Drift control and buffer zones for helicopter spraying of bracken (Pteridium aquilinum)

Application of asulam herbicide by helicopter, as a primary treatment of bracken, requires appropriate buffer zones to protect non-target plants, conservation areas and water catchments from spray drift. Drift from large-scale applications of asulam were monitored at two moorland sites where helicopters were fitted with Delavan RD ‘Raindrop’ drift-reducing, hollow-cone nozzles. Drift was measured on 10 m vertical ‘strings’ and on flat cards at intervals to 200 m downwind. Potted seedlings of Rumex acetosa, as a bioassay, were exposed over similar distances. At 200 m downwind, levels of drift were close to limits of detection by high performance liquid chromatography (HPLC). With wind speeds at the legal limit for helicopter spraying, total drift fallout (integrated to 1 km downwind) accounted for 0.61% of the applied asulam. In these conditions, fallout dropped below 1% of the applied dose rate approximately 35 m downwind of the sprayed area. In general terms, beyond 15 m from the sprayed areas, drift levels from the helicopters proved similar to those reported from tractor-operated boom-sprayers. Bioassay data correlated with asulam fallout. A zero-effect level of 100 g ha−1 after 7 weeks (2.25% of the dose for bracken eradication) suggested a buffer zone of 15 m which might be doubled to 30 m to allow for inaccurate flying, late shut-off, dosage errors and species with greater sensitivity than Rumex. On the basis of this work, and also to protect private water abstractions, the Environment Agency approved a 50 m buffer zone for helicopters fitted exclusively with RD Raindrop nozzles. © 2000 Elsevier Science B.V. All rights reserved.