Partitioning of etofenprox under simulated California rice-growing conditions

BACKGROUND: The pyrethroid insecticide etofenprox is of current interest to rice farmers in the Sacramento Valley owing to its effectiveness against the rice water weevil, Lissorhoptrus oryzophilus Kuschel. This study aimed to describe the partitioning of etofenprox under simulated rice field conditions by determining its Henry's law constant (H) (an estimate of volatilization) and organic carbon-normalized soil-water distribution coefficient (K_oc) at representative field temperatures. A comparison of etofenprox and λ-cyhalothrin is presented using a level-1 fugacity model. RESULTS: Experimental determination of H revealed that etofenprox partitioned onto the apparatus walls and did not significantly volatilize; the maximum value of H was estimated to be 6.81×10^-1 Pam³ mol^-1 at25°C,basedon itsair andwater method detection limits. Calculated values for H ranged from 5.6 × 10^-3 Pa m³ mol^-1 at 5°C to 2.9× 10^-1 Pa m³ mol^-1 at 40°C, based on estimated solubility and vapor pressure values at various temperatures. Log K_oc values (at 25°C) were experimentally determined to be 6.0 and 6.4 for Princeton and Richvale rice field soils, respectively, and were very similar to the values for other pyrethroids. Finally, temperature appears to have little influence on etofenprox sorption, as the log K_oc for the Princeton soil at 35°C was 6.1. CONCLUSION: High sorption coefficients and relatively insignificant desorption and volatilization of etofenprox suggest that its insolubility drives it to partition from water by sorbing to soils with high affinity. Offsite movement is unlikely unless transported in a bound state on suspended sediments.