“Think about what you may have left on the orchard floor,” former UCCE integrated pest management advisor Emily Symmes said about the extraordinarily large first flight of codling moth experienced by growers this year in many California walnut orchards.
Symmes, who is now a technical field manager with Suterra, said in June that the large first flight of codling moth (CM) caused high numbers of dropped nuts and even with well-timed sprays, crop damage is already being observed.
If a grower’s grade sheet from the huller is not that bad, he may forget that first flight was so large, Symmes said, but he should keep in mind the loss in yield.
As harvest approaches for the 2020 walnut growing season, evaluating the success of this season’s IPM program is an important first step to planning ahead for 2021. A thorough review of early season losses, damage assessments from in-orchard harvest samples, information provided on grade sheets, and the overall management program (sprays, timing, materials, application rates, non-insecticide approaches, etc.) will indicate what modifications may be necessary going forward to achieve desired crop quality measures.
Minimize Early Damage
The first generation of CM causes nutlets to fall from the tree. Evidence of nut loss by CM early in the season includes the frass found at the blossom end. Early season varieties, especially those with shells not well-sealed, are most susceptible to CM infestation. Toward the end of the first generation and with the second and third generation, nuts do not drop, but CM larvae boring into nuts damage the kernels. The CM also opens the door to navel orangeworm (NOW) infestations. A management program that minimizes early- and mid-season CM damage also reduces harvest damage by NOW.
Early maturing walnut varieties are the most likely targets for CM as they present feeding opportunities as the first flight appears in March and April. The flight of the overwintered generation may have two peaks and can last multiple 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 moth in the second flight lay their eggs, this starts the second generation.
A third-generation always occurs in the Central Valley. The fourth-generation may or may not be produced based on growing regions. Both of these generations can cause significant damage if populations are not controlled.
With early varieties, CM larvae can chew into poorly sealed nuts. Later season varieties like Chandler can also sustain larval feeding damage. To detect a CM infestation, look for frass at the point of entry into the husk.
An integrated pest management strategy for CM should be in place once a new orchard begins producing fruit. While CM can be mobile between orchards, it is not known to have a range of migration similar to other moth pests including NOW. Maintaining low resident populations of CM in individual walnut orchards and blocks can help populations remain low from year to year, and is an incentive to begin control early in orchard development.
There is a well-established and validated IPM program for walnuts. Decision tools developed by UC researchers provide very dependable information, Symmes said, allowing growers to predict flights and time insecticide applications. Use of mating disruption is also recognized as an effective part of a management strategy.
The recommendation from UC research is to begin hanging CM traps in mid-March. The traps with CM pheromone lures should be placed in the tree canopy. Orchards where mating disruption is used, and those near orchards with mating disruption, should also use traps baited with a combination of CM pheromone and pear ester plant volatile, as mating disruption will reduce trap counts in pheromone-only traps.
Traps should be checked to determine first flight biofix, the date where moth are consistently caught in traps and sunset temperatures are 62 degrees F or above. After that first flight biofix day is determined, begin tracking degree-day accumulations to schedule spray applications and predict the onset of subsequent flights. Symmes said the degree-day models for CM pair the known developmental requirements – heat units for a specific pest in a specific crop with the actual heat units.
The degree-day calculator for CM is on the UC IPM web site at ipm.ucanr.edu, in Identify and Manage Pests at weather and degree days. Temperatures can be obtained from the nearest CIMIS station. There is a range for each flight prediction and it should be confirmed with trap activity.
Determining the need for a spray application is based on orchard history, in-season trap catches, and for the second and third flights, damage evaluation including dropped nuts and canopy counts.
High crop damage due to CM in recent years is puzzling, according to Symmes, even with well-timed spray applications. Warmer, drier weather may be one factor in the increased CM damage seen. Another factor may be the loss of chlorpyrifos as a critical tool. Use of chlorpyrifos has dramatically decreased in recent years, she said, but it was a reliable tool that could be used in the case of a severe infestation. With chlorpyrifos applications, coverage of the tree canopy and precise timing to target the egg or early larval stage, while important, was not as critical as it is with more selective pesticides now in use.
There is one walnut growing region of California that does not have high CM pressure. The Lake County region, where about 4,200 acres of walnuts are farmed and the majority are in organic production, has some environmental advantages, according to UCCE Lake County pomology farm advisor Rachel Elkins. Late spring rains and frost events are common and suppress the resident CM populations, she said. In addition, the varieties grown in that region help lessen the susceptibility to CM damage. Finally, Elkins said, pears grown in the area are a preferred host for CM.
When growers did plant the earlier maturing varieties, they did have issues with CM damage, Elkins said. Once those were replaced with later varieties, the CM infestation lessened. An added benefit, she noted, was that without CM pressure in the orchards, their NOW damage disappeared.
Lower insect pressure in Lake County is one of the reasons growers there are able to use organic production practices, Elkins said.
The main insect pest in the area is the walnut husk fly that prefers later blooming and maturing walnut varieties, and can be managed with organic practices.