Resistance Management Reviews

Methyl Bromide: A Fumigant With a Future?

John Badmin, Chairman
IRAC Stored Products Pests SubCommittee
Shell Research Ltd.
Sittingbourne, Kent ME89AG
United Kingdom

All of the previous articles in this Newsletter have been devoted to problems associated with pests and diseases of growing crops. At present, world crop losses to pests are estimated to be about 35%, due to destruction by insects (12%), pathogens (12%), weeds (10%) and mammals and birds (1%). Post-harvest losses can be equally significant, with estimates ranging from 9% in USA to upwards of 20% in less economically advanced countries in the tropics (Gorham, 1991). Trade in stored grain and durable commodities has increased enormously in recent decades, in line with world population growth and forms a vital part of many countries' economies. Transfer of treated, insect-free commodities is a pre-requisite for commercial success. Shipments lost through insect contamination are expensive to replace and more importantly allow transfer of pests from one country to another. Rapid, simple methods of disinfestation are therefore required before food produce can be exported from a country of origin.

The treatment of stored grain and other durable commodities throughout the world continues to rely heavily on fumigation with methyl bromide or phosphine, the only fumigants now in regular use. Both compounds are highly toxic to man and strict conditions pertain to their use. Phosphine is usually preferred owing to its ready availability, ease of application and low cost. For fumigation to be effective, treatments should last a minimum of 7-14 days. Currently the main use of methyl bromide is for quarantine purposes and in countries where the temperature is too low for an acceptable fumigation period with phosphine. Fumigation with methyl bromide is technically more difficult but is achievable in 24 hours. The rapid action of this fumigant means that any produce found to be contaminated by insects, can be treated quickly, and shipped to its destination without undue loss of time.

A consequence of the Montreal Agreement has been to seek ways of reducing or banning known ozone depletors from commercial use. Methyl bromide, a readily identifiable ozone depletor has therefore come under increasing scrutiny and there are calls for its complete withdrawal. The point of this article is to emphasize the importance of methyl bromide as an effective, responsible way of disinfesting stored produce.

When the Insecticides Resistance Action Committee of GIFAP commissioned a world survey of insecticide resistance in 1986 the results on stored products pests revealed significant field resistance in a total of 16 arthropod species (Badmin 1991). No significant fumigant resistance was reported then, although low levels of phosphine resistance in the Flour beetle Tribolium confusum and Grain beetles Cryptolestes spp. were considered to be "a cause for future concern". In the intervening years the evidence suggests that some insects are acquiring an unacceptable degree of resistance to phosphine (Taylor, 1989). Thus there is a growing need to contain phosphine resistance where it occurs.

Store managers need to be able to integrate a resistance management strategy into their overall management plan, to know what options are available and to prioritize them according to local conditions. In the case of suspected phosphine resistance a manager may be able to treat his infested produce with an insecticide which will then require a period of time to achieve its full effect and for residues to decline to an acceptable level.

However he may need to treat more quickly and to use a fumigant. There are just two fumigants widely available for stored products use. The modes of action of methyl bromide and phosphine in insects are entirely different and so the former may be used to control pests showing resistance to phosphine (see Price, 1985 for a discussion of the mode of action of fumigants). The option of being able to substitute one fumigant for another with a different mode of action is crucial in delaying the selection of resistant strains. Thus it is vital that both fumigants remain available for disinfesting stored produce.

The amounts of methyl bromide used for disinfesting produce are relatively small by comparison with the amounts used for soil sterilization in certain parts of the world. An international committee, part of the United Nations Environmental Programme, has the responsibility of listing the various uses of methyl bromide and to obtain estimates of the quantities of material involved. This will include the practice of fumigating stored produce. Separate studies have indicated that more efficient fumigation procedures and the introduction of better designed fumigation chambers may reduce the quantities of methyl bromide and other fumigants used by a considerable margin. There are studies in progress looking at ways of recovering methyl bromide after fumigation. The net result of this will probably be to extend the period of fumigation since removal of very low concentrations of free gas or sorbed methyl bromide from treated produce may require lengthy extraction procedures.

At present there are no known fumigants which offer quite the same performance characteristics as methyl bromide, although a number of minor products has been used from time to time. Any other halide product, such as methyl chloroform or the old liquid fumigants are essentially covered by the Montreal Protocol and in addition have toxicology profiles which are far from ideal. Thus, there appears to be little chance of introducing alternatives in the near future, although people are re-examining the use of hydrogen cyanide. Removal of methyl bromide would have two immediate effects i) there would be an upsurge in the use of phosphine as the only commercially available fumigant, coupled with increasing pressure to use shorter exposure times leading inexorably to the selection of strains highly resistant to phosphine and ii) a search for novel fumigants or ways of treating stored produce. The effects of the first are difficult to assess, but it is likely that this would lead to a gradual breakdown in the trade of "pest-free" commodities around the world. The second effect is to be welcomed as it will focus our minds on finding new ways of protecting stored produce. Possible alternatives, currently under investigation, include irradiation and the use of "controlled atmospheres", but each of these has its limitations. For example, buildings which are designed for controlled atmosphere use offer scope for long term storage of produce as pest control is achieved over a period of months rather than days, but offer little hope for the vast bulk of produce which is transported daily around the world.

At the forthcoming 6th International Working Conference on Stored Products Protection to be held in Australia in April 1994, IRAC proposes to organize a Workshop aimed at discussing the major elements involved in storage design and management, as part of a co-ordinated strategy to delay the onset of resistance to stored products pesticides. Problems of insecticide resistance are increasing at a time when more restrictions are being placed on pesticide usage and registration costs are soaring. To improve the efficacy of pesticides (and that includes fumigants), there is a need to re-examine all aspects of the storage environment, and for all disciplines to be involved, store designers, construction engineers, managers, fumigation and pest control experts, and economists, to work together in a concerted effort to reduce the likelihood of infestation and to improve early detection of problems. Hopefully the Workshop will provide an opportunity for a useful dialogue between these groups.

REFERENCES:

Badmin, J.S. 1991. IRAC survey of resistance of stored grain pests: results and progress. Proc. 5th Int. Work. Conf. Stored Product Protection. 2:973-981.

Gorham, J.R. 1991. Ecology and management of food-industry pests, FDA Technical Bulletin. 4. Association of Official Analytical Chemists, Virginia.

Price, N.R. 1985. The mode of action of fumigants. J. of Stored Products Research. 24:157-165.

Taylor, R.W.D. 1989. Phosphine, a major grain fumigant at risk. Int. Pest Control. 31:10-14.

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