Sprayers to reduce spray volumes in mature citrus trees.
It was investigated 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 (OBS) applying 10 000 litres ha-1 was compared with 3 different air-assisted sprayers in a citrus orchard in Queensland, Australia. Spray efficacy was determined by measuring spray deposits on citrus leaves and fruit and off-target losses (canopy run-off) using a fluorescent dye tracer. Biological efficacy was determined by assessing Aonidiella aurantii and Planococcus citri control using Supracide 400 (methidathion). The Barlow tower sprayer operating at 6000 litres ha-1 produced equal spray deposits on leaves and higher spray deposits on fruit, compared to the OBS at 10 000 litres 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 OBS at 10 000 litres ha-1 to 59% for the Silvan tower at 500 litres ha-1. Canopy spray run-off increased with increasing volume from 2% for the Silvan tower at 500 and 1000 litres ha-1 and the Barlow tower at 1500 litres ha-1 to 28% for the OBS at 10 000 litres ha-1. The biological efficacy of the OBS on A. aurantii was matched by the Barlow tower at 6000 litres ha-1 and the Silvan tower at 500 litres ha-1. The Barlow tower at 6000 litres ha-1 produced a greater reduction in fruit infected with P. citri in the calyx than the OBS at 10 000 litres ha-1 and all other sprayer treatments. The reduction in insecticide dose rate by using lower spray volumes with registered rates based on volume of spray volume resulted in the pests not being controlled in some of the lower volume treatments.