Environmental Fate of Rice Pesticides, 2017


Project Leader

Ronald Tjeerdema, professor and associate dean, Dept. of Environmental Toxicology, College of Agricultural and Environmental Sciences, 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 in 2017 focused on three main areas: the active ingredient in the herbicide Butte® (benzobicyclon hydrolysate or “BH”); the insecticide Coragen® (chlorantranil­iprole); and modeling of the herbicide thiobencarb (Abolish® or Bolero®) leaving rice fields during drought years.


Recently approved for use in California rice fields by the U.S. Environmental Protection Agency (EPA), Butte® kills previously resistant weeds through the formation of the herbicide’s active ingredient, benzobicyclon hydrolysate. BH works through a new mode of action not previously used on California rice fields.

EPA has advised that a granular form of Butte® can be applied to 10% of California rice. Based on registration information and estimated physiochemical properties, EPA also recommends floodwater treated with Butte® be held in the field for 20 days prior to release into the Colusa Basin Drain (and ultimately the Sacramento River). The current Butte® label suggests a hold time of zero to five days. However, depending on when a field is drained, BH could be released into the Sacramento River before it is able to dissipate in the field.

Previous studies have experimentally characterized BH hydrolysis and behavior in soil, which indicate a significant portion of BH will remain in the aquatic fraction of a flooded rice field. Research in 2017 focused on photodegradation of BH in high-purity water, as well as filtered rice field water and Sacramento River water in both simulated and natural sunlight. Photolysis of Butte® in rice field water occurred rapidly and was enhanced by dissolved organic carbon.

Modeling of the 20-day water holding period with experimental data corresponded well with the EPA recommendation. Based on the model results, long term toxicity may be an issue for BH and its parent in rice field soil, as soil dissipation of both compounds is slow.


Coragen® was registered by the U.S. EPA for agricultural use in 2008 and by the California Environmental Protection Agency in 2016.

Chlorantraniliprole, the active ingredient in Coragen®, belongs to an emerging class of pesticides known as the anthranilic diamides. It has potent activity against rice water weevil larvae. Because of its specificity, chlorantraniliprole has lower toxicity than other currently used insecticides on nontarget species, including crayfish and pollinators. However, it exhibits high toxicity toward some aquatic invertebrate species. Thus, the environmental fate of chlorantraniliprole under California rice field conditions needs study to ensure water quality for both humans and endemic species in the environment.

Research in 2017 focused on characterizing the primary processes driving chlorantraniliprole water, soil and air partitioning under simulated California rice field conditions. Soil for this study was collected from three rice fields in the Sacramento Valley near Davis and Richvale.

Results showed that volatilization is not expected to be a significant contributor to field dissipation of chlorantraniliprole. Furthermore, soil–water partitioning experiments showed that the sorption of chlorantraniliprole to soils is a weak and reversible process affected by environmental factors such as temperature, salinity, and soil properties. Modeling suggests that chlorantraniliprole could mobilize into the Sacramento River with rice field tailwater.

Thiobencarb modeling

Thiobencarb, the active ingredient in Abolish® and Bolero®, is a systemic pre-emergence carbamate herbicide used to control annual grasses and broadleaf weeds. Approximately 84,000 acres of rice were treated with thiobencarb in California during 2013.

Abolish®, an emulsifiable herbicide, can be applied directly to soil both preflood and after draining. Bolero®, a granular herbicide, is applied to field water postflood. Water holding times are 30 days for Bolero® and 19 days for Abolish®. However, even with these holding times, thiobencarb has been detected in the Sacramento River, requiring that the environmental fate and regulations associated with thiobencarb be revisited.

Thiobencarb has moderate water solubility and is acutely toxic to aquatic organisms. It has been shown to bioconcentrate in some species of fish, clams, and shrimp. Thiobencarb is likely to adsorb to soils or remain partially in the aqueous phase, depending on the soil properties at the site of application.

In 2016 a model was developed that is capable of predicting thiobencarb concentrations downstream from rice fields based on application practices and water flow. The model fits well for the Colusa Basin drain. Further validation or extension of the model’s application would first require additional monitoring data at additional locations under varying water flow conditions.