One of the most important application of methyl iodide is that it is used in pesticide as a pre-plant biocide that helps in controlling soil borne pathogens, parasitic nematodes, weeds, and insects. Here are some other main applications of Methyl Iodide. It is used in the preparation of Methyl Magnesium Iodide (Grignard reagent). It is used as methylating agent in the methylation process during the organic synthesis of carboxylic acid like phenols. It is used to prepare Tetra Methyl Ammonium Hydroxide.