Technology▼Solutions No silver bullet to replace methyl bromide
© 2005 American Chemical Societ y
Jack Norton, manager of the U.S. Department of Agriculture’s (USDA’s) Interregional Research Project No. 4 Methyl Bromide Alternatives Programs, says that Flor ida tomato farmers are switching to Telone but that their progress lags behind that of California growers. As a result, Florida growers will seek to exempt 36,000 of the 44,000 acres to be planted this year for methyl bromide use. U.S. DEPARTMENT OF AGRICULTURE
Despite the Montreal Protocol’s ban on methyl bromide, the U.S. agricultural community remains dependent on the chemical—especially for fumigating harvested produce and sterilizing quarantined goods possibly harboring pests. Alternatives to methyl bromide exist, but questions about their effectiveness have left farmers seeking “critical use exemptions”. According to the U.S. EPA, U.S. farmers purchased 38% of the world’s methyl bromide in 1996. About 85% of the chemical is used to fumigate small-acreage plots against nematodes, fungi, and weeds. “We are by far the world’s largest consumer of methyl bromide,” says Bill Chism, EPA’s lead biologist on critical use exemptions. “It was developed here, and we have a number of agriculture systems designed around it.” Moreover, methyl bromide has proved to be effective and easy to use, so U.S. farmers have a lot at stake, say experts. “Farmers are really anxious,” warns Chism. The two high-value crops that have been most studied for methyl bromide replacements are Florida tomatoes and California strawberries (Pest Manag. Sci. 2003, 59, 814–826). The best replacements are the nematocide Telone (1,3-dichloropropene) and a new product called Inline, which is a 65:35 mixture of 1,3-dichloropropene and the fungicide chloropicrin. Although both products are cheaper than methyl bromide, the few studies available indicate that their effectiveness varies with growing conditions. California strawberry farmers have been slowly phasing out methyl bromide in favor of Inline, says Dan Legard, director of research and education with the California Strawberry Commission. Legard points out that in 2003, only 30% of the strawberry acreage had switched to the new product. “I expect that this year we’ll have switched 40% of the acreage over to the replacement,” he adds.
A California strawberry farmer tests methyl bromide alternatives in a portion of his field. Nevertheless, growers in this state will fumigate about 70% of this year’s strawberry acreage with ozone-destroying methyl bromide.
“We’re still learning how to use these products effectively,” says Norton. Florida has a huge problem with weeds, so Telone application is followed by an application of the weed killer metam sodium. However, metam sodium performs erratically in clay soils and is not as effective in northern states. Also, farmers have to wait almost two weeks after this second application, which can throw off their highly regimented schedules. “You could potentially miss a planting day, which is based on when you have to get the fruit to the packing shed,” says Norton. According to a review of field trials with tomatoes, switching to Telone and an herbicide cuts yields by up to 22.3%. Another study found that
Telone application followed by metam sodium decreased strawberry yields by up to 15% (Soil Crop Sci. Soc. Fl. 1996, 55, 16–20). Weeds are less of a problem in California, where fungal infections usually kill crops. There, farmers do not need the added herbicide. However, EPA considers Telone a probable human carcinogen, and application requires a wide buffer zone if the field abuts an occupied structure. Also, California communities have placed caps on the amount of Telone that can be sprayed. Just last year, two townships in California reached their annual caps for Telone during pepper planting, long before strawberry season had begun. Chism says that these two incidents forced EPA to seek more exemptions for strawberry growers. “We had already given [California growers] all they had wanted, so Florida just went ballistic.” Erin Rosskopf, a research microbiologist with USDA’s Agricultural Research Service, says that iodomethane may be a “drop-in” replacement for methyl bromide and give the same field results. Registration for the fumigant is expected this year. The gas has a much shorter atmospheric half-life than methyl bromide, but large field trials have not yet proven efficacy. Norton says that no replacement will ever be as effective as methyl bromide. Agricultural practices can vary widely across even one state, and he expects farmers to adapt with individual responses. Hidden problems may even arise in the future. Much of the testing for replacements has taken place on fields that have historically been sterilized with methyl bromide. Removing methyl bromide fumigation will completely alter the ecology of fields. “The pests we don’t see today might surface in the future,” Norton says. “It’s very possible. And we may fi nd ourselves coming back to revisit this subject at a later date.” —PAUL D. THACKER
JANUARY 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 13A
