Environmental Fate of Rice Pesticides, 1999

Knowledge of what happens to rice pesticides in the environment is crucial to decisions affecting their use and regulation.  The guiding objectives of this ongoing project are to investigate the factors governing the movement of rice pesticides; to estimate the relative importance of such factors; and to apply research results toward meeting regulatory requirements and improved management practices.

Shark®, Icon® Studies Concluded

A three-year investigation of carfentrazone-ethyl (Shark®) in California rice fields has been concluded.

Negligible amounts of the herbicide dissipated from treated fields through volatilization, soil adsorption or degradation by sunlight.  Instead, it reacts rapidly with field water to produce an acid that is stable, water-soluble and herbicidal.   This acid breaks down to some extent, but most of it moves laterally off-field in subsurface water flow.   Dilution eventually makes it undetectable, suggesting the need for untreated buffer strips between fields.  Off-field movement is undesirable.

A four-year study of the promising new rice water weevil insecticide fipronil (Icon®) also concluded in 1999.  This insecticide is under review for registration.  The California Department of Fish and Game is concerned about a toxic and persistent breakdown product formed in sunlight.  Environmental toxicologists found that this byproduct degrades in sunlight to low levels of several less persistent compounds.  Another major breakdown product, a sulfonate, has also been detected but is considered harmless.   While more persistent than fipronil, researchers conclude, these primary byproducts are not permanent and may be tolerated at the low concentrations existing in rice fields.

Flooding Study

Flooded rice soils quickly become anaerobic — that is, they contain relatively little oxygen.  With current trends toward longer flooding periods, there is some concern that these conditions could promote a type of pesticide breakdown quite different from those in previous, more oxidizing aquatic environments.  Anaerobic soil and soil water utilize natural sulfides and quinones to dechlorinate chlorine-containing pesticides, such as carfentrazone acid, to nontoxic products.  However, at environmental concentrations, the reaction may be slow compared to other pesticide dissipation routes.

Toxicologists conducted experiments to model the conditions required for anaerobic reactions of typical rice pesticides and to clarify how photoreduction occurs under such conditions.   They found that the presence of rice straw in field water significantly speeds up the breakdown of carfentrazone acid, probably by providing natural reducing agents.  Microbes in flooded soils already are known to carry out this reaction, but this is the first indication that sunlight contributes to the process.

Alternative Algicide

At least one class of nonpersistent algicide, the quaternary ammonium surfactants, might satisfactorily replace the copper sulfate currently used to treat rice fields.   However, a more concerted effort, primarily by plant biologists, will be required in order to establish an adequate algicide screening system.  Such a system could involve the USDA Aquatic Weed Research Program at UC Davis.

Project Leader and Principal Investigators

Donald G. Crosby, professor emeritus, Dept. of Environmental Toxicology, UC Davis

Ken Ngim, graduate research assistant, Dept. of Envoronmental Toxicology, UC Davis