Apr 7, 2007Phase-Out Continues; Critical Use Exemptions Continue
An international environmental agreement that may be the most successful of its kind in human history – a model of cooperation and achievement – continues to beleaguer farmers, who are having problems meeting its idealistic goals.
The agreement is the Montreal Protocol, and the problem for farmers is methyl bromide. Many producers, especially growers of tomatoes, bell peppers and strawberries, can’t find good alternatives for soil fumigation.
Under terms of the protocol, they keep looking and the world keeps exempting them ¬– because it wasn’t anybody’s intention to cripple producers. Nonetheless, the goal remains total phase-out of methyl bromide at some future time when critical use exemptions are no longer needed.
That’s the story told by Burleson Smith, director of pest management policy for the USDA. He spoke during a special session on the status of methyl bromide during the Southeast Regional Vegetable Conference in Savannah, Ga., in January.
The roots of this saga go back to the mid-1980s, when a hole was discovered in the ozone layer. Scientists quickly discovered the cause – certain chemicals in the atmosphere were breaking up the ozone. They projected dire effects – loss of the natural ozone shield that protected life on earth from destructive ultraviolet radiation.
What was more remarkable, the scientific discovery led to a serious worldwide discussion and an agreement on what to do about it. In 1988, Smith said, 189 nations signed the Montreal Protocol, agreeing to ban production and use of certain chemicals implicated in the ozone destruction process. Methyl bromide was one of several. Most of the chemicals were halogenated methane molecules ¬– formed when natural gas is treated with chlorine, bromine or fluorine. These latter elements are potent destroyers of ozone.
Not only was the level of cooperation in the Montreal Protocol unprecedented, so were the results. Most scientists now believe the ozone depletion has been diminished dramatically – perhaps reduced to zero, perhaps even reversed, Smith said.
It is hard to measure success. The chemical methyl bromide has a very short life in the atmosphere, he said, so reduction in use shows up very quickly.
That is not the case with chlorofluorocarbons (CFCs). It was CFCs that drew major attention in 1988, when the protocol agreed to ban their production and use completely. These man-made chemicals, discovered in the 1930s and broadly used by 1960, were used in refrigerators and air conditioners and in foam insulation.
“With CFCs, we were able to find a technological solution,” Smith said. “Replacement chemicals were found very quickly and the phase-out took place rapidly.”
But because CFCs are so persistent in the atmosphere, having half-lives of 50 to 100 years or more, the full effects of the ban won’t be seen for many years.
Methyl bromide is more complicated, he said. About half the methyl bromide is man-made. The other half is produced naturally, out-gassed from the oceans. And because methyl bromide is used by farmers to affect biological systems, the technological fix that worked with CFCs has been much harder to come by.
Because bromine is about 50 times as potent as chlorine in ozone depletion, the goal remains a complete phase-out of man-made methyl bromide. The schedule for elimination started with 1991 as the baseline, then a freeze at that level to 1999 and reduction each year after that until elimination in 2005. But the protocol contains provisions for critical-use exemptions for as long as they are needed, Smith said. Deciding how long they are needed is complicated.
Each year, the U.S. government puts together a list of “nominations,” stating what uses are critical, how much methyl bromide needs to be produced for those uses and how the search for alternative chemicals and methods is going. These nominations for exemption are passed on to the Montreal Protocol Technical Options Committee, which evaluates them.
The most recent nominations were for 2008 and were sent Jan. 4. The United States asked for exemption on 15 crops or uses, including tomatoes, strawberries, peppers, cucurbits, orchard replants and post-harvest uses. If this is approved, U.S. farm use of methyl bromide in 2008 will fall to 25 percent of the 1991 baseline level.
For 2007, the U.S. nominations would have resulted in 29 percent baseline use. In December, the final figure of 26.4 percent was agreed to by all parties meeting in Dakar, Senegal. The figures for 2006 were 35 percent nominated and 32 percent used. The 1991 baseline for U.S. use is 25,000 metric tons.
The U.S. government supports the Montreal Protocol in one important way. Each year, the EPA amends rules and pesticide labels to make farm use of methyl bromide conform to approvals on nominations.
To gain critical use exemptions, the Montreal Protocol requires an ongoing effort to look for alternatives to methyl bromide, Smith said, and a management strategy to reduce emissions into the atmosphere when it is used.
The process starts when growers, threatened by loss of the chemical, appeal to their land-grant universities, which then appeal to the USDA for research funding to look for alternatives, Smith said. Also important are the efforts of private companies motivated by a desire to market new products that may serve the purpose.
USDA looks for “commercial proof” – concepts that are tested in small plots, then proven to work reliably in large-scale production year after year, Smith said.
One important use of methyl bromide is for fumigation of orchard sites after old orchards are removed, Smith said. Since the supply of suitable orchard sites is limited, growers want to replant them rather than rotate fruit to other sites. Thorough fumigation is usually the solution for orchard replant disease, the mixture of diseases, viruses and nematodes that causes young orchards to do poorly on old sites.
While fumigants are usually thought of as cures for nematodes and soilborne diseases, weed control also is a big use.
“Nutsedge is 95 percent of the reason we have methyl bromide to use beyond 2005,” said Stanley Culpepper, a University of Georgia weed control specialist.
He told growers about his research during the Southeast Vegetable Conference.
“Methyl bromide alternatives for many diseases and nematodes do exist,” he said. “However, controlling weeds with these alternatives has been a struggle.”
Ted Webster, a research agronomist with USDA’s Agricultural Research Service in Tifton, Ga., agreed, saying that purple nutsedge is ranked as “the world’s worst weed” and yellow nutsedge is among the top 15 worst.
Nutsedge grows from both seed and tubers that form around the roots, he said. It is an aggressive weed that will pierce and grow through plastic mulches. While tomato growers might be able to get through the season with their tomato crop, the plastic mulched beds will be virtually destroyed by the end of the season and can’t be used for a second crop like cucumbers, Smith said. This is a serious economic disadvantage, since growers need the second crop to help recover the cost of the bed.
Culpepper has done research for several years looking for products that, when applied during the bedding process, will suppress nutsedge.
“After four years of research, our efforts suggest there are three potentially valid methyl bromide alternative fumigant combinations to manage moderate to severe infestations of nutsedge,” he said.
He compared the standard treatment of methyl bromide and chloropicrin with methyl iodide plus chloropicrin (MIDAS), dimethyl disulfide (DMDS) plus chloropicrin, and a combination of Telone II, chloropicrin and Vapam. All of these are older fumigants that are active against nematodes and soilborne diseases, but not noted for activity against weeds. The fumigants were applied under plastic mulch.
Culpepper found these materials adequately suppressed nutsedge for the first year, but lengthened the time required between fumigation and planting to about three weeks. Using VIF and metalized mulches kept the fumigants in the soil longer (compared to LDPE mulch) and improved weed control, but increased cost. Year after year consistency has been a problem.
He was not ready to proclaim a cure, but he said growers should start experimenting with materials other than methyl bromide. As uses for that chemical decline, so will availability, and price will increase, he said, even for growers who keep their critical use exemptions.
Carl Sams, a researcher from the University of Tennessee at Knoxville, told strawberry growers in Savannah about his work with biofumigants. These are crops growers might plant that would suppress weeds, diseases and nematodes in the following crop.
Research is underway worldwide, screening various crops for biocontrol properties, he said.
He is most impressed with Indian mustard, one of the many brassica species that reduce pathogens and nematodes when they are grown to near-flowering stage, then incorporated into the soil. As the plant tissues break down, they release glucosinolates, which further break down into isothiocyanates that are not unlike Vapam, metam sodium and Telone.
An advantage of having such crops, besides their suppressive benefits, is they become part of the crop rotation, adding organic matter to the soil and breaking insect and disease cycles. They don’t merely kill pests, they cause a shift in the microbe community in the soil, he said, one that is usually favorable to beneficial organisms. Fumigants like methyl bromide kill everything, starting a new race for repopulation.
A problem with cover crops is they may not have time enough to grow when introduced into a rotation. Ideally, a cover crop should grow well late in the year and produce sufficient biomass by planting time in spring, Sams said. That fits them between existing crops and doesn’t displace cash crops.
Byproducts of brassica production, like rapeseed oil meal, are being tested as biofumigants and appear to be active.
The goal remains to eliminate methyl bromide, but critical-use exemptions continue to be granted to growers who haven’t yet found a way to live without it. They should continue looking.