RESUMEN
The relative field hazards of insecticides to honeybees have been estimated by considering intrinsic toxicity levels and field application rates. This approach is extended here to a consideration of buffer zones downwind of sprayed areas by estimating the distance at which bees would encounter an LD(50) dose from spray drift. 'LD(50) distances' are determined for both ground and aerial spraying of ground crops in Britain using published data on spray deposition under various weather conditions. For ground spraying at low wind speeds (< or =3 m s(-1)), this zone of risk is up to 5 m for the great majority of compounds. Aerial spraying in unstable atmospheric conditions appears to produce drift deposits of about the same order of magnitude as from ground spraying at wind speeds of about 4 m s(-1), with maximum LD(50) distances of < or =40 m for chlorpyrifos, fenitrothion and triazophos. For aerial spraying in stable atmospheric conditions these distances would be much greater. Pieris brassicae larvae are contrasted with honeybees in their relative sensitivities to insecticides and consequent LD(50) distances.
RESUMEN
Two series of drift deposition measurements were carried out at different wind speeds using sodium fluorescein as a tracer dye sprayed over a grass field 6 m upwind of a hedge. Efficient receptors were placed below and above hedge height (1.6 m) between 1 and 20 m downwind from the sprayed area. Receptors below hedge height reflected a sudden decrease in deposition immediately behind the hedge, followed by a gradual increase again up to 15 m, i.e., nine times the height of the hedge. The sheltering effect of a hedge from the biological impact of spray drift was studied by bioassays using tomato and Lychnis flos-cuculi plants for the herbicide MCPA and young Pieris brassicae larvae for the insecticide cypermethrin. These demonstrated that the protection afforded to sensitive species in strong winds may be quite limited, and severe damage may be inflicted over considerable distances. In intermediate cases, a protected zone is followed by a zone of further significant damage before drift depositions cease to have further effect. In some cases, the sheltered zone may extend to a distance where drift deposition, even in the absence of a hedge, has minimal effect.