Vetiver Grass: a Natural Barrier to Protect Against Organophosphate Pesticides from Cabbage Fields

Abstract

The absorption of organophosphate pesticides by cabbage and vetiver grass (Vetiveria zizanioides Nash.) was investigated at Khunkong Watershed Research Station, Chiang Mai, between January and May 2003. The experimental plots with 12 cabbages per square meter and rows of vetiver grass about 0.5 m down slope below the plots were established on an area of about 30% slope. Two samples per plot of cabbage, vetiver grass and soil, were collected, twice a month at 15 days after application and at one day before next application, except the first collection, which was carried out right after the initial spray. Except methamidophos, the results showed that all the test compounds accumulated in cabbage, which was not found in any samples because of its low persistence. The investigation of test compounds in cabbage during the 139 day experiment showed that the average uptake rate was ethoprophos (0.78 mg/g lipid weight.day) < methyl-parathion (0.93 µg/g lipid weight.day) < methidathion (1.86 µg/g lipid weight.day) < EPN (7.7 µg/g lipid weight.day) and the concentration factor (CF = CB/CS, g organic carbon /g lipid weight) order was similar as ethoprophos (log CF = 0.54) was < methidathion (log CF = 0.98 ) < methyl-parathion (log CF = 1.18) < EPN (log CF = 2.17). The concentration factors of the test compounds were highly related to their hydrophobicity (log Kow) by log CF = 1.0696 log K_ow – 2.2347, r² = 0.87. In vetiver grass, only EPN was found in root samples with the absorbed concentration increasing with exposure time and in soil, EPN was the only compound which was found only in samples collected from the areas above vetiver grass rows. This result indicated that vetiver grass might be used to absorb organophosphate pesticides applied in agricultural areas.