Project Leader and Principal UC Investigators
Donald Crosby, professor, Department of Environmental Toxicology, University of California, Davis.
What happens to rice pesticides after they've been
applied to fields is a topic researchers have explored for many years
Growers and government agencies alike need to understand
how environmental factors, especially sunlight, break down pesticides
to make informed management and regulatory decisions
New testing methods
Earlier research showed that hydroxyl radicals, which occur naturally when ultraviolet light impinges upon field water, are an important ingredient in the breakdown of pesticides. During 1989 the environmental toxicologists on this project began developing a new testing method that simulates what happens to pesticides when sunlight hits field water.
Basically, the technique measures how quickly the various compounds used in rice culture degrade in the presence of a known quantity of hydroxyl radicals. If successful, the technique should eventually enable researchers to predict the rate of pesticide degradation in rice fields.
The rate at which pesticides break down is directly related to the intensity of sunlight that reaches the field. That amount changes from day to day, so the researchers developed another simple, inexpensive chemical test that measures the total amount of ultraviolet light hitting a field over a period of time.
One pesticide of particular interest is an insect growth regulator called Dimilin.® This compound is used to control the Rice Water Weevil, which was more prevalent during the 1989 growing season than any time in recent memory (see section on invertebrates).
Dimilin® is difficult to track in the environment because of its low application rate and peculiar physical properties. With a new method of analysis, however, the researchers discovered that while the chemical does not break down well in pure water, it does decompose rapidly in field-like conditions.
Research on another pesticide of interest, Londax,® confirmed earlier observations that this compound dissipates rapidly. Its half-life is between three and four days. The byproducts of Londax,® identified in separate laboratory tests, were undetectable in field water after two days.