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

Kaolin as a management alternative for insecticide-resistant navel orangeworm (Lepidoptera: Pyralidae)

The production of almonds and pistachios in California's Central Valley employs insecticides for the management of their primary pest, navel orangeworm. The pyrethroid Bifenthrin is commonly used, and now a strain of Amyelois transitella Walker (Lepidoptera: Pyralidae) (R347) obtained from Kern County almond orchards with a history of Bifenthrin use has acquired >110-fold resistance toward pyrethroids. One method to improve control is to use additives and spray adjuvants, which are applied simultaneously with an insecticide to increase coverage and/or duration of control. We tested 2 levels of the naturally occurring clay Kaolin as an additive, alone and in combination with either Bifenthrin or the diamide Chlorantraniliprole, to determine if it could reduce feeding damage and decrease survival of pyrethroid-resistant A. transitella on almonds in the laboratory and improve the efficacy of Chlorantraniliprole in the field. Larval performance was measured for the strains R347 and ALM (34.7-fold resistance compared to susceptible lab strain) reared on treated almonds. Strain R347 had 1.9-fold greater survival and caused 1.3-fold more feeding damage than strain ALM across all treatments, although both strains were susceptible to the combination of Kaolin + insecticide. Kaolin synergized Bifenthrin for R347, decreasing survival by 10.0%. Kaolin did not reduce feeding damage for either strain. When combined with insecticide, feeding damage was similar to insecticide alone, but the addition of Kaolin to the insecticide generally decreased survival more than the insecticide alone. In the field, the addition of Kaolin to Chlorantraniprole during application helped retain activity against this challenging pest.

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

Amyelois transitella is the primary pest of pistachios in California. The first A. transitella outbreak of the twenty-first century occurred in 2007 and a total of five outbreaks occurred between 2007 and 2017 (total insect damage >1%). This study used processor information to identify the nut factors associated with the outbreaks. Processor grade sheets were used to explore the relationship between the variables time of harvest, percent nut split, percent nut dark staining, percent nut shell damage, and percent adhering hull for Low Damage (82,537 loads) and High Damage years, (92,307 loads). Total insect damage (±SD) for the Low Damage years averaged 0.005 ± 0.01 and in High Damage years was three times higher, 0.015 ± 0.02. In Low Damage years the strongest correlation was between total insect damage and two variables, percent adhering hull and dark stain (0.25, 0.23) while in High Damage years the correlation between total insect damage and percent dark stain was the highest (0.32) followed by percent adhering hull (0.19). The linkage of these nut factors to insect damage suggests that outbreak prevention depends on early identification of premature hull split/breakdown in addition to the traditional emphasis on treating the standing population of A. transitella.

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

BACKGROUND

Sanitation, consisting of eliminating unharvested nuts (mummies) that serve as both harborage and food source for navel orangeworm (Amyelois transitella), is the foundation for control in almonds and pistachios. Sometimes sanitation is ineffective and needs to be augmented. Trials in almonds and pistachios were conducted early October 2005-2006 to determine if insecticides applied postharvest could augment sanitation, and in almonds late March to early May 2016-2020 to establish their duration of control.

RESULTS

Early October sprays reduced adult emergence by as much as 81.5% in pistachios, but reduction was only 32.4% in almonds. Trapping before almond hull split revealed that 55% of adult emergence occurred between 15 April and 1 June, indicating that an insecticide with a six week duration of control could reduce successful oviposition and neonate establishment. Five insecticides applied from late March to early May belonging to four Insecticide Resistance Action Committee (IRAC) groups decreased survival for at least six weeks, and some lasted as long as 10 weeks. By contrast, the duration of control in contact bioassays was far shorter.

CONCLUSION

Insecticides applied from late March to early May successfully reduced the survival of A. transitella in almond mummies for as long as 10 weeks postapplication, and there was a positive relationship between reduction in survival and kernel damage. If these insecticides are applied during mid- to late April, postbloom and before hull split, they can substantially eliminate almond mummies as a resource for 55% of the spring flight of A. transitella.

Toxicity, residue and risk assessment of tetraniliprole in soil-earthworm microcosms

Maize seed treatment with chemicals to control underground pests is a common agricultural practice, but inappropriate use of insecticides poses a considerable threat to plant development and soil nontarget organisms. In this study, the availability of tetraniliprole seed dressing to control the black cutworm Agrotis ipsilon (Lepidoptera: Noctuidae) in the maize seeding stage and its safety to earthworms (Eisenia fetida) were investigated. The selective toxicity (ST) of tetraniliprole between E. fetida and A. ipsilon was greater than 4000. No significant adverse effect of tetraniliprole seed treatment on the germination of maize seeds was observed at concentrations of 2.4-9.6 g a.i. /kg seed. Compared with the untreated control, seed treatment with tetraniliprole at 9.6 g a.i. /kg seed greatly reduced the percentage of damaged plants from 88.73% to 26.67%, and achieved the highest control effect of 69.91%. Tetraniliprole of 2.4 g a.i. /kg seed can effectively inhibit A. ipsilon until 14 days after seed germination, with the lowest mortality rate of 44.44%. During the entire exposure period, the maximum residual concentration of tetraniliprole detected in the soil (5.86 mg/kg) was considerably lower than the LC₅₀ value of tetraniliprole to E. fetida (>4000 mg/kg). According to the low-tier risk assessment, the highest risk quotient (RQ) of tetraniliprole seed treatment to earthworms at test concentrations was 2.8 × 10^-3, which was evaluated as acceptable. This study provided data support for tetraniliprole seed treatment to control underground pests in maize fields.

Differential regulation of cytochrome P450 genes associated with biosynthesis and detoxification in bifenthrin-resistant populations of navel orangewom (Amyelois transitella)

Pyrethroid resistance was first reported in 2013 for the navel orangeworm, Amyelois transitella, but the genetic underpinnings of pyrethroid resistance are unknown. We investigated the role of cytochrome P450 monooxygenases (P450s) belonging to the CYP3 and CYP4 clans using colonies derived from individuals collected in 2016 from almond orchards in two counties. One colony (ALM) originated from an almond orchard in Madera County with no reported pyrethroid resistance and the second colony (R347) originated from the same Kern County orchard where pyrethroid resistance was first reported. We used high-throughput quantitative real-time PCR (qRT-PCR) analyses of 65 P450s in the CYP3 and CYP4 clans of A. transitella to identify P450s induced by bifenthrin and associated with pyrethroid resistance. Nine P450s were constitutively overexpressed in R347 compared to ALM, including CYP6AE54 (11.7-fold), belonging to a subfamily associated with metabolic pesticide detoxification in Lepidoptera and CYP4G89 (33-fold) belonging to a subfamily associated with cuticular hydrocarbon (CHC) synthesis and resistance via reduced pesticide penetrance. Cuticular hydrocarbons analysis revealed that R347 produced twice as many total CHCs in the egg and adult stages as ALM. Topical toxicity bioassays for R347 determined that egg mortality was reduced at low bifenthrin concentrations and larval mortality was reduced at high concentrations of bifenthrin compared to ALM. Our discovery of both changes in metabolism and production of CHCs for R347 have implications for the possible decreased efficacy of other classes of insecticide used to control this insect. The threat of widespread pyrethroid resistance combined with the potential for cross-resistance to develop through the mechanism of reduced penetrance warrants developing management strategies that facilitate insecticide passage across the cuticle.

Evaluation of an unmanned aerial vehicle as a new method of pesticide application for almond crop protection

BACKGROUND

Unmanned Aerial Vehicles (UAVs), a new method of application to deliver pesticides, is rapidly being adopted for commercial use in crop protection in East Asia with increasing worldwide interest. Pest control in mature almond orchards with dense foliar canopies presents greater coverage challenges than field crops and smaller orchard or vineyard crops. We investigated the use of an electric hexacopter to provide acceptable spray deposition and canopy penetration to be considered credible for use in an almond pest control program.

RESULTS

The performance of the aerial and ground methods at different spray volumes were compared by analyzing spray deposition on water sensitive papers, insecticide residues on filter papers and residues on whole unhulled almonds at three canopy elevations. Overall residue levels of chlorantraniliprole insecticide on whole unhulled almonds across all pooled canopy strata were similar between UAV applied at 46.8 L/ha and 93.5 L/ha and the comparative air blast sprayer treatments applied at 935 L/ha. However, significant interactions between canopy elevation and spray method showed distinct residue patterns between the two application methods. Penetration and spray deposition at the lower canopy were observed and validated for the UAV application. Pest efficacy was evaluated by measuring nut damage at harvest.

CONCLUSION

This study presents promising data that support the potential innovative integration of UAV's into crop protection programs for large canopy crops such as almonds and may guide future research for developing relevant label recommendations.