Predicting Methyl Iodide Emission, Soil Concentration, and Pest Control in a Two–Dimensional Chamber System

Due to ever–increasing state and federal regulations, the future use of fumigants is predicted on reducing negative environmental impacts while offering sufficient pest control efficacy. To foster the development of a best management practice, an integrated tool is needed to simultaneously predict fumigant movement and pest control without having to conduct elaborate and costly experiments. The objective of this study was (i) to present a two–dimensional (2–D) mathematical model to describe both fumigant movement and pest control and (ii) to evaluate the model by comparing the simulated and observed results. Both analytical and numerical methods were used to predict methyl iodide (MeI) transport and fate. To predict pest control efficacy, the concentration–time index (CT) was defined and a two–parameter logistic survival model was used. Dose–response curves were experimentally determined for MeI against three types of pests (barnyardgrass [Echinochloa crus–galli] seed, citrus nematode [Tylenchulus semipenetrans], and fungi [Fusarium oxysporum]). Methyl iodide transport and pest control measurements collected from a 2–D experimental system (60 by 60 cm) were used to test the model. Methyl iodide volatilization rates and soil gas–phase concentrations over time were accurately simulated by the model. The mass balance analysis indicates that the fraction of MeI degrading in the soil was underestimated when determined by the appearance of iodide concentration. The experimental results showed that after 24 h of MeI fumigation in the 2–D soil chamber, fungal population was not suppressed; >90% of citrus nematodes were killed; and barnyardgrass seeds within 20–cm distance from the center were affected. These experimental results were consistent with the predicted results. The model accurately estimated the MeI movement and control of various pests and is a powerful tool to evaluate pesticides in terms of their negative environmental impacts and pest control under various environmental conditions and application methods.