Environmental Fate of Rice Pesticides - 2010
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Project Leader and Principal Investigators Ronald S. Tjeerdema, professor and chair, Dept. of Environmental Toxicology, UC Davis
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The overall goal of this continuing research project is to study how
pesticides important to rice culture dissipate under California rice field
conditions. These compounds break down in the environment through
interaction with soil, water, and air, as well as by degradation from
sunlight and microbial activity. Over the past decade the focus of research has shifted to different products of interest to the rice industry for weed and insect control. Early on the focus was on the delayed phytotoxicity syndrome caused by Bolero® (thiobencarb). More recently work focused on the mode of action and dissipation of Cerano® (clomazone). Characterization of the field dissipation properties of Granite®, Cerano®, and Trebon® (etofenprox) have now been completed. Research in 2011 will focus on the insecticide Poncho® and the herbicide imazosulfuron. Trebon® lab studiesTrebon® (etofenprox) is a pyrethroid pesticide under consideration for rice water weevil control. Previous research demonstrated the compound’s strong tendency to partition out of water and adhere to soil particles. Laboratory experiments in 2010 emphasized photodegradation under simulated rice field conditions.
Photolytic degradation rates, half lives, and pathways were analyzed on both flooded and air-dried soil surfaces. In summary, this insecticide is more efficiently degraded under flooded than under air-dried soil conditions (flooded soil half life 9.8 days; air-dried soil half life 17.6 days). Trebon® and its byproducts will most likely degrade by the action of light working in combination with soil microbes. Because of the lack of information on toxicity of breakdown byproducts, further study in this area is warranted. Cerano® lab studiesCerano® (clomazone) is a widely used herbicide for control of watergrass, barnyardgrass, and sprangletop in California rice fields. It was registered for use in California in 2002. Cerano® is characterized as strongly water soluble, weakly soil adhering, and minimally volatile. Previous laboratory experiments showed that Cerano® degrades rapidly under anaerobic conditions, with microbial degradation playing an important role in its environmental fate. Under aerobic conditions, Cerano® degraded more slowly. Recent research also indicates degradation of Cerano® may be influenced by soil organic matter. Laboratory research in 2010 focused on photolytic degradation rates, half lives, and pathways. In summary, Cerano® was characterized as “photo-recalcitrant” under simulated California rice field conditions. Thus, microbial degradation is the primary process governing Cerano®’s environmental fate.
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