Environmental Fate of Rice Pesticides, 2013

 

Project Leader

Ronald Tjeerdema, professor and chair, Dept. of Environmental Toxicology, UC Davis

The goal of this project is to characterize how pesticides important to rice culture dissipate under California rice field conditions. These chemical compounds break down in the environment through interaction with soil, water, and air, as well as by degradation from sunlight and microbial activity.

Research continued in 2013 on three compounds: the insecticide clothianidin, the herbicide imazosulfuron, and the herbicide benzobicyclon.

Clothianidin studies

Clothianidin, marketed as Belay®, is a neonicotinoid insecticide registered for foliar-spray and seed treatment applications for rice in Japan. It is currently being registered in California for pre- and post-flood aerial application to protect rice fields against the rice seed midge and rice water weevil.

Prior research showed that volatilization into the air will not be a significant route for dissipation of this compound. The objective of 2013 research was to complete characterization of clothianidin’s behavior in soil under California rice field conditions.

Laboratory experiments examined soils collected from rice fields in Davis, Biggs, and Richvale. Results indicate that clothianidin’s tendency to move into soil is low. Its capacity to do so decreases with the elevated temperatures typical of those on hot summer days in the Sacramento Valley. Under elevated temperatures, clothianidin remains primarily in the aqueous phase.

However, its ability to move from water to soil increases significantly in soils with higher concentrations of organic matter. Clothianidin has the potential to be mobile in soils, though surface degradation may prove to be a fundamental component in its dissipation in California rice fields. Thus, future research should focus on biological and photochemical processes, and the potential for offsite transport in tailwater.

Imazosulfuron studies

Imazosulfuron, marketed as League™, is a sulfonylurea herbicide of interest to California rice farmers because of its selectivity and effectiveness at low application rates. The U.S. Environmental Protection Agency registered imazosulfuron for commercial and residential use in 2010. The herbicide’s mode of action is to inhibit an enzyme critical to the production of amino acids necessary for cell generation.

Imazosulfuron is an herbicide with high water solubility. Previous work by project scientists suggests that once imazosulfuron is applied it will remain in field water because of its low potential for volatilization or soil sorption. Pesticides that partition into water generally have greater mobility in the environment and a higher potential to contaminate surface and ground water. Understanding the persistence of this compound in rice fields will allow for a better estimate of the potential for offsite movement.

Field water was collected for laboratory experiments from locations in Butte and Glenn counties. Half-lives for imazosulfuron were relatively short. However, statistically significant differences were observed between water sources, with degradation occurring faster in Butte County field water than in field water from Glenn County. Results suggest that photodegradation is an important process in the dissipation of imazosulfuron in a rice field. Future work will more thoroughly investigate how this herbicide breaks down with fully characterized water samples.

Benzobicyclon studies

The synthetic herbicide benzobicyclon could be an effective tool against sulfonylurea-resistant weeds such as the sedge Scirpus juncoides. Benzobicyclon has not been registered for use in California but has been used with great success in Japan. It kills weeds by inhibiting an enzyme involved in the biosynthesis of chlorophyll.

Research in 2013 sought to complete characterization work of this compound’s volatilization under California rice field conditions. Laboratory experiments showed that the herbicide’s ability to dissipate from rice field water through volatilization is minimal. This compound is considered to be a “pro-herbicide” that reacts with water to release the active herbicide, hydrolysate, into water, plants, and soil. Thus, future work will focus on benzobicyclon hydrolysate.