Evaluation of insecticides at hull split for navel orangeworm in almonds, 2023

Navel orangeworm (NOW) is a significant pest of almonds due to its ability to penetrate through the shell and feed directly on the harvestable kernel. Kernels infested by NOW are also at increased risk of the presence of aflatoxins that are highly regulated in certain export markets. In this trial, we evaluated the effects of 13 insecticide treatments, when applied twice during a 2-wk interval starting at the initiation of hull split, on the percentage of nuts infested at harvest. This trial was repeated twice: the first trial was done during the second NOW flight in the variety Nonpareil and the second trial during the third NOW flight in the pollinator variety Monterey.

Each trial was organized as a randomized complete block design with 8 replications of single-tree plots in an experimental almond orchard maintained by the University of California in Shafter, CA. The orchard was planted in 2009 using a 20 ft by 22 ft spacing of alternating rows on Nonpareil and Monterey. Applications for each product were made to Nonpareil on 7 Jul and 19 Jul, or to Monterey on 3 Aug and 17 Aug, to span the second and third NOW flights, respectively. Applications were made using a gas-powered wand sprayer calibrated to deliver 200 gpa of spray solution. All treatments were made with the inclusion of Dyne-Amic as a surfactant at a rate of 32 fl oz/acre (0.125% v/v).

Nonpareil and Monterey nuts were harvested on 14-15 Aug and 11-12 Sept, respectively, by placing a tarp under each tree and using a pole to knock down approximately 500-1,000 nuts, with care taken to collect from all elevations within the tree. Nuts on the tarp were gathered, mixed, and a subsample of approximately 250 to 300 nuts per tree was placed into a paper sack. The sacks were placed outdoors in an ambient-temperature warehouse for approximately 14 days for Nonpareil and 3 days for Monterey to dry, then all samples were placed into cold storage for future processing.

In Nov, all nuts were cracked open, and nuts with NOW were classified as having minor or major damage. Nuts with major damage were likely the result of eggs laid by moths from the second and third flights for Nonpareil and Monterey, respectively. Nuts with minor (pinhole) damage were likely the result of eggs laid by the third and fourth flights for Nonpareil and Monterey, respectively. Pinhole damage was most likely the result of eggs laid 1-2 wk after the second insecticide application was made. Data of the percentage of nuts with minor and major damage in each variety were evaluated by ANOVA after arcsine(sqrt(x/100)) transformation with means separated by Fisher’s Protected LSD (α = 0.05).

In the Nonpareil trial, there were no significant differences in minor damage with damage levels in the untreated check of 1.1% (Table 1). For major damage, significant reductions in kernel infestation compared to the untreated check (2.9%) were achieved by products containing methoxyfenozide (Intrepid, Intrepid Edge, Enkounter), 2 of the diamides (Altacor eVo and Shenzi), Plinazolin, and BasinFlex + Leprotec. Due to the disproportionate influence of major damage compared to minor damage on the total damage percentages, means separation for total damage paralleled the results previously described for major damage.

In the Monterey trial, very high NOW pressure led to control plots with 19.2% major damage and 3.3% minor damage (Table 1). When major and minor damage were combined, the most effective treatment was Plinazolin (5.3%). Products that were statistically equivalent to the best treatment included the industry standards Intrepid, Intrepid Edge, and Altacor eVo; as well as VCP-031, tetraniliprole, and Avaunt eVo. Each of these products reduced damage to < 10% compared to 22.5% in the untreated check. Products that had damage levels that were statistically higher than the best treatment but were significantly reduced compared to the untreated check included Enkounter, Shenzi, and BasinFlex + Leprotec.¹

Footnotes