Pesticide interception by emergent aquatic macrophytes: Potential to mitigate spray-drift input in agricultural streams

An increasing amount of research has focused on alternate methods to mitigate pesticide exposure in agricultural surface waters. The interception of spray-drift-derived azinphos-methyl (AZP) by emergent aquatic macrophytes was investigated in a tributary of the Lourens River, South Africa. The stream was dominated (80% coverage) by three species of emergent aquatic macrophytes; Juncus capensis, Fuirena hirsuta and Pycreus sp. During an application of AZP, drift deposition was determined on the surface of the vegetation, on the surface of the exposed channel of the stream and beneath J. capensis and F. hirsuta by means of drift collectors (n = 6). Drift deposition on the surface of the vegetation (1.5 ± 0.3 mg/m2) was well predicted by 90th percentile basic drift values (1.3 mg/m2), indicating that the sampling devices resulted in an accurate measurement of drift deposition. Drift deposition on the surface of the exposed channel (1.0 ± 0.3 mg/m2) was lower than measured on the vegetation surface indicating a positive shielding effect by the emergent plants. Drift deposition beneath J. capensis was significantly lower (p = 0.005) than on the vegetation surface and the exposed channel (p = 0.048), indicating highly effective interception of AZP. A simple formula was generated to make predictions of drift deposition reductions based on different percentage macrophyte coverage. Predictions showed that 50% macrophyte coverage in combination with a 5 m buffer strip resulted in as large a reduction in drift deposition as the combination of a 10 m buffer strip with 0% macrophyte coverage. Results thus indicate that emergent aquatic vegetation may be as effective a mitigation strategy for reducing spray-drift induced pesticide input as increasing the width of the no spraying buffer zone.