Confirm, Intrepid), depending on label instructions. If using a diamide insecticide (e.g., Altacor or Exirel), apply at or before peak egg laying of the targeted generation. Use pheromone (1 mg) or CM-DA combo traps, degree-days (DD), and sunset temperatures to monitor codling moth activity and determine when egg hatch occurs. If nearby orchards are using mating disruptants, use the CM-DA combo lure traps for monitoring.

The need for treatment and the timing of sprays is different for the different generations of codling moth. The degree-day model used in this guideline for codling moth reflects the concept that each subsequent codling moth generation time is longer than the preceding one.

First Generation

The first flight of codling moth can last a long time and have two peaks (1A and 1B). To minimize interference with the walnut aphid parasite and, in most cases, avoid the necessity for aphid insecticide applications, it is best to delay sprays until the second generation or the end of the first generation (1B), especially in later-season varieties.

Low populations

If damage did not exceed 3% the previous season and less than an average of two moths per trap per night are being caught with 1 mg traps, delay insecticide applications until the second flight peak (1B). If you see an increase in trap catches, spray when 600 to 700 degree-days have accumulated from biofix.

Moderate to high populations

If damage exceeded 3% the previous season or more than an average of two moths per trap per night are being caught with 1 mg traps, plan to spray both the 1A and 1B larvae:

Treating 1A larvae

Apply a pesticide when 300 degree-days accumulate after biofix, using a short-residual material to minimize disruption of the aphid parasite, Trioxys pallidus.

Treating 1B larvae

When you see an increase in moths caught in traps around 600 to 700 degree-days from the first biofix, apply a second spray when the residual period (i.e., the length of time the insecticide controls the pest) of the first pesticide ends. Residual periods for many of the pesticides are listed in the treatment table below. In most cases, a range of days is given. The actual length of a residual period is influenced by several factors, including the pH of the solution and the susceptibility of the population to that material. If the population has developed any resistance to the material, then the residual period will be shorter than it would be for a highly susceptible population.

Second and Later Generations

Codling moth has two to four generations a year. Continue monitoring with traps and accumulating degree-days (as outlined in TABLE 1) until the crop is harvested or numbers decline to below damaging levels in September. At the beginning of each generation, determine the biofix point for that generation in order to predict the best treatment timing during egg hatch for that generation.

To time an insecticide application for second-generation larvae, determine the biofix for the second generation. This generally occurs around 1060 degree-days from the first biofix point. However, any increase in trap catches after 800 degree-days can be considered the biofix. To better determine this biofix, clean and service the traps around 700 degree-days and start checking traps more frequently.

If there was no second peak in the first flight of codling moth, the number of dropped nuts can be used to determine if the second generation requires an insecticide application. All nuts damaged by codling moth early in the season drop, except when there is a second peak of the first flight or if the weather is not hot enough.

1. Look for frass at the blossom end of nuts to confirm that codling moth caused the drop.
2. Examine all the nuts under the same 10 trees in an orchard block each week during the nut drop period (4 to 6 weeks from the end of bloom).
3. Record the total number of damaged nuts per tree (not the percent damaged).
   • If an average of 4 or less infested nuts are found per tree, you can expect less than 5% codling moth damage by harvest without a spray.
   • If there are between 4 and 24 infested nuts per tree, spray at 250 degree-days from the second biofix and use a short residual pesticide.
   • If you collect more than 24 codling moth-damaged nuts per tree, apply an insecticide as soon as eggs of the second generation start hatching (250 degree-days from the second biofix), and use a long residual pesticide to cover the entire hatch period (about 1 month).

Third-generation Egg Hatch

A third (or fourth) generation of codling moth eggs does not occur every year in every location. Codling moth larvae normally go into diapause (winter dormant state) around August 22, but in warm years and warm locations they will have already started pupation before August 22, and these pupae will soon emerge as adults to produce a third generation. If 650 degree-days have accumulated between the peak of the second-generation flight and August 22, most of the codling moth will not go into diapause but will pupate and emerge in August to early September, depending on climate.

If degree-day accumulation data indicates a third generation will occur, use pheromone traps to establish a third biofix point around 1100 to 1200 degree-days from the second biofix. Apply a spray when 200 to 250 degree-days have accumulated from the third biofix unless trap catches are high, in which case spray at 160 degree-days. If needed, apply the second spray when the residual of the previous spray ends.

Additional Treatment Considerations

If there was a second peak in the first flight, not all of the infested nuts will have dropped, so visually inspect the tree canopy for infested nuts.

1. Look at a minimum of 10 nuts on each of 10 trees at least 10 feet up in the canopy.
   • If less than 2% are infested, don't spray.
   • If greater than 2% infested nuts are found, a pesticide application is necessary.
2. Apply an insecticide as soon as eggs of the second generation start hatching (250 degree-days from the second biofix).
   • Use a short residual material if the percent infestation is between 3 and 5, or
   • if greater than 5% infestation is found a long residual material to cover the entire hatch period (about 1 month).

For the third and fourth generation, the decision to spray must be based on a combination of factors including previous pesticide applications, number of nuts infested in the previous generation, trap catches, and the ability to harvest early. To determine the number of nuts infested in the previous generation, visually inspect 20 nuts at least 10 feet up in the canopy on 50 trees.

• If less than 2% are infested, don't spray.
• If greater than 2% are infested, spray.

If you spray, it is important to determine a biofix for third and fourth generations. The generation times get longer with each generation. Look for the third biofix around 1100 degree-days from the second biofix and for the fourth biofix around 1200 degree-days from the third biofix (the range for both biofix points is 800 to 1300 degree-days). Pesticide applications are often not needed at this time; but if stings are found on nuts, apply sprays 300 degree-days after the biofix.

• Degree-day table
• Degree-day calculator
• Identifying seasonal pests in harvest samples
•  Using degree-days to time insecticide applications