Possible replacements for methyl bromide

Photo Courtesy of U.S. Agriculture Research Service.
Measuring fumigant concentration at several distances above the soil surface, a student adjusts the rate of airflow through a charcoal sample collector. The charcoal is used to absorb and trap fumigant gases, which are then analyzed in the laboratory.

Most growers have relied upon the soil fumigant methyl bromide for years. The move away from this ozone-depleting chemical, a provision of the 1987 Montreal Protocol, has many scrambling for efficient and cost-effective alternatives.

Trying the Midas touch

On October 5, 2007, the Environmental Protection Agency approved a one-year registration of iodomethane (methyl iodide), marketed as Midas by Arysta LifeScience. The first new soil fumigant to win EPA approval in 20 years, it is expected to be widely available.

Both the EPA and Arysta say the product is safe when used as directed, and is effective in controlling a range of soil-borne diseases, nematodes, weed seeds and insects that threaten crops such as ornamentals, strawberries, tomatoes, peppers, stone fruit, nuts and vines, as well as turf. The EPA put the product to a rigorous risk assessment study over four years, evaluating it as a potential factor in over 50 medical conditions. Although there was an association with thyroid cancer, typical exposure is not expected at the levels that pose risk. The EPA concluded that its registration does not create unreasonable hazards.

However, the agency placed strict guidelines for its use. Arysta must provide training in proper Midas use; only certified applicators may purchase the product. Buffer zones vary depending upon the acreage treated and other factors, but the minimum is always 25 feet. Application is limited to 40 acres per day and buffer zones of areas treated within 48 hours cannot overlap. Use within a quarter-mile of occupied structures, such as schools and hospitals, is banned. Individuals involved in treatment must wear approved respirators; tractor drivers and co-pilots may opt to use a ducted fan or blower instead.

Despite the EPA’s evaluation and stringent precautions, some scientists oppose registration, citing possible health and environmental concerns. Fifty-four scientists signed a letter outlining their objections in September 2007, just days before the approved registration was announced.

Rob Welker and Frank Louws, both of North Carolina State University, reported on Midas’ effectiveness, as demonstrated under Experimental Use Permits (EUP), at a recent conference. The product earned a four on a one to five rating scale. By comparison, methyl bromide rated five. In each case, the efficacy ranking is for disease, nematode, nutsedge and annual weed control.

EUP studies have been underway in Midas use in Florida, Georgia, Michigan, North Carolina, South Carolina, Tennessee and Virginia since 2006. Test acreage in California was added in 2007. These trials evaluated efficacy, market yields and economic comparisons for several crops. Half of the growers who participated saw, on average, a 19 percent yield increase compared to methyl bromide in side-by-side trials on commercial acreage. The remaining growers saw results similar to those achieved with methyl bromide.

As to cost, Midas runs approximately $1,500 per acre, or $10 per pound, which is comparable to methyl bromide (chemical cost only). Midas is applied at the rate of 150 pounds per acre, as compared to 400 pounds with methyl bromide. However, expenses for other items, such as virtually impermeable film (VIF), must be added. The newer product does use conventional application techniques and equipment, allowing growers to avoid some new equipment purchases.

Other chemical alternatives

J. W. Noling of the University of Florida spoke to strawberry growers last fall about his institution’s efforts to develop substitutes. A number of substances, including chloropicrin alone, Telone C-17, Telone C-35, DMDS chloropicrin (Paladin, which is available under EUP in three states) and Midas, provided control comparable to that of methyl bromide. Telone C-35 has risen to the top because it offers the effective nematicide quality that chloropicrin lacks. That product, along with Telone InLine and Midas, is currently recommended as an alternative for strawberry growers.

Photo by U.S. Salinity Laboratory.
Test fumigant – Currently, it can take many years and many hundreds of thousands of dollars to test fumigant emissions in the field. ARS Scientist Scott Yates has been exploring the validity of smaller, simpler tests in the safety of a lab environment using stainless steel columns.

Research also indicated that efficacy is affected by the uniformity of application, mandating treatment with alternatives by standard shank, mechanical incorporation and more gas impermeable plastic mulch to reduce overall application rates and soil emissions.

Along with its one-year approval of Midas, the EPA has reviewed other soil fumigants such as methyl bromide, metam sodium, dazomet and chloropicrin. Although the public comment period ended in November, at press time decisions on risk mitigation measures had not been announced. That pending news, along with the limited approval period for iodomethane, may signal significant changes in the months to come.

Chloropicrin, which Noling considers a crucial component in alternative strategies, took quite a hit from the EPA. Recommendations for this and other fumigants includes expanded buffer zones and proximity to occupied structures and reduced applications, which may make the products virtually impossible to use in many areas. Emission reduction strategies may be increased, requiring more barrier and impermeable materials to be used.

Virtually impermeable film

Regardless of the chemical used, employing more effective barriers may be a good thing. It stands to reason that the efficacy will be improved when the fumigant remains within the soil for longer periods. In addition, greater impenetrability can lead to reduced chemical use.

For those reasons, virtually impermeable films continue to play important roles in the fumigation process. While both low-density polyethylene film (LDPE) and high-density polyethylene film (HDPE) are quite permeable to many fumigants, VIF reduces diffusion significantly. For example, methyl bromide is typically diffused in the range of 200 to 400 grams per square meter per hour. VIF may bring that down as low as 0.2 grams. Some growers have been able to reduce methyl bromide use by as much as 50 percent.

However, there is still a lot to learn about VIF. Some studies indicate that one VIF product may be highly effective with methyl bromide, but nearly useless with another fumigant. Requests for efficacy labeling on VIF products are starting to arise.

Cost is another factor to consider. VIF runs about $240 a roll [1.2 to 1.3 millimeter by 60 inches by 4,000 feet], approximately 80 percent higher than standard tarps. However, if significant reduction in fumigant use can be achieved, actual expenses may decrease. Powell Smith of Clemson University’s Extension Service compared the estimated costs of treating 1 acre with two rolls of film and methyl bromide 50:50. With LDPE, the total came to $1,010, based on 200 pounds of fumigant. Using VIF, the methyl bromide was reduced to 120 pounds, bringing the total expense down to $880.

Welker and Louws said the newest kid on the block is totally impermeable film (TIF), a multilayered product that effectively prevents all chemical movement from the soil. This product is so new, in fact, that little study of it has been made, so this will be one to watch.

They added that although many questions linger, now is the time to begin testing alternatives on your own farm. Your particular location and its disease and pest pressures require a unique solution that only on-farm assessment, evaluation and experimentation can provide.

Jenan Jones Benson is a freelance writer based in Greensboro, N.C.