THE EFFECTS OF HEDGES ON SPRAY DEPOSITION AND ON THE BIOLOGICAL IMPACT OF PESTICIDE SPRAY DRIFT

Two series of drift deposition measurements were carried out at differ ent wind speeds using sodium fluorescein as a tracer dye sprayed over a grass field 6 m upwind of a hedge. Efficient receptors were placed b elow and above hedge height (1.6 m) between 1 and 20 m downwind from t he sprayed area. Receptors below hedge height reflected a sudden decre ase 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. T he sheltering effect of a hedge from the biological impact of spray dr ift was studied by bioassays using tomato and Lychnis flos-cuculi plan ts for the herbicide MCPA and young Pieris brassicae larvae for the in secticide cypermethrin. These demonstrated that the protection afforde d to sensitive species in strong winds may be quite limited, and sever e 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 deposit ion, even in the absence of a hedge, has minimal effect. (C) 1994 Acad emic Press, Inc.