Status of Alternatives for Methyl Bromide in the United States
Methyl bromide (MeBr, CH3Br) is used in the agricultural sector as a broad-spectrum biocidal fumigant for soils, commodities, wood packing materials or structures, targeting pest insects, nematodes, weeds, pathogens and rodents. MeBr was identified as a chemical that contributes to
the depletion of stratospheric ozone, and its production and use are subject to regulation under the US Clean Air Act. As one of the original signatories of the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer, the United States ratified the Protocol in 1988. Amendments to
the Clean Air Act were enacted in 1990 to include Title VI on Stratospheric Ozone Protection to ensure that the United States would satisfy its obligations under the Protocol. The United States committed to a gradual reduction of MeBr use, leading to a near complete ban on January 1, 2005.
The Montreal Protocol and the US Clean Air Act allow yearly requests for Critical Use Exemptions (CUEs); Quarantine and Preshipment (QPS) applications as well as emergency uses are also exempt from the ban. Several alternative fumigants to MeBr as pre-plant treatments had been registered in
the US before the phase-out. These include chloropicrin (Pic), 1,3-dichloropropene (1,3-D), and methyl isothiocyanate generators which include metam sodium, metam potassium and dazomet. Two new fumigants have been registered recently, iodomethane and dimethyl disulfide (DMDS), though these
have not been registered in all states. These alternative fumigants are generally used in various combinations with one another. An economic analysis of alternative treatments for tomato production was conducted over a six year period in North Carolina. The authors concluded that Pic, 1,3-D
plus Pic and metam sodium provided a better economical return than the MeBr standard. Other combinations including iodomethane resulted in economic losses when compared to the standard. Postharvest use of sulfuryl fluoride (SF), which has nearly the same infrastructural requirements as MeBr,
has increased consistently since its registration in 2004. Numerous studies report that for post embryonic life stages of postharvest insect pests, SF is generally more toxic than MeBr for a given species. However, insect eggs are relatively more tolerant to SF than to MeBr, often requiring
many times the dosage required to control adults of the same species. Practical strategies for overcoming the tolerance of eggs to sulfuryl fluoride include multiple fumigations to allow for egg hatch or extending SF exposures beyond the duration of the egg stage. However, SF fumigations in
the US are restricted both by label requirements that specify a maximum dose (128 mg/L) and exposures (1500 mgh/L) as well as the resultant maximum residue levels (MRLs) on treated commodities. CUE requests from the dried fruit and nut sector continue because SF exposures required to control
eggs of target pest species exceed regulatory allowances, particularly when fumigations must be conducted <20°C. The technical and regulatory limitations of SF are exacerbated when processors need to treat large amounts of product rapidly to meet specific market demands, such as California
walnuts and dates intended for holiday markets. Ongoing research at ARS addresses these challenges by pairing SF with a potent ovicide. Unfortunately, due to concerns over fluoride residues in treated foods, the EPA has proposed withdrawing SF tolerances and gradually phasing out commodity
treatments. Obviously, this regulatory action, should it occur, would make SF unavailable as a MeBr alternative for US commodity treatments. Phosphine, in its various commercial forms (pellets, generators, cylinders), has been used in a postharvest capacity for decades with new application
technologies that serve to reduce fire hazard (a spontaneous ignition with atmospheric oxygen occurs at phosphine levels >18,000 ppm) and decrease the exposure time required for insecticidal efficacy. When applied at recommended doses (500–2000 ppm), complete mortality of all insect
life-stages is species specific and typically requires exposures of 2–7 days, as compared to 2–3 hours needed for MeBr.
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