Sprayers to reduce spray volumes in mature citrus trees

Abstract

Conventional high volume pesticide spraying in citrus crops with oscillating boom sprayers results in low levels of pesticide retention on trees and high levels of off- target losses. This study was conducted to establish whether lower volume pesticide spraying with an air-assisted low-profile sprayer and air-assisted sprayers fitted with tower air conveyors (air-towers) could replace conventional high volume pesticide spraying. The spraying efficacy of an oscillating boom sprayer applying 10 000 l ha−1 was compared with three different air-assisted sprayers applying volumes of between 500 and 6000 l ha−1. Spray efficacy was determined by measuring spray deposits on citrus leaves and fruit and off-target losses (canopy run-off) by using a fluorescent dye tracer technique. Biological efficacy was determined by assessing the control of two insect pests of citrus being California red scale (Aonidiella aurantii Maskell) and Citrus mealybug (Planococcus citri Risso) using Supracide 400 (methidathion). The Barlow tower sprayer operating at 6000 l ha−1 produced equal spray deposits on leaves and higher spray deposits on fruit compared to the oscillating boom operating at 10 000 l ha−1. The Hardi low-profile air-blast sprayer produced significantly lower spray deposits on both leaves and fruit as the height zone increased in the trees. The Silvan air-shear tower sprayer produced better distribution of spray than the low-profile air-blast sprayer through the height zones in the tree on both leaves and fruit. The percentage of leaf retention of spray volume increased with decreasing application volume from 14% for the oscillating boom at 10 000 l ha−1 to 59% for the Silvan tower at 500 l ha−1. Canopy spray run-off increased with increasing volume from 2% for the Silvan tower at 500 and 1000 l ha−1 and the Barlow tower at 1500 l ha−1 to 28% for the oscillating boom at 10 000 l ha−1. The biological efficacy of the oscillating boom sprayer on red scale was matched by the Barlow tower at 6000 l ha−1 and the Silvan tower at 500 l ha−1. The Barlow tower at 6000 l ha−1 produced a greater reduction in fruit infected with mealybug in the calyx than the oscillating boom sprayer at 10 000 l ha−1 and all other sprayer treatments. The reduction in dose rate of insecticide produced by using lower spray volumes with registered rates based on volume of spray volume resulted in pests not being controlled in some of the lower volume treatments. This clearly illustrates that changes to certain pesticide registrations will be necessary in the adoption of lower spray volumes in crops such as citrus.