Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 8, 1998
Publication Date: N/A
Interpretive Summary: U.S. State Department has recommended using herbicides to control the cultivation of cocaine-producing (coca) plants. The herbicides recommended to control coca plants are tebuthiuron and hexazinone at 3 and 6 pounds per acre. No information, however, is available on the herbicide residue levels that may occur in the leaves of plants treated with these herbicides at the recommended rate. Without these data, it is difficult to predict the potential health risk to individuals who may chew leaves from plants treated with these herbicides. In this study, field-grown coca plants were treated with tebuthiuron and hexazinone and the leaf tissue was harvested at the first signs of herbicide injury and when the leaves began to drop from the plants. The study demonstrated that the maximum amount of herbicide residue accumulated in the leaves was 7 micrograms per gram dry weight of leaf tissue. Based on the highest estimates of daily consumption of coca leaves by South American peoples, and the EPA's conservative estimates of safe exposure levels from a daily intake of these herbicides by ingestion from water over the course of one's lifetime, and if all the herbicide was extracted in the chewing process, a 45.4 kg (100 lb.) person could chew 71.4 or 28.6 kg per day of leaves containing 7 micrograms per gram of tebuthiuron or hexazinone, respectively, without any anticipated health risk due to the presence of the residues. Therefore, applications of tebuthiuron or hexazinone at recommended rates to control coca would not pose a health risk to individuals chewing the leaves. This information will be useful to those agencies or governments interested in controlling coca with these herbicides.
This study was conducted to determine the residue levels in leaves of Erythroxylum coca var. coca plants treated with soil applications of tebuthiuron and hexazinone at 3.36 and 6.72 kg active ingredient per hectare to determine the potential for human exposure to these residues from macerating the leaves. Field-grown plants were treated with a commercial formulation of tebuthiuron or hexazinone and leaves harvested at the first indication of herbicide injury (i.e. chlorosis and/or necrosis) and at the onset of leaf abscission. Herbicide residues in leaf tissue were determined using HPLC. Herbicide residues were detected in leaf samples from both harvests of all plants treated with tebuthiuron or hexazinone. At 3.36 kg per hectare, the recommended rate of tebuthiuron and hexazinone for control of coca plants, herbicide residue levels in the dried leaf tissue were less than 2 ug per gram when leaves were harvested at the either the first sign of herbicide injury or at the onset of leaf drop. The highest residue levels detected were 5.90 ug per gram for tebuthiuron and 7.17 ug per gram hexazinone in leaf tissue harvested at the onset of leaf drop from plants treated with 6.72 kg per hectare of herbicide. Based on published toxicity data and estimates of leaf consumption, the herbicide residues in leaves of E. coca var coca plants treated with tebuthiuron or hexazinone at twice the recommended control rates or less would have a negligible contribution to the health risks of consumers of the leaves.