|The Environmental Fate of
Pesticides Important to
Rice Culture - 96
Project Leader and Principal UC Investigators
Donald G. Crosby, professor, Department of Environmental Toxicology, UC Davis
Ken Ngim, graduate research assistant, Department of Environmental Toxicology, UC Davis
Dan Stewart, undergraduate lab assistant, Department of Environmental Toxicology, UC Davis
Amy Witter, graduate research assistant, Facility for Advanced Instrumentation, UC Davis
|Knowledge of what happens to rice pesticides in soil and water
is critical to decisions made about their regulation. Environmental toxicologists working
in this area of ongoing research are continually searching for improved analytical
methods; assessing factors affecting the practical use of rice pesticides; and applying
their results toward meeting regulatory requirements and improved management.
A case in point concerns the experimental insecticide fipronil. This promising chemical slated for use against rice water weevil undergoes rapid photodegradation into a product reported to be nondegradable in the environment. Consequently, the U.S. Environmental Protection Agency expressed concern about registering the chemical for use on paddy rice. In carefully constructed laboratory tests, however, toxicologists found those reports to be erroneous. The photoproduct is persistent but degradable, with a half-life of 5.6 days under controlled laboratory conditions and as short as one day in the field. This information should substantially assist registration of fipronil for use on rice.
In related fipronil research, toxicologists report:
Bluestone Alternative Suggested
Toxicologists have previously reported on the relatively high levels of copper in rice fields treated repeatedly with copper sulfate or "bluestone". Growers use the material to control algae and tadpole shrimp.
Researchers experimented with several chemical treatments to mobilize and potentially remove excess copper from the soil. However, bringing these agents into the necessary intimate contact with rice soil proved difficult. Further, the chemical fate of copper sulfate in field water is much more complicated than first thought. Most of the copper is bound to natural organic matter in the water and soil. Analysis of tailwater reveals that almost no copper leaves the field, so toxic level residues will continue to accumulate.
Because of this finding, toxicologists suggest that the water-soluble and biodegradable aquatic herbicide endothal may be worth reconsideration as a replacement for bluestone. Although endothal has been applied for weed control in California rice as recently as 1988, the amounts required risk toxicity to rice plants. The levels required for control of algae, however, appear to be lower.