Navel orangeworm mummy dip bioassay on almond, 2024

Each spring, overwintering navel orangeworm (NOW) adults emerge from remnant nuts from the previous season (mummies) to lay eggs back onto the mummies. Applications of pesticides at this time of year, commonly referred to as “May sprays” or “mummy sprays” target this oviposition period. During the spring of 2024, we conducted a mummy dip bioassay to evaluate 15 treatments (including 1 untreated check) for their impact on NOW eggs laid by first-flight moths. During the winter of 2023–2024 we collected mummies from Nonpareil trees by hitting them onto a tarp from a research almond orchard maintained by the University of California in Shafter, Kern Co., California. Individual nuts were manually picked up and shaken near an ear to listen for a rattling noise by the kernel to indicate that the mummy was not already infested by a larave. A total of 3,000 mummies that made a rattle sound were manually hulled, with the remaining mummy parts (shell + kernel) hot-glued in groups of 10 onto 16-inch pieces of flat drip irrigation tape. The 300 strips were randomly assigned to 1 of the 15 treatments. Insecticides were mixed in 5-gal buckets at a concentration equivalent to the product field rate applied in 200 gpa of water (Table 1). All treatments were made with the inclusion of Dyne-Amic as a surfactant at a rate of 16 fl oz per 100 gal of water (0.125% v/v). On 30 Apr 2024, 20 strips per treatment were dipped for 20 s in the insecticide solution and then allowed to dry. Once dry, they were hung in the orchard on individual almond trees with a binder clip and wire in a CRD. The twenty strips were left in the trees for 20 days, and on 21 May were removed. Upon removal, all strips were placed in a warehouse at ambient temperature for approximately 1 month to allow time for eggs to hatch and complete their development in the kernel. Nuts were cracked open the week of 8 Jul and inspected for live or dead larvae, and for the remnants of pupae that had emerged. Data of the percentage of nuts with dead larvae or emerged pupae were evaluated by ANOVA after arcsine(sqrt(x/100)) transformation with means separated by Fisher’s Protected LSD (α = 0.05).

All treatments except for the low rate of Plinazolin, Verdepryn, and MBI-306 caused a significant reduction in the percentage of mummies with emerged pupae, with all treatments (minus the exceptions) resulting in damage levels statistically equivalent to the best treatment (Table 1). Close inspection of mummies did not reveal any dead first-instar larvae in the untreated check. However, dead first-instar larvae were found in all other treatments, with the exception of VCP-031, suggesting that adult moths were able to lay eggs on treated nuts, and that at least a portion of the eggs remained viable through eclosion, after which the larvae died while still in the first-instar stage.¹

Footnotes