Technology News - December 22, 2004

No silver bullet to replace methyl bromide

The Montreal Protocol's ban on methyl bromide will go into effect this January, but the U.S. agricultural community will continue to depend on the chemical for some time—especially for fumigating harvested fruits and vegetables and sterilizing quarantined goods possibly harboring pests or pathogens. Alternatives to methyl bromide exist, but questions about their effectiveness have left farmers concerned and seeking "critical use exemptions".

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.

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 before high-value crops, such as flowers and nuts, are planted, estimates EPA. At the recent Montreal Protocol meeting this last November, 12 countries received critical use exemptions from the ban. According to the United Nations Environment Programme, the total exemptions came to 13,438 metric tons. The United States take of this amount was 8,942 metric tons, well more than the other 11 nations combined.

“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-studied 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 2002, only 15% of the strawberry acreage had switched to the new product, whereas the number increased to 30% in 2003. “The number of total acres planted [in strawberries] in 2003 was also higher,” he adds. “I expect that this year we’ll have switched 40% of the acreage over to the replacement.”

Jack Norton, manager of the U.S. Department of Agriculture’s (USDA’s) Interregional Research Project No. 4 Methyl Bromide Alternatives Programs, says that Florida 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.

“We’re still learning how to use these products effectively,” says Norton. Florida, for instance, has a huge problem with weeds. So Telone application is followed five days later 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 before they can plant. This 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 conducted by the University of Florida’s extension office, switching to Telone and an herbicide cuts yields by up to 22.3% compared with planting with methyl bromide. 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, California farmers compete with developers for land, and many fields border houses and buildings. EPA considers Telone a probable human carcinogen, with moderate toxicity to wildlife. Application also requires a wide buffer zone if the field abuts an occupied structure.

Fearing potential health threats, California communities have placed caps on the amount of Telone that can be sprayed in a given township. Just last year, two townships in California reached their annual caps for Telone during pepper planting, long before strawberry season had even begun. Chism says that after these two incidents, EPA had to seek more exemptions for strawberry growers to use methyl bromide. “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 that will give the same results in the field. Registration for the fumigant is pending, but it is expected to be approved later 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. Because of differences in the land, culture, plant cultivars, pests, and technology, 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, some of which have not had a nematode wriggling through the dirt in decades.

“The pests we don’t see today might surface in the future,” Norton says. “It’s very possible. And we may find ourselves coming back to revisit this subject at a later date.” —PAUL D. THACKER