Diazinon transport through inter-row vegetative filter strips: micro-ecosystem modeling

The efficacy of inter-row vegetative filter strips (VFS) for controlling runoff of the commonly used organo-phosphate insecticide (diazinon) from dormant-sprayed orchards was investigated through development of physical (micro-ecosystem) models. The micro-ecosystem consisted of a pesticide sprayer, rainfall simulator and orchard floor model with and without VFS. Diazinon was sprayed at a rate of 2.8 kg/ha, 24 h prior to rainfall simulation. Rainfall, at an intensity of 50 mm/h and 44% of the natural rainfall energy, was simulated for 60 min. Experiments were conducted for 0, 50 and 100% VFS soil coverage. Diazinon concentrations in runoff, interflow and baseflow, and also in soil and vegetative samples were measured in order to quantify transport/adsorption processes. Total diazinon losses as a fraction of applied pesticide mass from the orchard floor following rainfall-runoff simulation were 8.6, 5.8 and 2.3%, respectively, for the 0, 50 and 100% VFS cover treatments. Diazinon runoff concentrations decreased with time during the rainfall simulation, but at a slower rate in the VFS treatments as compared to the bare soil treatment apparently due to washoff from the sod leaves. The principle mechanism of diazinon runoff control in VFS was diversion of runoff, the primary pesticide carrier, into interflow through the rootzone and mainly vertical infiltration (baseflow) such that the diazinon was trapped on the VFS surface and in its rootzone. In fact, 37 and 88% of the applied diazinon remained as residue in the VFS vegetative matter and rootzone for the 50 and 100% VFS treatments, respectively, following rainfall simulation. Results from the micro-ecosystem suggest that inter-row VFS should be effective in reducing diazinon runoff from dormant-sprayed orchards. These results are used to calibrate a field-applicable numerical model for development of pesticide runoff control strategies, or best management practices (BMP's).