Moving toward almond harvest, it is important to take steps to effectively manage the navel orangeworm (NOW). This pest is a double threat to almond quality: In addition to causing direct damage to the kernel, NOW has broader quality implications because it opens the door to fungal infections and contaminants. Research shows the mold Aspergillus and the aflatoxin contaminant it produces is associated with reject kernels, particularly those damaged by navel orangeworm. Aflatoxin produced by Aspergillus mold is a known carcinogen and mutagen.
Aflatoxins are regulated the world over, with the European Union (EU) having particularly low maximum limits. The EU is the largest market for California almonds, even larger than the U.S. market, and has put pressure on the California almond industry to reduce aflatoxin contamination, which requires NOW prevention and control. Growers must take critical steps to minimize NOW damage to attain a goal of no more than 2 percent damage, which reduces the risk of aflatoxin contamination.
Early harvest, sanitation
The first two keys to NOW control and prevention are winter sanitation to remove and destroy mummy nuts from the trees and orchard, and early harvest. Knock as soon as possible after the nuts are mature — when 95 percent to 100 percent of the hulls have split along at least a portion of their suture as best measured at the 6–8 foot level of the tree canopy. When nuts are knocked to the ground, they escape late season NOW egg-laying and the resulting damage. Late season orangeworm populations starting 30-40 days after the very first nuts split can be explosive because the NOW has cycled through the new crop, which is a plentiful food source. After nuts have been shaken to the ground, remove them as soon as possible to avoid ant damage.
A third component of managing late-season NOW is hull-split sprays. Under higher pressure from NOW, a spray at hull-split initiation can be considered if winter sanitation is insufficient (e.g., average of more than one mummy left per tree by June) and/or mated female moths fly in from adjacent uncleaned areas (including pistachios and walnuts), particularly within a half-mile.
It is very important that hull-split sprays to susceptible varieties are applied at hull-split initiation at the same time as NOW egg laying. Just as hull split begins, egg laying can be confirmed by using egg traps, which are effective at the beginning of hull split and lose effectiveness as split progresses; and/or tracking day degrees from the biofix of the spring NOW flight (see ipm.ucdavis.edu for details). Balance egg-laying activity with spray timing, typically from about 1 percent to 5 percent hull split, which will be nuts in the upper outer portions of the tree canopy.
A number of new insecticides are being registered and the Almond Board of California is currently funding research to evaluate them. Studies under way by David Haviland (UCCE entomologist, Kern County), Brent Holtz (UCCE pomology farm advisor, Madera County) and Frank Zalom (entomologist, UC Davis) are evaluating their performance and best “fit” in-season (e.g., May or hull-split). In addition, their work is getting an “assist” from past and current research being conducted by Joel Siegel (USDA ARS, Parlier), Brad Higbee (Paramount Farming), and Walt Bentley (UC IPM advisor, Kearney).
This work is in progress, but there are some general observations. These insecticides fall into two groupings: 1) Reduced risk compounds (e.g., Intrepid, Delegate); and 2) Next generation pyrethroids (e.g., Brigade, Baythroid, Warrior). Traditionally, well-timed and applied hull-split sprays have offered 50 percent to 60 percent control. Historically, pyrethroids have given good insect control because they have contact activity against all stages (egg, larvae, and adult), while reduced risk compounds are not as active against adults; however, pyrethroids adversely affect mite predators and can prompt mite flare-ups.
When the active ingredient in Brigade was initially registered in crops like cotton and strawberries, it had a “mite suppressive” effect, but this subsided with use, and application resulted in mite flare-ups. Therefore, pyrethroids are probably best considered under high-pressure situations and later season application such as hull split. In contrast, reduced risk compounds may have appropriate “fits” for both earlier May and later season sprays. The ongoing research will fine-tune these observations.