Augmenting sanitation with insecticides to improve control of navel orangeworm (Amyelois transitella Walker) (Lepidoptera: Pyralidae) in California tree nuts

Authors: Siegel, Joel; GILCREASE, G - Syngenta

Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2022
Publication Date: 2/4/2022
Citation: Siegel, J.P., Gilcrease, G. 2022. Augmenting sanitation with insecticides to improve control of navel orangeworm (Amyelois transitella Walker) (Lepidoptera: Pyralidae) in California tree nuts. Pest Management Science. 78(5):2034-2042. https://doi.org/10.1002/ps.6827.
DOI: https://doi.org/10.1002/ps.6827

Interpretive Summary: The navel orangeworm (NOW) Amyelois transitella is the primary pest of California almond (estimated $7 billion farm gate value) and pistachio (estimated $2.5 billion farm gate value) and is also an important pest of walnut and fig. Sanitation is the cornerstone for its control in almonds because unharvested nuts, referred to as mummies, serve as both shelter and as a food source. At times, sanitation is insufficient because of wet conditions, lack of manpower and its cost. In this study we investigated a method to augment sanitation by applying insecticides to mummy almonds. The goal of this spray is to remove these nuts as a resource for egg laying adults by killing either the eggs after they are laid or the newly hatched larvae when they crawl on the nut and begin feeding. We found that for almonds, 55% of adult emergence from mummies occurred between April 15 and June 1, therefore an insecticide application that lasted from mid-April through May could have an impact on this flight. We tested insecticides belonging to four families, by spraying mummies in March and collecting them weekly and challenging them with eggs. All insecticides testes were effective for at least six weeks and some lasted 10 weeks. Although this is not a substitute for sanitation, we demonstrated that insecticides applied in mid-April could protect nuts through May and some insecticides applied in mid-April could last through June too.

Technical Abstract: The navel orangeworm (NOW) Amyelois transitella is the primary pest of California almond (estimated $7 billion farm gate value) and pistachio (estimated $2.5 billion farm gate value) and is also an important pest of walnut and fig. Sanitation is the cornerstone for its control in almonds because unharvested nuts, referred to as mummies, serve as both shelter and as a food source. At times, sanitation is insufficient because of wet conditions, lack of manpower and its cost. The consequences of sanitation failure are now greater because of the combination of the high value of tree nuts, increasing cost of infested nuts, and expansion in tree nut acreage (more than 1.5 million acres of almonds and 500 million acres of pistachios) resulting in a larger standing population of NOW that can exploit gaps in sanitation. Research conducted in 2005-2006 evaluated whether an insecticide sprayed on mummy nuts could reduce successful oviposition by adults and subsequent establishment of newly hatched larvae. Treatments in early October reduced adult emergence from pistachios by as much as 81.5% while adult emergence was reduced by 32.4% in almonds, indicating that they were infested during the summer. We shifted our focus to emergence from almond mummies in late winter through spring and discovered that 40% of adult emergence occurred in May and that 55% of adult emergence occurred between April 15 and June 1. We conducted almond challenge trials to determine how long insecticides were able to kill NOW in mummies and the duration of control for some insecticides lasted 10 weeks. All insecticides tested lasted for at least 5-6 weeks. Our study demonstrated that a mummy spray could provide up to 10 weeks of control, and if applied during the April-15 to May 1 window this application could target 55% of the overwinter flight.