Ethylene dibromide

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The insecticidal properties of ethylene dibromide (EDB) were reported by Neifert et al in 1925. It has become important as an insecticidal fumigant as a result of its specific value for the destruction of fruit flies (family Trypetidae) in fruit (Viel and Catelot-Goldman, 1957) and as a fumigant for grain in the tropics. It has also been found useful throughout the world as an ingredient of a number of liquid-type grain fumigants and "spot" fumigants. The role of EDB in fumigant mixtures will be discussed in Chapter 7.

Although EDB is a fumigant of considerable utility, it has a high boiling point and is sorbed by many materials, into which it does not penetrate well. It is thus more limited in usefulness than some of the more volatile fumigants. It has, however, found extensive use in soil fumigation, a subject outside the scope of this manual. It is also effective as an ingredient in very low proportions of dips to control fruit flies in fruit (Cohen and Nadel, 1958; Wolfenbarger, 1962; and Burditt et al 1963). In this use, the insecticidal effect is undoubtedly due to fumigant action.

TOXICITY

Ethylene dibromide is more toxic to human beings than methyl bromide. It is a severe skin irritant and can be absorbed through the skin as well as the respiratory tract. High concentrations can affect the lungs and injure liver and kidneys (Torkelson et al, 1966). When fed in small amounts to laying hens in fumigated grain, EDB was found to decrease the size and number of eggs (Bond) et al, 1955; Caylor and Laurent, 1960). In bulls, malformations of sperm cells appeared in the semen (Amir and Volcani, 1967). In addition, EDB has been investigated for its carcinogenic effects and shown to be capable of producing cancer in laboratory animals (Olson et al, 1973). Chemicals found to be carcinogenic in animal tests are generally considered, by the U.S. National Cancer Institute, to be a potential threat to human health (U.S. Department of Heath, Education and Welfare, 1979). Therefore, appropriate precautions should be taken to avoid exposure to this fumigant.

A serious toxic interaction between inhaled EDB and ingested disulphiram (tetraethylthiuram disulphide) has been demonstrated in experimental animals (Stein et al, 1978). Furthermore, Plotnick (1978) found that disulphiram in the diet of rats exposed to a low level of EDB (20 ppm) increased the incidence of tumours. As disulphiram is used in therapy for alcoholism, as well as in certain industrial processes, special precautions against possible exposure to the two chemicals together are indicated.

PROPERITES OF ETHYLENE DIBROMIDE
Alternative names : 1, 2-dibromoethane, ethylene bromide
Abbreviation used in this manual : EDB

Odour Like chloroform
Chemical formula CH2Br.CH2Br
Boiling point 131.6C
Freezing point 10 C
Molecular weight 187.88
Specific gravity  
gas (air = 1) 6.487
liquid (water at 4C = 1) 2.172 at 20C
Latent heat of vaporization 46.2 cal/g
Flammability limits in air Nonflammable
Solubility in water 0.431 g/100 ml at 30C
Pertinent chemical properties Stable
Method of evolution as fumigant By evaporation of liquid, often in mixture with other fumigants

Natural vapour pressure at different temperatures

0C (32F) 3.5 mm Hg
10C (50F) 6.0 mm Hg
20C (68F) 11.0 mm Hg
25C (77F) 14.0 mm Hg
30C (86F) 17.5 mm Hg
40C (104F) 28.5 mm Hg

Weights and volumes of liquid

1 lb (avdp) at 25C has volume 208.8 ml
1 U.S. gal weighs 18.11 lb (8.215 kg)
1 Imp gal weighs 21.72 lb (9.852 kg)
1 kg has volume 460.4 ml
1 litre weighs 2.172 kg

Dosages and concentrations of gas in air (25C and 760 mm pressure)

By volume

Weight per volume

Parts per million Percent g/m lb/l 000 ft
25 0.0025 0.19  
50 0.005 0.38  
100 0.01 0.77  
130 0.013 1.00  
200 0.02 1.54  
500 0.05 3.84 0.24
1 000 0.10 7.68 0.48
2 084 0.21 16.00 1.00
20 000 2.0 153.68 9.60

1 Ounces per 1000 cubic feet or milligrammes per litre

Among the fumigants commonly employed, EDB is one of the more toxic to insects (see Chapter 14, Table 16). Loschiavo (1960) found that female confused flour beetles, Tribolium confusum, were more susceptible than males to very low doses of EDB and that the fecundity and fertility of survivors were reduced. Confused flour beetle adults treated with sublethal doses of EDB laid sterile eggs, in contrast to those fumigated with methyl bromide, which laid fertile eggs (Kazmaier and Fuller, 1959). Insects affected by EDB may remain moribund many days before dying (Bond and Monro, 1961). A low level of resistance to EDB has been found in a population of Tribolium castaneum exposed to repeated treatments over a number of years (Bond, 1973). Ellis and Morrison (1967) described a simple technique for conducting small chamber tests with ethylene dibromide in order to assess the susceptibility of insects under local conditions.

EFFECT ON PLANT LIFE

Seeds

When EDB was used for insecticidal treatments, it appeared to have no effect on the germination of wheat, barley, maize, vetch, peas and beans (Amen et al, 1946). Seeds of high oil content, such as soybean, flax, sesame and groundnut, however, need prompt postfumigation aeration in order that the residual fumigant may not affect germination (Plaut,1957). Sorghum seed is low in oil content as compared with sunflower seed, but both require the same precautions (Lachover et al, 1958). Ethylene dibromide as a constituent of a fumigant mixture caused significant reduction in germination of maize, sorghum, barley, oats, wheat and rice seeds after 12 months' storage, especially under combined conditions of high moisture content and high temperature, e.g. above 27C King et al, 1960). C.H. Richardson (1951) also found that EDB was detrimental to the germination of maize.

It is clear that the use of EDD for seed fumigation should be undertaken with caution, preferably after preliminary trials under local conditions.

Growing Plants and Nursery Stock

There is little information on the susceptibility of plants to the vapours of EDB. It has been stated that it is strongly toxic to growing plants but less injurious to dormant ones (Negherbon,1959; Pritchard, 1949). Monro (1955) found that ethylene dibromide in concentrations toxic to the European pine shoot moth, Rhyacionia buoliana (Schiff.), in dormant pine nursery stock caused considerable injury and retardation of subsequent growth. The species of pine tested were red, Scotch, mugho and white, and of these the latter was extremely susceptible. However,EDB has been found effective for dipping, or for injecting soil balls around the roots of nursery stock to prevent the spread of the

Japanese beetle (Fleming et al,1958; Richardson and Balock, 1959) and of the European chafer (Tashiro,1962). Wolfenbarger (1957) reported that dips or surface applications of EDB emulsions did not injure a wide range of nursery and glasshouse plants.

EFFECT ON PLANT PRODUCTS

Fresh Fruit

The use of EDB for the fumigation of fruit came into prominence as a result of the work of Balock (1951) and Balock and Lindgren (1951) on the control of the oriental fruit fly in Hawaii. Generally speaking, fruit appears to be more tolerant to EDB than to methyl bromide at insecticidal concentrations.

An incipient off-flavour is noticed in fruit immediately following fumigation with EDB but it disappears as soon as the gas diffuses from the fruit (Claypool and Vines, 1956).

In the fumigation of fruit, particular attention must be paid to the relationship between dosage and load in the fumigation chamber. EDB is rapidly sorbed and initial residues have been found to be proportional to concentration and exposure time (Chalutz et al, 1971). Packing materials can also have considerable influence on sorption of fumigant and on postfumigation effects. Swaine et al (1976), after fumigating mangoes in cardboard cartons, concluded that the residual fumigant in the carton contributed significantly to the effectiveness of the treatment.

Peel injury, which sometimes occurs in citrus fruit, is related to persistence of residual unchanged fumigant in the fruit. The incidence of peel injury is found to be highest in fruit stored at low temperatures or wrapped in polyethylene bags (Chalutz et al, 1971). Some reduction in peel injury may be obtained by application of the fungicide thiobendazol (Chalutz et al, 1973).

In a study of the tolerance of avocados to EDB for control of Mediterranean fruit fly, Wolfenbarger (1962) found that treated fruit ripened more rapidly than unfumigated fruit. Akamine et al (1954) discussed the factors influencing tolerance or injury in bananas fumigated with EDB to control fruit flies. Farooqi and Hall (1972) concluded that some injury could occur on Australian cavendish bananas when the dose required to kill the Queensland fruit fly (15 g/m for 2 hours at 21.1C) was used.

Tests on apples have shown that EDB will effectively control apple maggot (Sanford, 1962 a, b) and eggs of European red mite (Bond et al, 1973).

Details of treatments with EDB for fruit are given in schedule J.

Venetables

Several varieties of vegetables are tolerant to EDB in treatments against fruit flies. Tomatoes may be delayed in ripening (Pratt et al, 1953).

Details of some treatments are given in Schedule K.

Cereals and Milled Foods

Used alone, EDB has found only limited application for cereals and milled food products because it does not penetrate well into large masses or stacks of these materials. In India, EDB has been injected directly into grain stored in bags (Muthu and Pingale, 1955). Flour fumigated with EDB had normal baking properties and bread made from it had a normal taste and odour (Plant and Zelchuch, 1953).

Because of the persistence of EDB in cereal grains, its use has been discouraged for cereal fumigations in some countries (FAD/WHO, 1980).

EFFECT ON PAINTS AND METALS

EDB in vapour or liquid form attacks many paints and some metals (particularly aluminium). This characteristic is important when the material is used in fumigation chambers, as special finishes may have to be used and precautions taken to prevent damage and corrosion. It has been found that catalyst-type paints are not affected during fumigation by the vapours of EDB, whereas standard enamels may soften and wrinkle (Grierson and Hayward, 1959). Gray (1959) described an inhibitor which renders a grain fumigant containing EDB noncorrosive to mill machinery.

RESIDUES IN FOODSTUFFS

Ethylene dibromide, in contrast to methyl bromide, does not normally react to any significant degree with the constituents of foodstuffs, but there is the possibility of the formation of small amounts of inorganic bromide. Other reactions may occur, such as the breakdown of EDB to form ethylene glycol, which may react with the methionine in the wheat protein (Bridges, 1956; Olomucki and Bondi, 1955).

When considering the question of inorganic bromide residues, it must be borne in mind that naturally occuring bromides are found in many foodstuffs (Heywood, 1966) and, therefore, analysis based on inorganic bromide alone may not give a true indication of fumigant residue. Because of the widely differing toxicological effects, it is necessary to determine residues of unchanged fumigant and of bromide ion separately.

The main problem with EDB is that, because of its comparatively low volatility, it is physically sorbed by fumigated materials; considerable aeration and a long interval are required before the vapours are completely dissipated.

Fruits with thick skins are likely to retain small amounts of the fumigant almost indefinitely (Sinclair et al, 1962). Studies by Hargreaves et al, (1978) showed that EDB residues up to 4 mg/kg could occur in fruit and vegetables (e.g. capsicum, mango, papaya, passion fruit, pumpkin and zucchini) after fumigation and they suggested a witholding period of at least five days to allow for Resorption. The levels of residual EDB in apples bested with 12 mg/l of the fumigant for four hours at 13C declined rapidly in the first two days after treatment but required nearly four weeks at 13C to desorb to 0.1 mg/kg (Dumas and Bond, 1975). In studies on grapefruit treated to eliminate possible infestation by larvae of the Coribbean fruit fly, King et al (1980) showed that residues were higher in fruit held at lower temperatures but, after three to six days of storage, residues were less than 1 mg/kg in fruit held at 13 or 21C.

In cereals, the uptake of EDB increases significantly with an increase of moisture content from 9 to 18.5 percent (Berck, 1965b). Uptake is greater in seeds with high fat content and grinding or milling increases sorption. Amuh (1975), using C labelled EDB, found that six weeks were required to remove sorbed EDB from fumigated maize but about 40 percent remained in a chemically bound state for 14 weeks after treatment. On milling of aerated wheat, Sidhu et al, (1975) showed that 18-38 percent of residual EDB was lost but significant residue remained, particularly in bran and shorts. Sensitive analytical methods now available have shown that a minute part of unchanged EDB can be detected in baked goods made from treated wheat. Flour and biscuit samples from commercial channels have been found to contain up to 4 mg/kg residual EDB in flour and 0.26 mg/kg in biscuits (Rains and Holder, 1981).

The problem of residual EDB is likely to be greatest in animal foodstuffs or in foods that are intended for consumption without cooking. Although some surveys of cargoes of imported grain have shown only low levels of residual EDB, at or below 1 mg/kg (FAD/WHO, 1980), the possibility of significant residues remaining in foodstuffs that have not been adequately aired is viewed with some concern. Jagielski et al (1978) indicated that grain treated in a well constructed farm bin or in a tightly sealed bog may retain residual fumigant at a high level for a considerable period of time.

It is clear that great care must be exercised to ensure that residual vapours of ethylene dibromide are fully dissipated from fumigated foodstuffs before they are consumed. This is particularly important in countries where the material may be treated at temperatures lower than 25C.

An annotated list of references to residues of EDB found in 8 range of foodstuffs is given in Schedules J and K.

Evaluation of Residues

In the past, residue tolerance recommendations for EDB have been based on bromide ion present in the food material. However, as there is no way of determining the source of the bromide (naturally occurring bromides are present in some foods), the FAO Panel of Experts on Pesticide Residues in Food (FAD/WHO, 1980) indicated that it is unrealistic to regulate residues of EDB or other bromide fumigants on the basis of bromide ion.

Because of the adverse effects of EDB, as demonstrated in various tests on animals and with the availability of more sensitive analytical methods, the FAO panel recommend that the guideline level of EDB for cooked products should be reduced to a figure at or about the new low limit for determination (0.01 mg/kg). For cereal products intended for consumption without cooking, they recommend that the grain be selected from lots not treated with EDB. The guidelines for fruit and vegetables were set in line with residue levels known to occur at the end of suitable withholding periods after treatment e.g. 0.5 mg/kg for citrus and passion fruit, 0.1 mg/kg for other fruit and vegetables.

METHODS OF ANALYSIS

Determination of Vepours

Concentrations of EDB in air can be determined with considerable precision by gas chromatography using thermal conductivity or flame ionization detectors (Berck, 1965a; Dumas and Bond, 1975; Swaine et al, 1976). If necessary, samples can be taken in suitable containers for transport to the laboratory; however, due allowance for sorption on the walls of the sampling container may be necessary. Jonsson and Berg (1980) described a method for rapid and simultaneous determination of trace concentrations of EDB and EDG in ambient air, using a porous polymer for collection of the sample and gas chromatography for analysis. Dumas and Bond (1982) outlined a method for direct microdetermination of EDB in air.

The infra-red gas analyser (described in Chapter 3) is described as being capable of measuring concentrations of EDB from 0.1 to > 8 100 ppm and the portable gas chromatograph is sensitive down to 0.01 ppm or less. Chemical methods based on the Volhard titration have been successfully used for determining concentrations of EDB (Sinclair and Crandall, 1952; Kennett, 1954). A colorimetric method for analysis of levels as low as 1 ppm in air and 0.5 ppm residue in grain has been described by Rangaswamy et al (1976).

The thermal conductivity analyser described in Chapter 4 is not recommended for use with EDB because high sorption from the materials in the guard tubes results in an extremely slow response.

Determination of Residues

Low levels of residual EDB in commodities can be determined by gas chromatography using electron capture detectors. A cold extraction procedure developed by Heuser and Scudamore (1969b) was established, by a Panel on Fumigant Residues in Grain (1974), to be suitable for residue determination in maize and wheat. For residues in fruit, steam distillation procedures followed by gas chromatography have been found suitable for determinations down to levels of 0.01 mg/kg (Bielorai and Alumot, 1966; Dumas and Bond, 1975). King et al (1950) described an electron capture gas chromatographic method for determination of EDB residues in grapefruit down to 0.00038 mg/kg and Hargreaves et al (1974) gave a method for estimation of EDB in vegetables.

EDB residues in whole and milled wheat have been determined by benzene extraction followed by azeotropic disillation of the extract with water and iodometric estimation as bromide ion after breakdown by alcoholic potash (Sidhu et al, 1975).

Inorganic bromide can be determined by the method of Kennett and Huelin (1957). For determinations intended to differentiate between organic bromide and bromide ion, a non-aqueous solvent extraction procedure should be used rather than a heating process that can break down EDB.

APPLICATION

This fumigant is often a component of liquid-type fumigant mixtures, as described in Chapters 7 and 10. Owing to its sorption by grain, its use alone should be undertaken with caution.

When the fumigant is used in a conventional fumigation chamber, it is necessary to volatilize it by heating. This is done by pouring it onto an enamel or stainless steel pan heated by an electric hotplate, or other convenient means without a flame, or by glowing wires exposed to the gas (Richardson and Balock, 1959). This gas is more than six times as heavy as air and vigorous circulation by fans or blowers is needed to provide even distribution. Condensation of the vapour can occur if air movement is insufficient for thorough mixing of the fumigant with air.

Muthu (1964) devised a method of applying EDB in small amounts for treatment of individual Lags or of individual small storages, such as split bamboo bins, used in Indian homes or farms. The required amount of EDB is impregnated in small cardboard discs, which are then sealed in aluminium foil envelopes.

In Ghana (Hall, 1963), an experiment was carried out in which maize was stored in jute bags with 500-gauge polyethylene liners, 149 of ethylene dibromide being applied to each bag on a pad of cotton wool before the liner was tied and the bag sewn. This treatment gave 100 percent control and will, it is claimed, give protection for as long as the polyethylene remains intact.


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