Sprayers to reduce spray volumes in mature citrus trees

Conventional high volume pesticide spraying in citrus crops with oscillatin g boom sprayers results in low levels of pesticide retention on trees and h igh levels of off- target losses. This study was conducted to establish whe ther lower volume pesticide spraying with an air-assisted low-profile spray er and air-assisted sprayers fitted with tower air conveyors (air-towers) c ould replace conventional high volume pesticide spraying. The spraying effi cacy 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 depo sits on citrus leaves and fruit and off-target losses (canopy run-off) by u sing a fluorescent dye tracer technique. Biological efficacy was determined by assessing the control of two insect pests of citrus being California re d scale (Aonidiella aurantii Maskell) and Citrus mealybug (Planococcus citr i Risso) using Supracide 400 (methidathion). The Barlow tower sprayer opera ting 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 00 0 l ha (- 1). The Hardi low-profile air-blast sprayer produced significantl y lower spray deposits on both leaves and fruit as the height zone increase d in the trees. The Silvan air-shear tower sprayer produced better distribu tion of spray than the low-profile air-blast sprayer through the height zon es in the tree on both leaves and fruit. The percentage of leaf retention o f spray volume increased with decreasing application volume from 14% for th e oscillating boom at 10 000 1 ha (- 1) to 59% for the Silvan tower at 500 l ha (- 1). Canopy spray run-off increased with increasing volume from 2% f or the Silvan tower at 500 and 1000 l ha (- 1) and the Barlow tower at 1500 1 ha (- 1) to 28% for the oscillating boom at 10 000 1 ha (- 1). The biolo gical 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 low er spray volumes in crops such as citrus. (C) 1999 Elsevier Science Ltd. Al l rights reserved.