Toxicity and residues of endosulfan isomers.

The continued availability of endosulfan is desirable for the production of cotton, and various other crops, because of its particular suitability for use in IPM and resistance management strategies. However, ongoing residue problems threaten the availability of the insecticide. Data described here suggest a beta-enriched insecticide is worthy of investigation as a useful alternative organochloride insecticide, having the advantages of reduced environmental and health concerns. The alpha-isomer of endosulfan is an order of magnitude more volatile than the beta-isomer, which is reflected in its initial rapid disappearance in the field. Approximately 70% of endosulfan is lost within 2 d of application due to volatilization. Based on volatilization kinetics, the volatilization of a beta-endosulfan formulation would be less than 5% that of commercial endosulfan. However, while it has been established that endosulfan vapor does lead to contamination of the riverine environment, this contamination is below residue levels recorded in rivers during the cotton growing season and, as such, reducing the volatility of the insecticide will only partially alleviate residue problems. Initial field trial results suggest that beta-endosulfan insecticide can achieve equivalent efficacy to commercial endosulfan at half the recommended label application rate; presumably this is a reflection of its containment on site in comparison to the more volatile commercial mix of isomers. An insecticide composed primarily of the beta-isomer would have reduced volatility and equivalent efficacy at lower application rates compared to the commercial mix of isomers, reducing offsite endosulfan residues. An important advantage of a beta-enriched insecticide would be its potential to minimize endosulfan residues in locally grown production animals. The predominant endosulfan residue in animal fat is endosulfate, accumulated after the animal ingests the metabolite while grazing on pastures contaminated by endosulfan spray drift. As the beta-isomer is oxidized on the surface of plants and by microbes at much lower rates than the alpha-isomer, endosulfate levels would be lower as a result of a contamination event with a beta-endosulfan-based insecticide compared to the commercial mix. Finally, acute toxicity against mammals of the alpha-isomer is more than three times that of the beta-isomer, and the neurotoxicity of the insecticide has been attributed to the alpha-isomer. Therefore, a beta-enriched insecticide will be less acutely and chronically toxic to agricultural workers than the commercially available insecticide. In conclusion, these properties suggest that the alpha-isomer contributes more significantly to the residue problems associated with the insecticide than the beta-isomer and that the use of a beta-isomer-based insecticide would reduce residue problems yet retain the advantages to IPM and resistant management strategies unique to the current endosulfan formulation.