Introduction

Methyl bromide (CH3Br) is a fumigant that was first listed as an Ozone Depleting Substance (ODS) under the 1992 Montreal Protocol, an international environmental agreement that aims to eliminate chemicals that reduce the ozone layer. Methyl bromide is a versatile chemical with a wide range of applications. For more than 40 years it has been used commercially to control pests such as fungi, bacteria, soil-borne viruses, insects, mites, nematodes and rodents. CH3Br has sufficient phytotoxicity to control many weeds and seeds in soils. Used mostly for soil fumigation, a moderate amount is also used for disinfestation of durable and perishable commodities and a minor amount is used for disinfestation of buildings, ships and aircraft. Its action is usually sufficiently fast and it airs rapidly enough from treated systems to cause relatively little disruption to commerce or crop production.

Ozone depletion

Methyl bromide is a significant ozone depleter with a 0.4 Ozone Depletion Potential (ODP), according to the most recent Scientific Assessment on Ozone Depletion [Kurylo et al., 1999]. This means that, on a per molecule basis, its effectiveness in depleting ozone is about four times that of methyl choroform and 40% that of CFC-11, two chemicals banned in 1996. Thus, the bromine in CH3Br, known to deplete 40-50 times more ozone than chlorine (atom for atom), makes CH3Br one of the more hazardous substances for the ozone layer listed under the Protocol, despite its low atmospheric concentration.

In the international schedule to phase out production of CH3Br, governments under the Protocol agreed that developed countries would cut CH3Br consumption by 25% in 1999, 50% in 2001, 70% in 2003, with phase out by 2005. Developing countries need to reduce their consumption by 20% in 2005 and phase out by 2015. CH3Br used for quarantine and pre-shipment (QPS) treatments is exempt from these controls, as the governments under the Protocol considered that CH3Br was strategically useful for protecting countries against unwanted pests in imported agricultural foods. The volume of CH3Br consumed for this purpose is relatively small. Similarly, CH3Br used for feedstock in industrial processes is also exempt, as most of it is consumed in chemical reaction.
Despite CH3Br being a useful pest management agent in specific instances, its listing under the Protocol necessitates ultimately that its production must cease.

Consumption

Of the 1996 global production of methyl bromide of 71,425 tons, approximately 2,759 tons (3.9%) was used as a feedstock for chemical synthesis with the remaining 68,666 tons produced for fumigation. Based on official data, the 1996 global consumption for fumigation (including that used for quarantine treatments) was estimated as 66,750 tons, with approximately 76% of this used as a treatment to control soil pests, weeds and diseases immediately prior to planting a crop; 15% for fumigation of durable commodities (such as walnuts, rice and grain) and structures (such as flour mills and food processing facilities); and 9% for perishable commodities (such as apples and leafy vegetables).

There has been consistent evidence in recent years showing that the Montreal Protocol system is working. Scientists are reporting for the first time a consistent downward trend since 1994 in effective chlorine loading in the lower atmosphere [Montzka et al., 1999; Montzka et al., 1996]. As the upper atmosphere lags behind by about 6 years, it is predicted that a similar trend will be recorded there next year.

However, the stratosphere is most vulnerable to ozone depletion today and will remain so for the next couple of decades [Hofmann et al., 1999]. There is no evidence of replenished levels of ozone and this is not expected for another 20-30 years. For these reasons, countries are constantly reminded not to be complacent and to make every endeavor to continue to phase out ozone-depleting chemicals.

Development of alternatives to CH3Br

One of the challenges of the last decade has been the ongoing need to develop and implement alternatives to CH3Br for all its diverse uses. Research institutes and agencies in both developed and developing countries are finding that there is no one-shot replacement for CH3Br and that combinations of practices or treatments will often be required. This results in farmers and pest control operators having to learn new ways of controlling pests and, unlike most chemical control methods, these new ways are knowledge intensive.

A report detailing alternatives available or being developed to substitute for CH3Br was recently submitted to the United Nations Environment Programme (UNEP) for the governments under the Montreal Protocol [UNEP, 1998]. The report was written by UNEP's Methyl Bromide Technical Options Committee (MBTOC), which currently consists of 35 experts-12 from developing countries and 23 from developed countries. (This report is available on http://www.teap.org, a website that reports on alternatives for a number of ozone depleting substances including CH3Br.)

The report assesses alternatives available in four categories of use and discusses opportunities for capturing and recycling this fumigant. The categories are treatments for fumigating the soil before planting a crop, for quarantine, for durable commodities, and for food processing facilities.

Soil treatments

Soil fumigation with CH3Br is the single largest use category, accounting for about 76% of global use. Methyl bromide is used as a pre-plant soil fumigant in locations where a broad complex of soil-borne pests limits economic production of crops and particularly in situations where the same crops are grown repeatedly on the same land. Methyl bromide has been successfully used under a variety of cropping systems. The major current categories of use include vegetables, fruits, ornamentals, tobacco and some nursery crops.

Significant progress has been made in the past four years identifying alternatives to CH3Br for soil fumigation. In spite of the widespread use of CH3Br as a soil fumigant, the MBTOC did not identify a single crop that could not be produced successfully without the use of this fumigant. However, increased investment in research and technology transfer would be necessary to fully implement alternative pest management systems worldwide.

Integrated pest management (IPM) strategies, which combine various pest management methods (non-chemical and/or chemical), were considered the best alternative to CH3Br, as they are likely to be the most sustainable and environmentally benign. IPM programs have one or more of the following key elements based on incorporating as many biological control methods as possible:

• Cultural practices such as crop rotation, soil-less culture, organic amendments, biofumigation, planting time, water management and flooding, mulching, cover crops and sanitation
• Biological control such as plant growth promoting rhizobacteria, resistant plant varieties and grafting of annual and perennial crops
• Physical methods such as soil solarization and steam treatments
• Strategic applications of selective pesticides

Quarantine treatments

Methyl bromide is typically applied to perishable commodities as a quarantine treatment to prevent pests from becoming established in an area where they are not currently present. Perishable commodities include fresh fruit and vegetables, cut flowers, ornamental plants, fresh root crops, and bulbs. About 9% of global CH3Br consumption is used for disinfestation of perishable commodities, with about half used for disinfestation of fruit for quarantine purposes.

The MBTOC identified at least thirteen different categories of alternative treatments (e.g., heat, cold, irradiation) that are approved by regulatory agencies in one or more countries for disinfestation of perishable commodities, but only for very specific applications. Only a small proportion of commodities in commercial trade are treated in the export country using these alternatives, as most countries have specific requirements for proving the efficacy of any treatments for each commodity-pest combination. Post-entry alternative treatments used by the importing country are particularly problematic because many alternatives have neither been approved for treating a specific product on arrival, nor would they be easy to implement. To solve this problem, a range of alternatives is urgently needed to cope with the large and varied volume of produce entering via multiple air and seaports. Such treatments would need to be able to treat perishable commodities quickly in order to avoid congestion at air and seaports.

In the future, if CH3Br for quarantine treatments is not permitted and no alternatives are available, infested consignments may be prohibited until satisfactory 'on-arrival' treatments are developed and approved, or the consignments may be re-shipped or destroyed. Alternatively, import of consignments considered high risk for pest infestation may be prohibited until an alternative treatment has been implemented in the exporting country to reduce pest contamination to a level acceptable to the importing country.

The MBTOC identified a range of alternatives for perishable products that were either in use or under development. These included non-chemical treatments that kill pests by exposure to changes in temperature and/or atmospheric conditions; high energy processes such as irradiation and microwaves; or physical removal using air or water jets. A combination of treatments may be required to kill pests if they are tolerant to single exposure treatments.

Commercialization of these replacements for CH3Br will depend on a number of considerations that include proven treatment efficacy, commodity tolerance, equipment design and commercial availability, cost competitiveness, regulatory approval, logistical capability, availability and agreement on the scientific research required for regulatory approval, and technology adoption. As there are many aspects to complete for any new quarantine treatment, the time from conception to implementation can vary from 2 to 15 years.

Durable commodities and food processing facilities

Currently, CH3Br is primarily used to disinfest grain stacked in bags; to disinfest food processing facilities such as mills; to treat durable commodities such as cocoa, grain, certain dried fruit and nuts at the time of export and post-entry; and a variety of quarantine applications, notably treatment of logs.

It is estimated that approximately 12% of the global consumption of CH3Br is used for the disinfestation of durable commodities and about 3% for structures. Some of the CH3Br uses for durables, wood products and structures fall within the QPS exemption.

A variety of alternatives to CH3Br exists for disinfestation of durable commodities and structures. The principal ones are phosphine (PH3), heat, cold and contact pesticides for durable commodities; sulfuryl fluoride, chemical wood preservatives and heat for wood products; and sulfuryl fluoride and heat for food processing facilities. The choice of appropriate alternatives depends on the commodity or structure to be treated, the situation in which the treatment is required, the accepted level of risk, the speed of action required and the cost.

Emissions reduction

Emissions from fumigation operations occur through leakage and permeation during treatment (inadvertent emissions) and from venting at the end of a treatment (intentional emissions). Estimates of the proportion of CH3Br used that is released into the atmosphere vary widely because of differences in usage patterns, the condition and nature of fumigated materials, the gas-tightness of enclosures, and local environmental conditions. Also, some CH3Br may react during use, so it is incorrect to equate production with emissions as at least part of CH3Br applied is converted in use to non-volatile materials.
Under current usage patterns, the proportions of applied CH3Br eventually emitted to the atmosphere globally are estimated by the MBTOC to be: 32–87% of applied dosage for soil, 85–95% for perishable commodities, 69–79% for durable commodities and 90–95% for structural treatments. These figures correspond to a range of 43–87% overall emission from agricultural and related uses, with a best estimate of overall emissions of 73% or 50,240 tons (based on 1996 production data).

There has been some limited implementation of recovery and recycling for CH3Br, mainly in North America. A plant has recently been installed at an airport in Texas to recover CH3Br from fumigation facilities. The recovered CH3Br is transported to production facilities where the bromine is removed with heat. Recovery and recycling systems are generally complex and expensive to install compared with the cost of the fumigation facility itself. Some systems would also have high running costs associated with energy requirements and many would require a level of technical competence not normally found at fumigation facilities. For these reasons, there are few examples of recovery and recycle in current commercial use.

Methyl bromide for quarantine and pre-shipment treatments

The MBTOC reported that consumption of CH3Br for quarantine and pre-shipment treatment (QPS) has increased to about 22% of global fumigant use for both developed and developing countries. A pre-shipment treatment is one that is used mainly to kill non-quarantine pests in products such as grain, typically within 14 days of shipment. The quantity of CH3Br used for QPS has increased owing to increased import and export trade throughout the world.

Many countries are now considering that the exemption for QPS is not in the best interests of the ozone layer. There also is limited development of alternatives for CH3Br used in QPS, as funding agencies are focusing on near-term solutions to pest problems that must be controlled with limited volumes of CH3Br. In contrast, CH3Br for QPS currently has no volume restrictions and therefore the exemption under the Protocol acts as a disincentive for promoting expenditure on research.

Representatives of governments under the Montreal Protocol are currently meeting in Beijing, China to consider a range of further measures for all ozone depleting substances, including CH3Br. One proposal under consideration is more strict use of CH3Br for QPS treatments, a freeze in consumption at some time in the future, and official data reporting of the quantities of CH3Br used in this area. If parties accept a freeze on consumption as a form of control, funds will be made available under the Protocol to finance the development and implementation of alternatives to QPS in developing countries. This is a difficult decision for representatives to consider. Accepting limits on the availability for QPS at some time in the future may result in less being available for fumigation of food crops imported by countries with strict pest control measures. On the other hand, limits on availability will result in funds being deployed for developing alternatives. Only one thing is for sure-the representatives will decide only by consensus on any new measures under the Protocol.