The Status of MeBr Alternatives

Methyl bromide's reign as the gold standard of soil fumigants became tarnished as its ozone depleting properties surfaced. In 2005, methyl bromide will be lost to growers as a soil treatment option, and other practices and compounds must fill the void. ARS and others have put significant efforts into researching various ways to fill the gap that will be left when methyl bromide is completely unavailable. This report is a compilation, or a status of sorts, of the leading options available to growers in the United States.

Before the methyl bromide mandatory reductions began in 1999, U.S. agriculture used about 60 million pounds of the fumigant. Soil fumigation accounted for 75 percent of the methyl bromide used, with about 11 percent used to fumigate harvested commo-dities during storage and import or export. Another 6 percent of methyl bromide was used to fumigate structures such as food processing plants, museums, transport vehicles, and warehouses. The last 8 percent was used to produce other chemicals.

While quarantine use is currently exempted from the upcoming ban, the Montreal Protocol Technology and Economic Assessment Panel may reconsider the exemption. In 1998, the latest year for which statistics are available, the United States used approximately 570,015 pounds for quarantine purposes. In exporting U.S. goods to other countries, fumigation with methyl bromide may be required. Methyl bromide is also used at U.S. ports-of-entry to disinfest commodities found, on inspection, to be infested with nonindigenous pests, the introduction of which may irreparably harm U.S. agriculture.

The primary focus for the U.S. Department of Agriculture in facing the loss of methyl bromide is research, although technology transfer and education activities with the private sector are also included. Additional funding has been given to assist in the registration of potential alternative fumigants. The Agricultural Research Service (ARS), which serves as the lead agency in these activities, has established a national program called Methyl Bromide Alternatives which includes all ARS research on this topic. Information about the program and the research being conducted can be found at the web site:
http://www.nps.ars.usda.gov.

ARS Research on Alternatives

Preplant Soil Fumigation

Research on preplant soil fumigation consists of investigations of alternatives that use other chemicals, biological controls, disease/pest resistant plants, modified cultural practices, and integrated pest management practices to control weeds, pathogens, nematodes, and damaging insects. Viable methyl bromide alternatives are difficult to identify compared to replacements for other ozone depleters because many factors affect the efficacy: crop and soil type, climate, and target pests. These factors change from one geographical location to another; therefore, it is important to be aware that technology developed in one location is most successfully used in the areas in which the research was conducted. Two chemicals, iodomethane (methyl iodide) and propargyl bromide, have shown promise in field studies. Iodomethane has an advantage over the chemically similar methyl bromide—it photodegrades before reaching the stratosphere, so it is not an ozone-depleting substance. At this time, however, neither iodomethane nor propargyl bromide are registered as pesticides and will have to go through the process of registration, which is costly and time-consuming, before they are available to producers. Also, restrictions on use may be implemented due to concerns about its environmental fate since many of the growing regions are close to schools and residences.

Because the bulk of preplant methyl bromide, nearly 80 percent, is used on strawberries, tomatoes, ornamentals, peppers, and nursery crops, ARS' primary research focus has been on these crops with special emphasis on tomatoes in Florida and strawberries in California as model crops. Additional research has addressed replant difficulties often associated with perennial crops such as grapes, apples, and peaches.

ARS' strategy for evaluating potential alternatives is to first test the approaches in controlled experiments to determine efficacy and follow up with field tests of those determined to be effective. The impact of the variables that affect efficacy is addressed by conducting field trials at multiple locations with different crops and against various diseases and pests. Those alternatives that are effective in field trials are further tested in field-scale validations, often by growers in their own fields. Research teams that include ARS and university scientists, extension personnel, and grower representatives meet periodically to evaluate research results and plan future trials.

Vegetable Crops

In the United States, tomato crop production uses 24 percent of preplant methyl bromide, more than any other crop. Peppers account for 12 percent. Methyl bromide is used for preplant fumigation of soil in both nurseries and fields before transplanting. Target pests include soil-borne pathogens, particularly Fusarium oxysporum, Phythium spp., and Phytophthora spp.; nematodes, especially root knot nematodes (Meloidogyne spp.); and weeds such as purslanes, spurges, and nutsedges.

The most promising fumigant alternatives for control of nematodes, pathogens, and weeds are a combination of metam-sodium plus chloropicrin, Telone (1,3-dichloro-propene, or 1,3-D) C-17 (83 percent 1,3-D and 17 percent chloropicrin), Telone C-35 (65 percent 1,3-D and 35 percent chloropicrin), propargyl bromide, pebulate (Tillam) plus Telone C-17, and iodomethane (methyl iodide), according to research results in 2000 from Dr. Joseph Noling, an extension nematologist with the University of Florida's Institute of Food and Agricultural Sciences at Lake Alfred, Florida.

According to a 1997 U.S. EPA study, metam-sodium and plastic mulch controlled all weeds tested in the southeastern United States except nutsedge, the most bothersome weed in horticultural crops. Noling reports that metam-sodium displays erratic control, with excellent control in some studies and very little in others. Although this variability is attributed to differences in application, it still presents a major shortcoming. The use of paper mulch, a cultural approach, is also being studied for nutsedge control. Preliminary small-scale tests have demonstrated good nutsedge suppression, and this approach also eliminates the need for polyethylene mulch. Although this research has just been started, it reflects that long-term, more sustainable approaches are being studied in addition to short-term, chemical alternatives.

While biological controls show promise for control of pathogens and weeds in vegetable nurseries and production fields, effective control may require several years of continuous treatment. Another possible biological control is competitive displacement of pathogenic organisms in soil through the use of nonpathogenic organisms and soil amendments. In another approach, Nancy Kokalis-Burrelle at USDA's U.S. Horticultural Research Laboratory at Fort Pierce, Florida, uses plant growth promoting rhizobacteria (PGPR) that have enhanced the growth of tomato and pepper transplants and increased tomato yields. Incidence of Fusarium was decreased in this study in some PGPR plots. Studies conducted by C. Douglas Boyette at USDA's Southern Weed Science Research Unit at Stoneville, Mississippi, indicate that a fungal pathogen, Myurothecium verrucaria, shows promise for controlling weeds in pepper and tomato plots without affecting the transplants. Additional biological control agents that are under development include Dactylaria higginsii, which, after having been shown to be extremely effective in controlling purple nutsedge in small-scale field trials, is now under evaluation by Erin Rosskopf in large-scale plots through a cooperative research project between USDA and the University of Florida. Rosskopf, a research microbiologist at USDA's U.S. Horticultural Research Laboratory in Fort Pierce, Florida, is also developing a biological control agent, Phomopsis amaranthicola, for use in vegetables and ornamentals. This fungal plant pathogen significantly affects the growth of pigweeds (Amaranthus spp.) and has been field-tested in small-scale trials over several seasons.

Cultural controls, another nonchemical approach, show promise and are under continual testing. A period of several years is necessary for this approach to work at an optimal level since pest population levels are reduced each successive year. Bell and chili peppers that demonstrate high levels of resistance to root-knot nematodes have been developed and commercially released. Crop rotation has reduced nematode populations in pepper fields, resulting in increased yield. Soil solarization, though it works well on occasion, has performed inconsistently. Water quality issues have limited the practice of flooding fields to kill pests in regions that have a high water table, such as some parts of Florida.

Strawberries

Producers of strawberries are the second largest preplant users of methyl bromide in the United States, targeting pests such as soil-borne pathogens, nematodes, and weeds. Soil-borne pathogens of particular note are Phytophthora, Phythium, Rhizoctonia, and Verticillium dahliae. Methyl bromide is used for preplant treatment of soil in nurseries and in production fields before transplanting strawberries. In the nursery industry, the use of methyl bromide is critical because healthy, vigorous plants provide growers with a fighting chance to control disease problems in the field.

In the field of nonchemical approaches, several PGPR strains seem to enhance the growth of strawberries and are being further tested in field studies. While resistance to fungal pathogens is under evaluation, only low levels of resistance have been identified. Soil solarization, in limited field trials in Florida, has shown yields that are comparable to those of methyl bromide treated plots, although unpredictable weather events will limit its usefulness. In California, where most commercial strawberries are grown, soil solarization is not an option due to a lack of solar radiation and low temperatures.

Thomas Trout of ARS' Water Management Research Laboratory in Parlier, California, and Hussein Ajwa (formerly of the Water Management Research Laboratory, now with University of California-Davis) found that various mixtures of 1,3-D/chloropicrin resulted in yields comparable to methyl bromide fumigation. Continuing studies are being conducted to determine minimum effective rates, optimum application conditions, and the impact of virtually impermeable film on efficacy and rate of application. Ajwa and Trout, in conjunction with the California Strawberry Commission, conducted 4 years of field trials in growers' fields to test and demonstrate the most likely effective alternatives to methyl bromide fumigation for preplant soil treatment for strawberries. Nearly all California strawberries are grown with preplant fumigation with methyl bromide/chloropicrin mixtures, which typically doubles marketable yield. Mixtures of 1,3-D and chloropicrin and chloropicrin alone, in Ajwa and Trout's research plots in 2000, seem to give good response with slightly smaller yields (5 to 15 percent) compared to those with methyl bromide. These chemical alternatives have disadvantages that include greater weed problems, especially with chloropicrin alone; longer waiting times before planting; and regulatory limitations on use. Drip application, according to current studies, may reduce emissions and worker risk. Although fungicides, such as metalazyl (Ridomil) and fosetyl-aluminum (Aliette), slowly reduce a portion of the fungal spectrum encountered in fields over a period of years, the lack of control of other pathogens does not support this approach. It appears that in the near future, alternative chemical fumigants are the only technology likely to replace methyl bromide for control of soil-borne pathogens in strawberry nurseries and production fields.

Grapes, Fruit Trees, and Nut Trees

Methyl bromide has been used for fumigating soil before planting or replanting to kill pathogens and plant parasitic nematodes in the soil. Methyl bromide is capable of destroying most life stages of soil-dwelling organisms as well as the roots of old trees and vines. Without fumigation, roots are likely to remain present for at least 3 years after tree removal. These woody roots provide a food source for a variety of soil-borne nematodes, insects, and plant pathogens. Viruses, bacteria, and other microorganisms, like fungi, will remain in the soil for as long as the food source is available.

Developing alternatives to methyl bromide for replant of vineyards, as well as fruit and nut trees, is particularly challenging because the results of these studies can only be ascertained over a period of years. Scientists have not been able to isolate pathogens that consistently cause replant disorder. Various species of nematodes are present in the soils, such as ring nematodes in sandy soils and pin nematodes in sandy-loam soils. In some instances of replant disorder, small roots are evident but no obvious pathogens are present.

A number of nonchemical approaches are being explored, including biocontrol and growth enhancement agents. Results of these studies are several years away. A number of cultural control methods appear promising. Some field studies have been set up to determine whether fallowing for several months to several years and the use of cover crops may alleviate replant disorder. The use of some wheat cultivars as a cover crop seems to reduce levels of pathogenic microbes in apple orchards, according to Mark Mazzola, a plant pathologist in ARS' Tree Fruit Research Laboratory in Wenatchee, Washington, and Yu Huan Gu, formerly of the Tree Fruit Research Laboratory, in studies conducted in 2000. Under certain conditions, preplant soil solarization also appears to be a viable alternative for control of the ring nematode (Criconella xenoplax) in orchards. Alterations in cultural practices, such as soil excavation in the fall prior to planting and subsequent exposure of this soil to freeze/thaw cycling during the winter, or establishing new plants in the old aisle rather than the old row, appear effective in reducing replant problems.

Another viable alternative to methyl bromide for some trees seems to be host-plant resistance; for example, "Deep Purple" rootstock for fruit and nut trees appears resistant to pathogens in orchards. However, graft compatibility determinations require waiting for symptoms to appear, which can take years. "Guardian" rootstock demonstrates exceptional resistance to peach tree short life induced, in part, by the ring nematode and may prove to be a viable alternative to preplant methyl bromide fumigation for nematode control.

At ARS' Water Management Research Laboratory in Parlier, California, Cynthia Eayre is investigating chemical controls. Preliminary results from 2 years of data, reported in 2000, show that iodomethane effectively controls peach replant disorder. Combinations of resistant rootstock and either 1,3-D or metam-sodium also appear to be possible alternatives for methyl bromide for control of peach replant disorder. Fungicides to control soil-borne pathogens have been used unsuccessfully.

Sally Schneider, also of ARS' Water Management Research Laboratory, treated a 65-year-old vineyard that was infested with significant plant parasitic nematode populations with drip-applied Telone II EC and shank-applied iodomethane. The field was then replanted with three grape rootstocks that demonstrated broad resistance to most nematodes in grape production areas. Three years after planting, the Harmony rootstock supports minimal populations of the root knot nematode, even in untreated plots, but supports higher populations of the citrus nematodes than either Thompson Seedless or Teleki 5C. Iodo-methane and Telone/Vapam combinations appeared to act as adequate replacements during the 3-year evaluation.

Postharvest Research

Developing alternatives for controlling pests of stored and exported commodities is the realm of postharvest research. Many commodities cannot be exported legally without methyl bromide treatment to eradicate quarantine pests and certify the commodities pest free. Alternative fumigants, heat and cold treatments, modified atmospheres, and combinations of treatments are various research approaches being explored.

Dried Fruits and Nuts

Low-temperature (10 °C) storage and controlled-atmosphere (5 percent O₂) storage were found to effectively control pests of dried fruits and nuts.

Fresh Fruits

In a number of crops, including citrus and papaya exported from Hawaii and other places and limes imported into Florida, the use of forced hot air for quarantine treatment of fresh fruits has been commercialized. Hot water immersion for quarantine treatment has been commercialized for litchi exported from Hawaii and guavas exported from Florida. Conversely, cold treatments are used commercially for avocado and starfruit exported from Hawaii. Large-scale tests have shown irradiation to be effective for disinfestation of sweetpotato weevils in sweetpotatoes exported from Florida, Malaysian and other fruit flies in fruits exported from Hawaii, and maggots in blueberries exported from Florida. Field-study results are used to establish and maintain pest-free zones in lieu of postharvest treatments for fruit flies in citrus exported from Texas. Approximately two-thirds of the fruit, in most years, can be harvested under the requirements of a pest-free zone so that methyl bromide treatment is not required.

Processing Facilities and Warehouses

Methyl bromide is used by many food companies to fumigate processing facilities and storage areas to rid these areas of insect pests. A proven alternative treatment is raising the temperature of the facilities to near 50 °C for 2 to 3 days. Also, a combination of heat and diatomaceous earth seems particularly effective in areas that may be difficult to heat.

Emissions Reductions

A methyl bromide recapture and recycling system was designed by ARS and commercialized, and is now in operation at the Dallas-Fort Worth International Airport. The pilot recapture system vastly reduced the amount of methyl bromide released to the atmosphere—93 to 96 percent of recoverable methyl bromide is captured by the carbon in field tests.

Conclusions

Currently registered alternative fumigants such as metam-sodium, chloropicrin, and 1,3-D seem likely to be the most reliable replacements. However, it is probable that the replacements will not be as reliable as methyl bromide in all cases. These alternatives were once the standard chemicals used but in many cases were replaced by methyl bromide.

For preplant purposes, biological control and host-plant resistance may demonstrate effectiveness in some cases in the future. Much of the research on these approaches is still to be conducted, and it is unlikely that economic use of these alternatives will be available for many uses before the 2005 phase-out has occurred. Due to the multiple combinations of crop, pest or pathogen, environmental conditions, weed species, soil type, etc., combinations of approaches will be necessary to effectively manage diseases and pests.

For postharvest and export uses, several nonchemical approaches, such as heat/cold treatments and modified atmospheres, demonstrate some possibilities as methyl bromide alternatives. A methyl bromide recapture system has been developed and is in use at one airport in Texas.

Research is only one cog in the machinery to bring reliable, effective, and economical methyl bromide alternatives to farmers and other users of methyl bromide. A critical role must be filled by the chemical industry: registering and marketing any promising unregistered alternatives. It is currently uncertain if there is adequate financial incentive for the chemical industry to bring new products to market in light of the minor-use nature of methyl bromide. Users of methyl bromide will ultimately make decisions, based on several factors including economic considerations, as to whether to utilize any alternative technologies that become available.

Last Updated: July 17, 2002