Methyl bromide (CH₃Br) is a broad spectrum pesticide used in the control of insects, nematodes, weeds, pathogens, and rodents since 1932. It is primarily used as a fumigant in stored product pest management. It is also widely used as a preplant soil fumigant to control nematodes, insects, pathogens and weeds. Methyl bromide is a preferred fumigant for most of the quarantine authorities around the world, because of its good penetrating ability, noncorrosive and non-flammable nature, rapid action, and high toxicity to a broad spectrum of insets from egg to the adult stage. After its intended use in fumigation, a high proportion of the methyl bromide eventually enters the atmosphere. The “Montreal Protocol on Substances that Deplete the Ozone Layer” established in 1987, recognised methyl bromide as an ozone depleting substance along with chlorofluorocarbons (CFCs) and halons. Each bromine atom from methyl bromide was found to destroy about 60 times more ozone molecules than each chlorine atom from CFCs on an atom-per-atom basis. The Montreal Protocol proposed phase-out schedule of methyl bromide both for developed and developing countries.  The schedule of reductions for developed countries are 50% level by 2001, 70% by 2003, and 100% by 2005, while the developing nations have to freeze methyl bromide usage by 2002 at average 1995-98 levels, reduce by 20% by 2005, and complete phase out by 2015 (TEAP, 2000). The restricted use and phasing out of methyl bromide has forced pest control operators, plant managers in the food industry, and entomologists to find alternatives to the fumigant. However, the use of methyl bromide for phytosanitory purposes is currently exempt from the protocol’s phase-out provisions because of difficulties in identifying technically and economically feasible alternatives. 

In India methyl bromide fumigation forms an essential component in quarantine processing of import/ export consignments by disinfestation and salvaging. The Directorate of Plant Protection, Quarantine and Storage, established under the Ministry of Agriculture is the regulating authority for granting approval for restricted use of methyl bromide and accreditation of fumigation agency. The directorate plays a major role in guidance and control of fumigation activities in the country under its “National Standards for Phytosanitary Measures (NSPM)” and the NSPM-11 deals about the “Quarantine Treatments and Application Procedures - I. Methyl bromide fumigation”. The phasing out of the fumigant necessitates the impetus for research in finding a suitable alternative for the Indian quarantine system. Several chemicals such as phosphine, sulfuryl fluoride, carbonyl sulfide, carbon disulfide, ethyl formate, ethylene oxide, hydrogen cyanide, methyl iodide, and methyl isothiocyanate are being considered as alternative fumigants to methyl bro­mide. But among them very few match the rapid action and penetration capabilities of methyl bromide on target pests. Some taint the fumigated product, few are phytotoxic, leave unacceptable residues, or may be unlikely to be registered for health or economic reasons ).  

Alternatives fumigants for methyl bromide

Phosphine: This is the most widely used fumigant for insect con­trol in the durable commodities throughout the world. It is increasingly used as a treatment to re­place methyl bromide especially because of its low cost, fast dispersion in the air and low residues. Versatility of use is a major advantage for phosphine, as it can be used in a variety of storage buildings, during transit (e.g. in ship holds) or in plastic sheet enclosures. It is close to an ideal fumigant except for few drawbacks: slow activity, the rapid increase in insect resistance, flammability at higher concentrations (>900 ppm) and corrosion of copper, silver and gold. The phosphine resistance among the insect populations was found to be the result of selection pressure caused by inadequate fumigations in the storage units; storage facilities not adequately sealed before fumigation; and fumigant concentrations not being monitored. The understanding of phosphine resistance mechanism, improved monitoring tactics and management of resistance are the priorities in tackling the problem (Rajendran, 2001). The other problems like corrosion and flammability were found to be limited by using the combination of heat (30–36^◦C), carbon dioxide (3–7%) and phosphine at 80–100 ppm, while achieving a complete insect control.

 Sulfuryl fluoride: Another promising chemical is sulfuryl fluoride, which is widely used for control of termites, and stored-product insects. This chemical shows considerable promise and is undergoing registration procedures as replacement for methyl bromide in several countries including the United States, Great Britain, Italy, and France. The fumigant has vapour pressure of about ten times higher (13,442 mm of Hg at 25ºC) than that of methyl bromide and hence it is more penetrative into treated commodities which is a significant benefit in situations where insect infestations are located deep within cracks or machinery in premises such as flour mills . The fumigant has several other advantages as a replacement for methyl bromide; it is non- reactive with structural materials and components of electronic equipment, is non flammable and has not caused problems of taint following treatment of a wide range of materials. However the sulfuryl fluoride was found to be less effective in controlling the insect eggs.  

Carbonyl sulfide: The use of carbonyl sulfide as a fumigant for durable commodities and structures was patented worldwide by Australia in 1992. It is effective on a wide range of pests, including the common stored product species at reasonable concentrations (less than 50 gm^-3) and exposure times (1-5 days) . However, the egg stage of several insects showed tolerance to the fumigant. The other problems associated with the use of carbonyl sulfide include its high tainting odour on the treated products and reduction in the germination of seeds. Hydrogen sulphide, an impurity, present in fumigant product supply was reported to be responsible for the off-odour problem. Selective removal of hydrogen sulphide using absorbents like tertiary amine may solve the tainting issues with this fumigant.

 Ethyl formate: Plant volatiles such as ethyl formate have been shown to have insecticidal properties as fumigant. The efficacy of ethyl formate against insect pests of food commodities, bagged cereals, spices, pulses, dry fruits and oilcakes had been proved. The fumigant was known to provide a high mortality of mixed stage cultures of the key stored product pests, with limited efficacy against the pupal stage of few pests like S. oryzae. The advantages of ethyl formate include natural occurrence in food; rapid kill of insects (2-4 hours); fast breakdown of residues to natural products and low human toxicity. However the fumigant exhibits poor penetration characteristics and high doses (>120 g/t of grain) were required to control internal feeders, which was higher than the flammable limit of 85 g/t. Formulations of ethyl formate in liquid carbon dioxide were found to overcome the problems of flammability and poor penetration.

 Ozone: The powerful oxidizing property makes ozone, an attractive candidate as a fumigant for controlling insects and fungi in stored products. It is currently used to disinfect fruits, vegetables, and other foodstuffs of microorganisms and viruses; as a means of reducing odor; and for removing taste, color, and environmental pollutants in industrial applications. Ozone controlled the major stored-grain insects viz., S. oryzae, Oryzaephilus surinamensis, Tribolium spp., and E. elutella  and various fruit flies. The advantages of ozone as a fumigant include lower hazards, practically no residues in treated commodities, reduction in transportation costs as it can be generated on site and its efficacy against wide range of pests. The main disadvantages are its poor penetration capabilities, requirement of longer exposure time, corrosive properties on metals and phytotoxicity to live plants. The efficacy of ozone fumigation was reported to be enhanced by higher temperatures, lower oxygen levels, and longer treatment times and modifying the method of application like closed-loop recirculation 

Hydrogen cyanide: Hydrogen cyanide was a widely used insecticidal and rodenticidal fumigant against the stored product pests on durables and perishables for many years. The high dermal toxicity of the gas makes it hazardous to applicators, and residues are often a concern. However, recent restrictions on the use of methyl bromide are resulting in the phytosanitary uses of old fumigants being considered again and some countries started using hydrogen cyanide to disinfest oranges of surface pests with insignificant injury to the fruit  

Carbon disulphide: This is one of the oldest fumigants in use, and its use in France during 1869, against the grape phylloxera was a landmark in the history of applied entomology. Carbon disulphide was widely used as a soil or space fumigant in many countries and owing to the properties of high flammability, requirement of longer exposure period and toxic residues in treated commodity, its usage had been discouraged. 

Botanicals: Plant essential oils and their components, especially monoterpenes, have the potential for the development as alternative fumigants and they possess several advantages over conventional fumigants in terms of low mammalian toxicity, environmental friendly, rapid degradation and local availability. Essential oils derived from more than 75 plant species have been evaluated for fumigant toxicity against stored product pests and had been found to possess ovicidal, larvicidal, and repellent properties against various insect species 

Non-chemical alternatives: Irradiation; controlled atmospheres utilizing nitrogen and carbon dioxide, heat and cold are the widely used non-chemical alternatives for methyl bromide fumigation. The FAO promotes the use of radiation for phytosanitary purpose, recommending minimum doses of <4000 Gy for nematodes, <300 Gy for most insects, 100 Gy for Diptera, and 150–320 Gy for mites. Irradiation treatment has the potential to evolve as an alternate for the methyl bromide fumigation and is the most tolerated treatment by fresh commodities in general. Gamma rays, X-rays, or accelerated electrons as the source of irradiation are being used for the control of quarantine insects. The main drawbacks of using irradiation are the initial cost to set up a facility, regulations in the operational management of the facility and the uncertainties regarding consumer acceptance of treated product. 

 In India only phosphine (from aluminium phosphide and magnesium phosphide) is registered for use under the Insecticides Act, 1968 apart from methyl bromide. The country is committed to reduce the ozone depleting substances under the signed international treaties and its own act viz., the Ozone Depleting Substances (Regulation and Control) Rules, 2000 under the Environment (Protection) Act, 2000. An impetus for the research on alternative fumigants is obligatory to cater the needs of phytosanitary measures after 2015. Few research programmes were initiated in the Indian Council of Agricultural Research and Central Food Technological Research Institute on the alternatives and there is a long way to go in finding substitute or replacement to the methyl bromide. A decade of research in the developing countries failed to identify a single alternative fumigant. However encouraging results have been obtained in the enhancing the efficacy of alternate fumigants by different combinations of treatments like sequential fumigation, heat and irradiation, heat followed by cold storage, cold and low oxygen, heat and high carbon dioxide, low temperature and fumigation depending on the product and the pest.