Environmental Fate of Rice Pesticides - 92 



Home.gif (3162 bytes)

Next.gif (3180 bytes)

Back.gif (3162 bytes)

Project Leader and Principal UC Investigators

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

J.M. McFarland, staff research associate

S.A. Mabury, graduate research assistant

C. Schrenk, postgraduate research chemist

U.B. Cheah, visiting scientist (Malaysia)


What happens to pesticides used in the cultivation of rice is another ongoing area of critical research. Environmental toxicologists continue their search for ways to identify, investigate and model factors that affect the movement and chemical fate of these pesticides; estimate the relative importance of these factors to the practical use of rice pesticides; and apply their research results toward meeting regulatory requirements and improved management practices.

Analytical techniques

In previous research, these scientists had identified a direct relationship between the rate at which pesticides degrade in the field and their reactivity to hydroxyl radicals, which are created naturally in field water by sunlight. Their next step was to develop a satisfactory method for quantifying this relationship. They have done so with a technique involving high-pressure liquid chromatography. However, they discovered that the dissolved copper derived from-the "bluestone" that growers sometimes used to control algae and tadpole shrimp may influence results obtained from this method. Consequently, they encourage further studies of the relationship between timing and concentration of copper sulfate applications to pesticide persistence.

Facet® studies

Ongoing studies of Facet® (quinclorac) continue to show that this herbicide moves rapidly through rice field water. This phenomenon poses a potential barrier to Facet®'s eventual registration since this characteristic increases the likelihood that it will escape from flooded fields.

Tests on clay soil and sandy loam in field conditions and at UC Davis showed that the herbicide's rate of movement is primarily dependent upon the rate at which field water flows over the soil. Researchers found that Facet® would move across checks in virtually any direction the water did. In these experiments, half the herbicide escaped in less than 50 hours under field conditions and in just three to six hours under laboratory conditions.

Researchers also say their experiments indicate that Facet@ degrades slowly by the action of sunlight into small nonpersistent fragments. A portion of the herbicide breaks down completely into harmless carbon diode.

Rice toxicity explored

Japanese researchers have raised concerns that under certain flooded conditions, Bolero® (thiobencarb) will degrade into byproducts that are toxic to rice seedlings. There was some speculation that this phenomenon might be behind recent unexplained die off in parts of some California rice fields.

Researchers took soil samples from three major California rice soil typesWillows clay, San Joaquin loam and Stockton clay adobe. After extensive tests in their UC Davis laboratory, the toxicologists found no evidence that Bolero® had broken. down into toxic byproducts in these soils and concluded, therefore, that it was unlikely this compound was responsible for the recent diebacks.

Dimilin® studies

Researchers also continued experiments on the insecticide Dimilin® (diflubenzuron), which if. approved for registration could be used to' control rice water weevil. This compound's mode of action is to kill insects and other arthropods by inhibiting their skeletal development.

Although Dimilin® breaks down readily in sunlight, its byproducts are both very-persistent and very toxic to aquatic invertebrates. Researchers recommend monitoring Dimilin® treated field water with bioassays in further studies.

Experimental chemicals

Researchers also continued their . examination of two new experimental herbicides from a Japanese company.

The first, KI-2023, is characterized by low volatility and stability toward sunlight. It nonetheless dissipated rapidly through subsurface water and moved quickly down into impermeable soil.

The other compound, KI-6127, also moved with subsurface water but dissipated primarily from volatilization and sunlight.

In field tests both compounds displayed very short half lives ranging from five hours to 18 hours. Neither compound was detected in water flowing into an adjacent plot.

Microlayer explored

In another aspect of this research project, scientists further examined the nature of a thin "microlayer" of organic liquid on the surface of field water. This part of the water column is comprised primarily of fats from decaying plants and animals. . Concentrations of pesticides had previously been observed to be higher, these than in adjacent water.

In a comparison of Furadan® and Bolero®, the researchers last year found initial pesticide levels very high compared to underlying water. Although pesticide concentrations, eventually declined in the water column, they persisted in the microlayer.

Literature review

Finally, the scientists conducted a literature review-on research into the movement and fate of the six most commonly used rice pesticides Londax®, Ordram®, Bolero®,. Furadan®, MCPA and methyl parathion  and their potential for reducing residues in tailwater through artificial wetlands. Copies of the review are available from the Colusa office of the Soil Conservation Service and from Agronomy Extension at UC Davis.

Home.gif (3162 bytes)Next.gif (3180 bytes)Back.gif (3162 bytes)