- Author: Iqbal Pittalwala
Methyl iodide – yes, that volatile chemical that could find use as a soil fumigant – has been in the news lately, and mostly in a negative light. In April, California’s Department of Pesticide Regulation made a preliminary decision to approve the use of methyl iodide as a fumigant under strict conditions, and soon after received more than 50,000 public comments.
Methyl iodide, it turns out, is not only toxic, like all fumigants, it “can cause cancer, brain damage and miscarriages.” Its potential use in California’s strawberry fields in place of its ozone-depleting and toxic cousin, methyl bromide, is what shot the compound into news orbit.
Why phase out methyl bromide as a soil fumigant? Answer: Ozone loss up in the stratosphere, leading to more ultraviolet light penetrating the Earth’s atmosphere, leading, in turn, to increases in skin cancer. When methyl bromide is released in the lower atmosphere, a fraction gets transported into the stratosphere where it undergoes a series of chemical reactions leading to stratospheric ozone depletion.
Cousin methyl iodide, on the other hand, is photolyzed quickly in the lower atmosphere, leaving none of it to escape into the stratosphere.
While public opinion currently leans heavily against the use of methyl iodide as a soil fumigant replacement, James Sims, a professor emeritus of plant pathology at UC Riverside, supports its use. For one thing, it delivers results similar to methyl bromide, he says, and, second, it can be used safely. He explains that it is not applied directly to plants; it is injected into the soil two weeks before any plants are planted.
Methyl iodide is a liquid boiling at 42.5 degrees C or 108.5 degrees F. Sims says it is therefore safer for workers to handle than a gas like methyl bromide. Moreover, it can be applied using the same equipment with few or no modifications, and it is effective in reducing pest, weed and plant disease problems. Other alternatives — solarization, anaerobic soil disinfestations, crop rotation, steaming, etc. — are not effective, Sims says.
Critics are not convinced of methyl iodide’s relative safety and growers, meanwhile, continue to insist that their fields need a fumigant that poses no harm to our precious ozone layer.
How then might the controversy play out? We will have to wait and see – for several weeks at least. After processing the public comments received, state officials plan to weigh in on the “controversial effort to register the fumigant methyl iodide.”