Nut factors associated with navel orangeworm, Amyelois transitella (Lepidoptera: Pyralidae) damage to pistachio (Pistacia vera) in California (2007-2017) and implication for control

Authors: Siegel, Joel

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/27/2023
Publication Date: 4/11/2023
Citation: Siegel, J.P. 2023. Nut factors associated with navel orangeworm, Amyelois transitella (Lepidoptera: Pyralidae) damage to pistachio (Pistacia vera) in California (2007-2017) and implication for control. Journal of Economic Entomology. 116(3):882-890. https://doi.org/10.1093/jee/toad051.
DOI: https://doi.org/10.1093/jee/toad051

Interpretive Summary: In California, pistachios are a relatively new and valuable crop and the USA grows 50.8% of the world supply. Pistachios are primarily grown in five counties in the southern San Joaquin Valley, Kern, Fresno, Tulare, Madera and Kings, accounting for 93.8% of the California production in 2020. The navel orangeworm is the primary pest in California and the population dynamics of this moth are complicated because it has three to four generations per year and develops on both nuts left over from the previous harvest and new crop nuts. Its first outbreak of the twenty-first century occurred in 2007 and during the 11-year period between 2007-2017 there were five outbreaks. Currently, adult trapping does not provide sufficient lead time to adjust insecticide application schedules to prevent an outbreak. The goal of this study was to determine if there are nut associated factors associated with A. transitella outbreaks that can be identified earlier in the season to allow sufficient time to make additional insecticide sprays or alter the timing of scheduled treatments. This study used processor grade sheets because they provide information on a scale that cannot be matched by sampling individual orchards. This paper reports the relationship between time, the nut factors percent split, percent dark staining and percent shell damage, on total insect damage for low damage years, 73,611 loads and high damage years, 88,803 loads as identified by the processors, for the period 2007-2017.

Technical Abstract: The navel orangeworm, Amyelois transitella is the primary pest of pistachios in California. The first A. transitella outbreak of the twenty-first century occurred in 2007 and during the 11-year period between 2007-2017 there were five outbreaks. The goal of this study was to determine the nut factors associated with these outbreaks that can be identified earlier in the season to alter the timing of scheduled treatments. This study used processor grade sheets to explore the relationship between the variables time, percent nut split, percent nut dark staining and percent shell damage on total insect damage for low damage (73,611 loads) and high damage years, (88,803 loads). Total insect damage for the Low Damage years averaged 0.005 and for High Damage years it was 0.015. For Low Damage years the strongest correlation was between total insect damage and percent dark stain, 0.23, while the correlations between total insect damage and both time and percent shell defect were equivalent, 0.15 and 0.14, respectively. For High Damage years the correlation between total insect damage and both percent dark stain and time were equivalent, 0.35 and 0.36, respectively. When Low and High Damage years are contrasted, the correlation between total insect damage and time increased as did the correlations with time and percent dark stain. A focus on nut factors shifts attention to the tree rather than to the standing population of A. transitella, and outbreaks then become a consequence of pistachio orchard problems that cause tree stress and premature hull breakdown.