For years, University of California Cooperative Extension (UCCE) farm advisers have reminded almond growers about the importance of winter orchard sanitation as part of year-round Navel orangeworm (NOW) control.
Now they are extending the same advice to walnut growers, many who have recently seen increased damage to the crop.
The recommendation comes as NOW pressure is higher this season, partly due to a mild winter followed by an early spring which allowed the pest to have four generations instead of the usual three.
“In most years, the timing doesn’t work out (for a fourth generation), but this year it's a little bit of a perfect storm,” said Dani Lightle, UCCE farm adviser for Glenn, Tehama, and Butte counties.
This has made walnut more of a potential host. In most years, walnuts are not as susceptible.
During winters with more normal rainfall, many mummies left on trees after harvest fall on the orchard ground, consumed later by mold. Yet after the last dry winter, Lightle walked into walnut orchards this spring and found good quality walnuts still attached to trees.
These same nuts provide overwintering habitat for NOW larvae.
An early spring hastened moth development. It included first generation maturation in about 750 degree days rather than the predicted 1050 degree days. Coupled with the early spring biofix, this meant fourth-generation eggs were laid in mid-to-late August.
Emily Symmes, UC area integrated pest management adviser for the northern Sacramento Valley, says increased walnut and almond acreage also could affect NOW populations.
“In the upper Sacramento Valley, farms are typically smaller than in the south Central Valley, but we're getting more contiguous acres of almonds and walnuts,” Symmes said.
“I think that's why we're seeing more Navel orangeworm. I believe the pest in walnut is pretty much here to stay. Just how bad it could be I just don't know.”
She is conducting trials this year to try to better understand how trap catches equate to NOW orchard populations.
Symmes work will continue next season, hoping to learn more about NOW population dynamics, and whether the problems are caused by native populations within walnut orchards, or whether the moths are migrating in from nearby host crops.
“Right now, there’s a lot of speculation,” she said.
As long as the hull remains sound and intact, walnuts are protected from NOW, says Lightle. Yet once an entryway is provided through hull split, then sunburn, codling moth feeding, or blight nuts can become susceptible to larval feeding.
This is one reason why farm advisers recommend a prompt nut harvest after hull split.
Once inside the hulls, worms enter the stem end of the nut through softer tissue and feed on the kernel - producing frass and webbing - which renders the nut unmarketable.
Hulls of early walnut varieties were already splitting in Northern California in mid-September. Symmes hoped to conduct demonstrations on later varieties to examine various hull split sprays to protect the nuts until harvest.
Unlike almonds, Lightle says UC does not have current insecticide recommendations for NOW in walnut.
Reviewing post-harvest grade sheets is one way to determine whether a problem exists, yet the information only notes the insect damage percentage and does not differentiate between NOW and codling moth.
Symmes encourages pest control advisers and grower who suspect damage to collect nut samples at harvest to crack out and identify the pests. The samples also can be stored in a refrigerator for later inspection.
NOW can be differentiated by a crescent shape behind its ear and shoulder, or head capsule, which is not found on codling moth.
A codling moth-infested nut also has a limited amount of frass and webbing, compared to a NOW-infested nut which may have multiple worms. Regardless of the number, NOW tends to be a messier pest.
As part of year-round NOW management in almonds, UC IPM recommends that growers shake or poll trees during the winter to remove un-harvested nuts, and leave no more than two mummies per tree.
Growers also should flail or chop afterwards to destroy nuts on the ground.
Lightle does not know how realistic this might be for walnut growers as walnut trees tend to be taller and not as many growers own shakers. The university does not have a mummy nut threshold for walnuts.
Lightle and Symmes agree that growers should remove and destroy trash and nuts around orchards, hullers, and dryers to help reduce the overwintering NOW habitat.