Environmental Fate of Rice Pesticides-90



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Project Leader and Principal UC Investigators

D.G. Crosby, Dept. of Environmental Toxicology, UC Davis

D.B. Alger, visiting scholar

K.L. Armbrust, graduate research assistant

S.A. Mabury, graduate research assistant

C.R. Mourer, staff research associate

J.R. Gever, undergraduate assistant


With carbofuran under increasing scrutiny, interest in how the rice industry will deal with rice water weevil is high. Diflubenzuron (Dimilin®) has been proposed as a replacement, but no information had been available until this time on the fate or persistence of this insect growth regulator under field conditions.

The researchers were able to determine that Dimilin® has a half-life of 27 hours in rice field water and can, therefore, be classed as a non-persistent pesticide when applied at the usual rate of 0.25 pounds per acre. Their results also suggest the insecticide would be more effective against RWW if applied in late afternoon or early morning to avoid chemical breakdown by sunlight.

Tests on the persistence of bensulfuron-methyl, the active ingredient in Londax®, were consistent with tests a year ago. The half-life of Londax® in rice field water was determined to be 48 hours, due either to sunlight degradation or to binding with sediment.

In another test, however, Londax® persisted much longer, with a half-life of five to seven days. Even after 42 days, 10 ppb (parts per billion) bensulfuron-methyl remained in the soil.

Using a newly developed method, the researchers analyzed the relative reactivity toward oxidation of nine pesticides in typical field water conditions and in the laboratory. Carbaryl and carbofuran were most reactive, propanil and MCPA intermediate, and atrazine and hexazinone were least reactive. The results indicate that the more reactive a given pesticide is, the less its persistence will be in the environment. Other factors, such as volatilization, can come into play.

Field dissipation of bensulfuron-methyl in sediment and water.

In response to state concerns over potential toxic effects of carbofuran, methylparathion, and malathion on larval fish and the zooplankton that serve as their primary food source, the researchers developed a suitable gas chromatographic method to analyze residues as low as 0.01 ppb (10 parts per trillion). This will allow research on the unexplored persistence of such pesticides at extremely low concentrations.

A chemical measure of solar ultraviolet light in field water was also developed, information that will greatly enhance the ability to predict pesticide persistence.

Publication manuscripts on previous work on the analysis and environmental fate of quinclorac (Facet) herbicide in rice fields have been prepared and provided to BASF Corporation to support their registration for Facet on California rice.

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