FIELD MOVEMENT AND PERSISTENCE OF FENAMIPHOS IN DRIP-IRRIGATED PINEAPPLE SOILS

The persistence and mobility of fenamiphos and its toxic oxidation products in soil with pineapple cropping were studied in three field experiments on the islands of Oahu and Lanai, and by sampling three commercial fields on Oahu. Fenamiphos was applied by drip irrigation after planting, following pre-plant application of 1,3-dichloropropene (1,3-D) in most treatments. The Oahu experiments, conducted on highly weathered Oxisol and Ultisol soils, evaluated the impact of restricted irrigation to reduce leaching and also the effect of 1,3-D on fenamiphos behavior. Total toxic residue (TTR) concentrations of 1000-mu-g-kg-1 or greater were maintained in the root-zone with fenamiphos applications of 3.4 kg ha-1 tri-monthly on the Oxisol (Kunia site) and bi-monthly on the Ultisol (Whitmore site). The relatively high applications, along with weekly irrigations and unseasonably high rainfall, resulted in leaching of fenamiphos TTR out of the root-zone with concentrations between 10 and 100-mu-g-kg-1 at 3 m depth. Reduced leaching with restricted irrigation was evident in the early months of both Oahu experiments. The potential to minimize leaching of fenamiphos by reduced application rates and limited irrigation during rainy periods was evident from nematicide concentration profiles measured on three commercial pineapple fields on Oahu; little fenamiphos TTR was detectable below 1 m.