Environmental Fate of Rice Pesticides, 2014

 

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 2014 on three compounds: the insecticide clothianidin (Belay®), the herbicide imazosulfuron (League®), and the herbicide benzobicyclon (Butte®).

Clothianidin studies

Clothianidin, marketed as Belay®, is a neonicotinoid insecticide recently 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 demonstrated that volatilization into the air will not be a significant route for dissipation of this compound. In 2014 research showed that clothianidin undergoes rapid photolytic degradation under simulated California summer field conditions. Observed degradation rates were faster in pure Sacramento River water than in rice field water.

Results suggest that photodegradation of clothianidin is primarily controlled by turbidity and light attenuation. Given clothianidin’s moderate solubility and relatively low affinity for rice field soils, photodegradation is expected to dominate dissipation when applied to a flooded field.

In order to fully characterize this compound’s persistence and potential for offsite transport in field tailwater, further investigation will now focus on biological degradation of residues in field soils.

Imazosulfuron studies

Resistance of several weeds to current herbicides is a prevalent issue affecting California rice production. Herbicides with alternative modes of action are being sought to battle these weeds.

Imazosulfuron, marketed as League®, is a sulfonylurea herbicide registered for pre- and post-emergent use on rice. Sulfonylurea herbicides are valued for their broad-spectrum effectiveness, low mammalian toxicity, and good crop selectivity.

Imazosulfuron has high water solubility. Previous work 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.

The goal of 2014 research was to determine the photolysis rate of imazosulfuron under simulated California rice field conditions and to characterize the influence of several environmental factors on degradation, including natural organic matter, temperature, and high salinity.

League® degraded relatively rapidly under simulated California growing conditions. Degradation products were observed but did not accumulate because of their susceptibility to photoreaction.

Photolysis proceeded fastest in pure laboratory water. Field relevant differences in salinity and temperature did not influence photolytic rates significantly. Future studies will examine microbial degradation.

Benzobicyclon studies

Resistance of several weeds to current herbicides is a prevalent issue affecting California rice production. Herbicides with alternative modes of action such as benzobicyclon, marketed as Butte®, are being sought to battle these weeds.

Butte® inhibits an enzyme involved in the biosynthesis of chlorophyll. This compound is a “pro-herbicide” that reacts with water to release the active herbicide, benzobicyclon hydrolysate (BH), into water, plants, and soil.

Previous experiments suggested that volatilization does not play a significant role in the dissipation of Butte® under field conditions. Loss of Butte® through hydrolytic degradation appears to be a more significant contributor. Thus, characterization of hydrolysis rates and half-lives of Butte® were needed to better understand its fate under field conditions.

Preliminary studies suggest that Butte® rapidly degrades under aqueous conditions, regardless of pH. Hydrolysis appears to be a significant, if not dominant, dissipation process. With evidence of rapid conversion of Butte® to BH, future studies will investigate the dissipation process and transformation behavior of BH under California rice field conditions.