Evaluating insecticide coverage and determining its effect on the duration of control for navel orangeworm (Amyelois transitella Walker) (Lepidoptera: Pyralidae) in California almonds

Authors: Siegel, Joel; STRMISKA, MATHEW - Adaptiv; Walse, Spencer; NIEDERHOLZER, FRANZ - University Of California - Cooperative Extension Service; GILES, KEN - University Of California, Davis

Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/17/2019
Publication Date: 3/29/2019
Citation: Siegel, J.P., Strmiska, M.M., Walse, S.S., Niederholzer, F.J., Giles, K.D. 2019. Evaluating insecticide coverage and determining its effect on the duration of control for navel orangeworm (Amyelois transitella Walker) (Lepidoptera: Pyralidae) in California almonds. Pest Management Science. https://doi.org/10.1002/ps.5413.
DOI: https://doi.org/10.1002/ps.5413

Interpretive Summary: In California almond orchards, the navel orangeworm (NOW), Amyelois transitella Walker (Lepidoptera: Pyralidae), is the primary insect pest. Management of this pest is extremely challenging because of its ability to develop on multiple hosts grown in close proximity to almonds, combined with its mobility, which in turn requires a strategy to manage immigrating pests as well as internal populations. NOW is currently managed by a combination of sanitation (timely removal and/or destruction of mummy nuts), insecticide sprays, and, more recently, mating disruption. Protecting almonds with narrow spectrum insecticides is exceptionally challenging because the only target is the nut. The focus of this study was to improve application using orchard airblast sprayers by quantifying what percentage of the spray is on target. Our studies established that there is less insecticide deposited in the upper canopy than in the lower canopy, and that it breaks down sooner in the upper canopy than in the lower canopy. There is an apparent loss of insecticide throughout application, because insecticide deposited on the leaves and bark does not count as hitting the target; as little as 10% of the insecticide (90% apparent loss) was deposited on the almond hull, and as little as 1% of the insecticide was deposited in the almond suture. Control failure was most apparent in orchards treated at ground speeds above 2 mph, and coverage was greater using 150 gallons per acre than at 100 gallons per acre. In contact toxicity assays, control in the upper canopy failed after two weeks. Our data underscore the importance of all aspects of application including sprayer calibration, ground speed, insecticide concentration and volume in order to ensure that more insecticide is deposited on the nuts and less winds up in the foliage.

Technical Abstract: Insecticide application currently plays a vital role in the control of navel orangeworm (Amyelois transitella) in almonds, but obtaining proper coverage is problematic. We report the results of studies that quantified insecticide deposition over height, identified the amount of insecticide deposited in the suture, and established the duration of control in the field, using a combination of bioassays and chemical analysis. Companion laboratory studies evaluated the impact of photolysis and reduced insecticide deposition on contact toxicity. There was less insecticide deposited in the upper canopy than in the lower canopy, and contact toxicity failed sooner (14 days) than ingestion toxicity (stable during a 21-day collection period). Protecting nuts with narrow spectrum insecticides is particularly challenging because insecticide deposited elsewhere in the tree is off target. In our study, as little as 10% of the insecticide was deposited on the almond hull, and only 1% deposited in the suture. The cumulative effect of this apparent loss was demonstrated in almond challenge studies, when control failed at two weeks in the upper canopy. This failure was most obvious in orchards treated at ground speeds above 2 mph. Our data underscore the need to improve all aspects of application, including sprayer calibration, ground speed, insecticide concentration, water volume, and adjuvant choice in order to maximize control of A. transitella, as well as the need for additional studies focusing on insecticide degradation in the upper canopy.