Transport and Fate of Methyl Iodide and Its Pest Control in Soils

Abstract

For fumigants, information on transport and fate as well as pest control is needed to develop management practices with the fewest negative environmental effects while offering sufficient pest control efficacy. For this purpose, a 2-D soil chamber with a surface-mounted flux chamber was designed to determine volatilization, real-time soil gas-phase concentration, degradation, and organism survivability after methyl iodide (MeI) fumigation. Three types of pests were used to give a broad spectrum of pest control information. An infected sandy loam soil with a volumetric water content of 10.6% was packed carefully into the 2-D chamber to a bulk density of 1.34 g cm(-3). After MeI fumigation at a rate of 56.4 kg ha(-1) for 24 h, about 28.9% of MeI was emitted into air, 6.8% remained in the soil, and 43.6% degraded in the soil (based on the residual iodide concentration). The uncertainty in the measured MeI degradation using iodide concentration was thought to mainly contribute to the unrecovered MeI (about 20%). The citrus nematodes [Tylenchulus semipenetrans] were effectively eliminated even at low concentration-time (CT) values (<30 microg h mL(-1)), but all Fusarium oxysporum survived. The response of barnyardgrass seeds [Echinochloa crus-galli] spatially varied with the CT values in the chamber. To fully control barnyardgrass seeds, CT of greater than 300 microg h mL(-1) was required. Using this experimental approach, different fumigant emission reduction strategies can be tested, and mathematical models can be verified to determine which strategies produce the least emission to the atmosphere while maintaining sufficient pest control efficacy.