Evaluating insecticide coverage in almond and pistachio for control of navel orangeworm (Amyelois transitella) (Lepidoptera: Pyralidae)

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

Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/4/2018
Publication Date: 11/15/2018
Citation: Siegel, J.P., Strmiska, M.M., Niederholzer, F.J., Giles, K.D., Walse, S.S. 2018. Evaluating insecticide coverage in almond and pistachio for control of navel orangeworm (Amyelois transitella) (Lepidoptera: Pyralidae). Pest Management Science. 75(5):1435-1442. https://doi.org/10.1002/ps.5265.
DOI: https://doi.org/10.1002/ps.5265

Interpretive Summary: Insecticide application is an essential component of insect control in the California almond and pistachio industry. Excellent insect control is essential because tree nuts are valuable (estimated farm gate value exceeding $7 billion), and improving nut quality is an ongoing challenge that must be met to ensure continued demand. The acreage planted in tree nuts has increased dramatically over the past decade, but the purchase of spray rigs to manage these acres has lagged behind. Over the past 6 years, the temperature during the growing season has increased, and this change has been favorable to a major pest of tree nuts, the navel orangeworm. The research presented in this paper focuses on describing the insecticide coverage produced by ground and air application for control of this pest. Several consistent patterns were identified: ground applications fail first in the upper canopy of the tree, and air applications fail first in the lower canopy. In the best ground applications coverage was consistently high throughout the canopy; this was not observed for application by air. Uniform coverage helps ensure that all nuts are protected by insecticide while uneven coverage results in gaps that can lead to increased damage. The reduced coverage in the lower canopy may not be important if most nuts are growing in the upper canopy, but if the nuts are evenly distributed gaps in coverage are unacceptable. We also investigated the effect of water volume and speed on ground application. Using a volume of 200 gallons per acre provided better coverage than a volume of 100 gallons per acre, and coverage in the upper canopy failed when the speed exceeded 2 miles per hour. However, there are tradeoffs associated with the high volume of water; application will take longer because more stops will be necessary to refill the spray tank. Application in the field requires balancing several factors including the time to complete a spray, therefore lower volumes may be used, but applicators must be made aware that they are sacrificing coverage. Finally, insecticides may be applied at several rates, and we investigated the effect of both minimum and maximum rate on contact toxicity. Toxicity in our assay system was highest when the maximum label rate was used. Growers facing high insect pressure should get the best control using the maximum label rate of insecticide applied at 2 miles per hour.

Technical Abstract: In California almond and pistachio orchards, navel orangeworm, Amyelois transitella Walker (Lepidoptera: Pyralidae), is the primary insect pest and is also a serious pest of walnuts, figs, and pomegranates. Amyelois transitella damage to tree nuts has increased since 2012 due to a change in the landscape caused by increased host availability (>1,241,000 ac of almonds and >660,000 ac of pistachios and walnuts) combined with increased heat unit accumulation during the growing season, in addition to increased crop value. Currently there is no standard to assess application efficacy, other than the economic damage assessed by the processor, and our purpose in this paper was to gather the information necessary to develop objective standards in order to improve control. Our focus was on describing the insecticide coverage produced by ground and air application. Several consistent patterns were identified: ground applications fail first in the upper canopy of the tree and air applications fail first in the lower canopy. The decline in air application was logarithmic. In the best ground applications coverage was consistently high throughout the canopy; this was not observed for application by air. The reduced coverage in the lower canopy may not be important if most nuts are growing in the upper canopy, but if the nuts are evenly distributed gaps in coverage are unacceptable. We also investigated the effect of water volume and speed on ground application. Using a volume of 200 gallons per acre provided better coverage than a volume of 100 gallons per acre, and coverage in the upper canopy failed when the speed exceeded 2 miles per hour. Finally, insecticides may be applied at several rates, and we investigated the effect of both minimum intermediate and maximum rates on contact toxicity. Toxicity in our assay system was highest when the maximum label rate was used. When this information is combined with our data on insecticide distribution and loss we believe that the best coverage will be attained using the maximum label rate at 2 miles per hour.