How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

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 flight of moths 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 60 eggs on leaves or nuts. Eggs are disc 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. The larvae leave the nuts when they are fully grown 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. At harvest 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. 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.

DAMAGE

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 with 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

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 depend on the size of the trees and the degree of codling moth infestation. In all cases, a program of monitoring with pheromone and/or kairomone traps and checking for damage is necessary to follow codling moth generations and assess the degree of infestation and 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.

Biological Control
Alone, natural enemies are not able to keep codling moth populations 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 may not be economically feasible. They are most effective when the orchard's codling moth population is low.

Organically Acceptable Methods
Use of hand-applied mating disruption products and organically approved insecticides 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 with phytotoxicity with oils, especially when weather is hot, and oils have been shown to kill the walnut aphid parasite, Trioxys pallidus.

MONITORING AND TREATMENT DECISIONS IN A MATING DISRUPTION ORCHARD
Unless the orchard is isolated, mating disruption is most successful in large (more than 10 acres) uniform orchards on flat ground, with moderate canopy height (25 feet or less) with a square shape (as opposed to a narrow rectangular shape), and with relatively low-to-moderate codling moth populations. It is less effective in orchards with tall trees and/or susceptible varieties (e.g., Ashley, Payne, Serr, Vina) with a large codling moth population history or a previous history of 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 to reduce population levels.

Establish Female Biofix
Time the application of mating disruptants to the female biofix, which is determined with the use of traps that contain either the DA lure or the CM-DA combo lure. The DA lure contains an attractant (kairomone) made of pear essence, which attract both males and females. The CM-DA combo lure also contains codlemone pheromone (male attractant) and attracts larger trap catches than the DA lure, and often the pheromone lure, alone. The female biofix date is the first day that female moths are consistently caught in traps for at least 2 days in a row.

DA traps. Males are attracted to DA traps only after females have emerged; before female emergence, males are not attracted to these traps. Therefore, any moths captured in DA traps indicate female emergence, biofix, and relative flight intensity. It is not necessary to determine the sex of these moths.

CM-DA combo traps. Because the CM-DA combo lure is composed of both pheromone and DA kairomone, it performs both to attract males before female emergence and to later attract females (because of its DA component).

To use CM-DA combo lures to set female biofix, the moths captured in these traps must be sexed by examining the tip of the abdomen. Males have a scale-covered bullet-shaped tip, whereas the tip of the female tapers and has a copper-colored, heart-shaped bilobed pad.

Setting Out Mating Disruptants
Place hand-applied dispensers in the upper third of the tree canopy before female biofix determined by a DA or CM-DA combo lure in mid-March in the central and south San Joaquin Valley to mid-April further north.

Sprayable pheromone should be applied after female biofix and when leaves have started growing and are partially expanded. Sprayable formulations have short residual activity and must be applied at 3- to 4-week intervals for sustained mating disruption. Research to date has not shown a benefit from applications applied at longer intervals. Make additional applications shortly after the female biofix of the second and third flights.When populations are high in an orchard, sprayable pheromones have been shown to reduce codling moth damage when added to a conventional spray program (see below).

Monitoring with Traps
Air currents entering the windward sides of orchards adjacent to open areas may break down mating disruption along orchard edges. In addition, the edges of orchards adjacent to other walnuts not under mating disruption may be infested by immigration of mated female from those blocks. These situations should be monitored closely. 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.

Population monitoring with DA lures. Traps with DA or CM-DA combo lures placed around mid-canopy in the trees will provide information on the overall population levels of codling moth in the orchard. High counts of codling moths in these traps will help determine the need for supplementing mating disruption with insecticides.

Mating disruption assessment with regular (1 mg or 1X) pheromone traps. After mating disruptants are applied, monitor weekly with standard 1X pheromone traps to assess the effectiveness and longevity of the mating disruptant. Hang these traps at 6 to 8 feet in the trees, which is lower than DA lures. If moths are caught in these traps consistently for 2 consecutive weeks, the mating disruptant may have broken down or expired, and chemical treatments may be necessary. Change trap lures and bottoms at the frequency recommended by the manufacturer.

Nut Sampling
Check nuts for damage during each generation, particularly near the end of the generation when it is easier to see the frass (excrement). Examine 1000 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 following egg hatch period, which is 250 degree-days after the next female biofix.

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

Supplemental Treatments
When making the transition from managing codling moth with insecticides to mating disruption in orchards with high codling moth populations (i.e., where damage in the previous season's harvest sample was over 4%), supplement mating disruption with insecticide applications to reduce the population. In the second year or in orchards with moderate populations (i.e., where the previous season's damage was 2 to 4% at harvest), supplement mating disruption with sprays of insecticides that will not disrupt populations of natural enemies. In orchards with low populations (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 time applications for maximum effectiveness, which is when the larvae are emerging from the eggs.

MONITORING AND TREATMENT DECISIONS IN A CONVENTIONAL ORCHARD
In orchards treated with contact or ingested insecticides (e.g., spinosad and oil, organophosphates pyrethroids, and carbamates), time all treatments to kill larvae as they emerge from eggs. If insect growth regulators are used, apply treatments before egg laying (Dimilin) or egg hatch (Confirm, Intrepid), depending on label instructions. Use pheromone 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 DA or CM-DA combo lure traps for monitoring.) 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.

Establishing Biofix and Accumulation of Degree-Days
In mid-March, place traps in your orchards to determine first moth emergence. Put traps in the southeast quadrant of the tree about 6 to 7 feet high for l mg lure traps (or at mid-canopy if using DA-based lures). Traps placed high in the tree canopy catch more moths, which may be useful in orchards with low codling moth populations.

Biofix is the first date that moths are consistently found in traps and sunset temperatures have reached 62°F. 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.

Treatment Decisions
The need for treatment and the timing of sprays is different for the different generations of codling moth.

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 treatments, it is best to delay treatment 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 2 moths per trap per night are being caught, delay treatment until the second flight peak (1B). If you see an increase in trap catches, treat when 600 to 700 DD have accumulated from biofix.

Moderate to high populations: If damage exceeded 3% the previous season or more than an average of 2 moths per trap per night are being caught, plan to treat both the 1A and 1B larvae.

Treating 1A larvae: Use the degree-day (DD) accumulations below to time this application and use a short-residual material to minimize disruption of the aphid parasite, Trioxys pallidus.

Pheromone lures - Apply a treatment when 300 DD accumulate after the first male biofix.

DA or CM-DA combo lures - Apply a treatment when 250 DD accumulate after first female biofix.

Treating 1B larvae: When you see an increase in moths caught in traps around 600 to 700 DD 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 treatment materials are listed in the Treatment Table. 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 populations decline to below damaging numbers 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 a treatment for second generation larvae, determine the biofix for the second generation. This generally occurs around 1060 DD from the first biofix point. However, any increase in trap catches after 800 DD can be considered the biofix. To better determine this biofix, clean and service the traps around 700 DD 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 treatment. All codling moth damaged nuts drop except when there is a second peak of the first flight or if the weather is not hot enough. (Look for frass at the blossom end of nuts to confirm that codling moth caused the drop.) 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). Record the total number 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 DD from the second biofix and use a short residual pesticide. If you collect more than 24 codling moth-damaged nuts per tree, apply a treatment as soon as eggs of the second generation start hatching (250 DD from the second biofix), and use a long residual pesticide to cover the entire hatch period (about 1 month).

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

For the third and fourth generation, the decision to treat must be based on a combination of factors including previous treatments, 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 treat. If greater than 2% are infested, treat.

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 DD from the second biofix and for the fourth biofix around 1200 DD from the third biofix (the range for both biofix points is 800 to 1300 DD). Treatments are often not needed at this time; but if stings are found on nuts, apply sprays 300 DD after the biofix.

• Summary of Important Links

IMPORTANT LINKS

• Degree-day table
• Run models and calculate degree-days
• Identifying seasonal pests in harvest samples

PUBLICATION

UC IPM Pest Management Guidelines: Walnut
UC ANR Publication 3471
Insects and Mites
C. Pickel, UC IPM Program/UC Cooperative Extension, Sutter/Yuba counties
W. J. Bentley, UC IPM Program/Kearney Agricultural Center, Parlier
J. A. Grant, UC Cooperative Extension, San Joaquin Co.
J. K. Hasey, UC Cooperative Extension, Sutter/Yuba counties
W. W. Coates, UC Cooperative Extension, San Benito Co.
R. A. Van Steenwyk, Insect Biology, UC Berkeley
D. Light, USDA, Albany, CA.
Acknowledgment for contributions to the insects and mites section:
W. H. Olson, UC Cooperative Extension, Butte Co.
L. C. Hendricks, UC Cooperative Extension, Merced Co.
G. S. Sibbett, UC Cooperative Extension, Tulare Co.
D. Light, USDA, Albany, CA

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