The Environmental Fate of
Pesticides Important to

Rice Culture - 96



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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:

  • Development of a "laboratory microcosm" containing field water and soil for more rapid evaluation of factors affecting the fate and persistence of fipronil.
  • A significant part of fipronil loss in flooded rice fields appears to be due to volatilization - contrary to other accounts in scientific literature.
  • A comparison of granular, dry flowable and water-soluble formulations of fipronil showed the water-soluble formulation to be least persistent and produced the least photoproduct.
  • Field-scale applications of water-soluble fipronil at the three-leaf stage resulted in a half-life of just 24 hours in water, with no detectable residue within only 2.5 days, a "very favorable" result.

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.

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