The mission of this ongoing project is to study the environmental dissipation of pesticides important to rice culture.  Studies in 2006 concentrated on etofenprox, an experimental insecticide for rice water weevil control, and clomazone, a popular grass herbicide.

Insecticide studied

Etofenprox belongs to a new class of ether-based pyrethroid pesticides and is not yet registered in California. It has been used effectively for pest control in Spain and Japan. The insecticide is effective against rice water weevil, as well as green leafhoppers and plant hoppers resistant to commonly used organophosphates.

Experiments in 2006 investigated how etofenprox moves through soil and water. It is characterized as extremely insoluble in water, making measurement of its potential to volatilize in rice fields very difficult. A commonly-used “gas stripping” apparatus method was used in the experiments, but because of the low solubility and volatility of etofenprox, experimental measurement of its volatilization was difficult. Therefore, a new non-equilibrium method is planned for use to try again to measure its potential for volatilization under simulated rice field conditions.

Etofenprox is more likely to cling to soil particles, as is the case for other agricultural pyrethroids. However, due to the chemical’s “hydrophobic” nature, movement into groundwater is thought to be highly unlikely. Work continues on this objective with alternative analytical methods.

Herbicide studies

Studies continued in 2006 on clomazone, a popular and versatile herbicide used against barnyardgrass, sprangletop and water grasses. Herbicide uptake is through plant roots and disrupts chlorophyll production in leaves. It is described as moderately water soluble and stable in the environment.

Little specific information is available on its movement in soil, so experiments with soil samples from near Princeton and Richvale were examined. A Princeton rice farm had historically incorporated rice straw but the last four years had burned its post-harvest waste. The Richvale site produced organic rice and has always incorporated rice straw into the soil.

Clomazone had a slightly greater tendency to adhere to soil from the Princeton site than the Richvale site. Organic matter content may explain the difference, but other factors such as the role of burned residue could also be at play.

Clomazone is not expected to adhere appreciably to the clay- and silt-rich soils of the Sacramento Valley. Experiments are continuing to characterize the compound’s degradation by soil microbes present in California rice fields.