Ronald S. Tjeerdema, professor and associate dean, Dept. of Environmental Toxicology, CAES, 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. The research emphasis in 2018 was exclusively on the insecticide Coragen® (chlorantraniliprole).

Coragen® was registered by the U.S. EPA for agricultural use in 2008 and by the California Environmental Protection Agency in 2016 for use on California rice fields as a preflood treatment to control rice water weevil.

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. Furthermore, other nontarget organisms have shown reduced activity, reproduction, and feeding after short-term exposure to chlorantraniliprole. This indicates the potential for ecological impacts at sublethal concentrations that could result from spray drift, weir leakage, or tailwater releases.

Previous research concluded that chlorantraniliprole is unlikely to volatilize and exhibits weak sorption to soil. Therefore, the degradation of chlorantraniliprole in California rice field soils needs to be determined to ensure adequate dissipation prior to the release of tailwater.

Pesticide degradation in soil is dominated by the microbial community present and the properties of the local environment that influence the microbes. When a rice field is flooded, anaerobic conditions are rapidly established, causing dramatic changes in the dominant microbial communities. As a result, pesticide degradation rates under preflood aerobic conditions can differ greatly from those measured under flooded anaerobic conditions.

Extended flooding periods—combined with elevated temperature and rapid evapotranspiration in California rice fields during the growing season—also influence salinity levels (as indicated by elevated electrical conductivities). Increased salinity has been shown to negatively affect microbial and enzyme activity in soils. An investigation into the impact of salinity on the degradation of pesticides in soil was needed.

Therefore, research in 2018 sought to characterize chlorantraniliprole degradation in soil under simulated California rice field conditions and to analyze the impact of soil properties, flooding, and field salinization on degradation rates. Degradation half-lives were determined through microcosm experiments in the laboratory with two rice field soils from the Sacramento Valley.

The half-life of chlorantraniliprole ranged from 59 to 100 days and was not statistically correlated with soil, redox state, or salinity. Thus, degradation half lives in soil are not expected to vary greatly from field soil properties, flooding practices, or salinity.

The results also showed that chlorantraniliprole degraded for the first 63 days in all treatments. After that, no further degradation was observed. Nutrients such as nitrogen and phosphorous can play an important role in the degradation process in soil. It is possible that depletion of soil nutrients in the laboratory experiments resulted in death or dormancy of soil microorganisms and could explain this observation.

Future research should be conducted to determine the impact of specific soil nutrients on the biotic degradation of chlorantraniliprole. The impact of photolysis and abiotic degradation on chlorantraniliprole also needs further study.