Submitted to: Hayes' Handbook of Pesticide Toxicology
Publication Type: Book / Chapter
Publication Acceptance Date: 10/1/2009
Publication Date: 6/13/2010
Publication URL: http://www.ars.usda.gov/SP2UserFiles/Place/53102000/pdf_pubs/P2319.pdf
Citation: Ashworth, D.J., Yates, S.R. 2010. Methyl Iodide. Hayes' Handbook of Pesticide Toxicology. UC Riverside, CA: Elsevier. p. 2307-2317. Interpretive Summary: With the phase out of methyl bromide, growers need new crop protection chemicals to continued production of fruit and vegetable crops. As a preplant soil fumigant, methyl iodide appears to be an effective potential alternative to methyl bromide. This chapter provides a review of research currently available on the fate and transport of methyl iodide after soil fumigation. While offering the environmental advantage of not being reactive with stratospheric ozone, existing research suggests that MeI is comparable to, and often more efficacious than, MeBr to a wide range of weed, nematode, and microbial pathogens. However, its time-limited registration (and nonregistration in certain states, e.g., California) reflect its potential for environmental impact. Its propensity to convert to the gaseous phase (high Henry’s constant) and relatively high vapor pressure ensure that MeI diffuses rapidly through soil and is readily emitted from soil to air, even in the presence of surface containment such as plastic tarps. Coupled with its relatively long degradation half-life, the potential exists for high emissions over more extended periods than might be expected for fumigants with shorter half-lives. It is apparent that there is a need for field studies to more realistically determine the flux dynamics of MeI emissions from soil over time. There is also a need for methods to reduce emissions to the atmosphere to alleviate concern over use of MeI as an agricultural fumigant and subsequent exposure to agricultural workers and local populations. Although reducing this exposure is implicit in the guidelines covering the use of MeI (e.g., limits on application rates, minimum distances from occupied structures), research is needed to minimize the risk. This study should provide up to date information that will be useful to fruit and vegetable commodity groups, scientists, regulators and the public.
Technical Abstract: Methyl iodide (MeI, iodomethane, CH3I) was reported as a potential alternative to the stratospheric ozone-depleting fumigant methyl bromide (MeBr) in the mid-1990s (Sims et al., 1995; Ohr et al., 1996). It has since received significant research attention to determine its environmental fate and transport as well as its pesticidal efficacy. As a preplant soil fumigant, MeI can be used alone or in combination with chloropicrin to control plant pathogens, nematodes, insects, and weeds on crops such as strawberries, tomatoes, peppers, ornamentals, turf, trees, and vines (U.S. EPA, 2008). In addition to offering a similar pesticidal efficacy to MeBr, MeI also offers the major advantage of having the very short atmospheric half-life of 4–8 days, compared to 1.5–2.0 years for MeBr (Gan et al., 1997), due to rapid photolysis when exposed to UV radiation (Gan and Yates, 1996). Importantly therefore, it is not considered to degrade stratospheric ozone and so its emissions from soil are of less concern compared to methyl bromide. Nevertheless, MeI fumigation has the potential to increase human health risks through direct inhalation of the MeI gas and due to its potential role in the formation of near-surface ozone.