7. Fumigant mixtures

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Fumigants are sometimes marketed in mixtures with other compounds.

There are a number of practical reasons for combining fumigants, the more important being:

1. The flammability risk of the toxic ingredient is reduced or prevented altogether by the addition of another chemical. Examples are the addition of carbon dioxide to ethylene oxide or methyl formate and of carbon tetrachloride to acrylonitrile or carbon disulphide.

2. With liquid-type fumigants in grain fumigation, mixtures may be made to provide ingredients whose vapours have differing rates and patterns of diffusion. After the liquid has been applied to the surface the ingredients evaporate. The distance the ingredients diffuse downward depends largely on the extent to which they are sorbed by the grain. It may, therefore, be necessary to have various types of ingredients in the mixture to kill insects at various depths and locations in the grain (Kenaga, 1957a). An example is a mixture of ethylene dibromide, ethylene dichloride, carbon disulphide and carbon tetrachloride, which is used for grain fumigation in farm storage units and country elevators.

Similarly with gaseous fumigants, mixtures may be used to obtain complementary advantages. Phosphine is used in combination with methyl bromide in treatments of materials such as oil-seed expeller; the phosphine penetrates deeply into the material while the methyl bromide gives more control on the outer surfaces (Wohigemuth et al, 1976).

3. A highly volatile fumigant such as methyl bromide may diffuse downwards too rapidly, so that the upper part of a load of infested goods may not receive an adequate insecticidal treatment. Another less volatile fumigant, such as ethylene dibromide, is added to ensure that the material at the top is properly fumigated. A mixture of this type may be useful under tropical conditions when commodity temperatures may range as high as 38C (Majumder and Muthu, 1964).

4. The principal toxic ingredient may be diluted so that its distribution becomes more uniform. Carbon tetrachloride, though only moderately insecticidal in itself, aids in the distribution of other fumigants, such as ethylene dibromide, which does not diffuse well through masses of grain (Berck, 1958).

Although fumigant mixtures are very useful for many types of application, especially for the treatment of grain in bulk and for local or spot fumigations, it should be pointed out that their use involves certain complications. As indicated above, the ingredients may settle out in different parts of the fumigation system. This may result in a situation whereby the more toxic material is acting against one part of the insect population while the less toxic ingredients are acting against the remainder. Erratic results may thus be recorded unless full provision is made to allow time for the less toxic materials to exert their maximum effect.

Another objection, which is possibly more serious, is the fact that commonly used methods of field analysis of vapours, such as the thermal conductivity analyses, will not differentiate between the components of the mixture so that is is not practicable to attempt to keep a check on the effective distribution of the toxic ingredients. Any reading taken with this type of instrument is meaningless under these conditions. A gas chromatograph or an infra-red analyser would provide the necessary information in the field, or samples could be brought to the laboratory for immediate analysis. Kenaga (1957a) has suggested the use of the mass spectrometer for the analysis of mixtures but this expensive instrument would have to be used in the laboratory.

Some fumigants that have been Found effective in mixtures are discussed below. They are given in the alphabetical order of the more insecticidally active ingredient of the mixture. The proportions of fumigants in a liquid state at normal temperatures are usually expressed in terms of volume. Mixtures containing volatile ingredients, such as methyl bromide, are expressed by weight of the components.

Acrylonitrile

Because of its flammability, acrylonitrile is never applied alone but always in admixture with nonflammable materials. Mixed with carbon tetrachloride it has been used as a local fumigant and for the fumigation of tobacco. Mixed with chloroform or methylene chloride it has been employed with some success for fumigating buildings against dry wood termites (Young, personal communication, 1967). Although in comparison with methyl bromide these mixtures do not have a deleterious effect on houshold materials, such as sponge rubber, they do not aerate after fumigation as rapidly as methyl bromide or sulphuryl fluoride.

Ruppel et al (1960) concluded that mixtures of acrylonitrile with carbon tetrachloride were not fully satisfactory against pests of stored maize in Colombia as compared with the EDC : CT 3 to 1 mixture commonly used in that country.

Carbon disulphide

In mixture with carbon tetrachloride, carbon disulphide (CS2) is very useful for grain fumigation, especially in tall, upright silo storage units and in all storage units equipped with adequate recirculation systems. Enough carbon tetracilloride should he incorporated in the mixture to eliminate the fire hazard. A mixture commonly marketed for these purposes contains, by volume, 16.5 percent CS2 82.5 percent carbon tetrachloride and 1.0 percent inert ingredients.

Another mixture useful for gravity distribution in concrete or metal grain bins contains 24 percent CS2, 71 percent chloroform and 5 percent ethylene dibromide by volume.

Chloropicrin

The highly insecticidal and lachrymatory properties of chloropicrin have been put to use by mixing this chemical with other materials for the fumigation of grain in farm or bulk storage units. It has been marketed in a mixture with methyl bromide or carbon tetrachloride (Cotton, 1963). A mixture of chloropicrin and methyl chloride 85 : 15 is recommended for forced distribution fumigation of grains (USDA, 1967).

Ethylene dibromide

Ethylene dibromide (EDB) is an ingredient of many important fumigant mixtures. As indicated in Chapter 6 it is highly insecticidal, and under normal conditions the residues remaining in foodstuffs are of a low order. These advantages are offset by the fact that EDB is considered to be very hazardous to mammals and is highly sorbed by materials undergoing fumigation. As a result, there may be poor penetration during actual treatment and prolonged persistence of the vapours in the fumigated commodity during the aeration process. Attempts to utilize the advantages and overcome the disadvantages have resulted in the formulation of a wide variety of mixtures. It is not possible to describe these in detail here. For the present purpose the subject may he discussed unifier two headings: liquid formulations and mixtures with methyl bromide.

LIQUID FORMULATIONS

Alone or with other toxic fumigants EDB is commonly mixed with carbon tetrachloride (CT) as a grain or local spot fumigant. The inclusion of CT as a high proportion of the formulation appears to add greatly to the effectiveness of EDB, principally because CT acts as an eluant for EDB in a column of grain and assists effectively in its downward migration (Berck, 1958). The following are some liquid formulations containing EDB in admixture with other materials (percent by volume) that have been widely used:

Grain Fumigants

EDB 5: CT 95 - General grain fumigant
EDB 7: Ethylene dichloride (EDC) 29: CT 64 - Gravity distribution grain fumigant
EDB 7: Carbon disulphide (CS2) 12: CT 81 Gravity distribution in flat storages
EDB 3.5: EDC 10: CS2 10: CT 76.5 - Gravity distribution in flat storages
EDB 5: CS 24: Chloroform 71 - Gravity distribution grain fumigant

Local (spot) fumigants

EDB 20: EDC 20: C! 60
EDB 59: EDC 9: CI 32
FDB 15: CT 85

ETHYLENE DIBROMIDE-METHYL BROMIDE MIXTURES

Mixtures of EDB with methyl bromide have been found useful for three main purposes:

- in the tropics to treat bagged plant products under gas-proof sheets;

- as local fumigants for treating food handling equipment in mills and food processing plants generally;

- as a fumigant for grain stored in bulk.

Bagged goods. In India, mixtures of the two fumigants in various proportions have been recommended for application under sheets or in chambers to treat different types of commodities in bags under warehouse conditions (Majumder, 1962). Treatment with the fumigant mixtures accompanied by simultaneous prophylactic applications of liquid insecticides to prevent reinfestation is known as the "Durofume" process (Majumder and Muthu, 1964). These authors give detailed recommendations for the proportions of the two chemicals required to achieve the best results with different materials.

A special applicator for administering fumigant mixtures of EDB and methyl bromide, which is particularly suitable for treating stacked commodities under gas-proof sheets, has been described by Majumder et al (1962). This consists of a brass or stainless steel tube held horizontally, containing the EDB. Above this there is a vertical scaffold into which a can of methyl bromide may be inserted. The methyl bromide can is pierced at the bottom by a probe, which connects to one end of the tube containing the EDB. The methyl bromide with its natural vapour pressure forces the EDB out through a discharge nozzle at the other end of the horizontal tube so that the mixture may be led through a distribution system above the stacked commodity.

Local fumigants. A mixture of seven parts of EDB and three parts of methyl bromide (weight to weight) has found wide use as a local or spot fumigant. This is best introduced by special applicators into holes bored at strategic points in the food handling equipment. This subject is dealt with in more detail in Chapter 8.

Bulk grain fumigation. The mixture has been found effective for bulk grain. The proportions of the two fumigants may vary according to the material being treated and the method of application. For cereal grain treatments the ratio of 70 : 30 EDB to methyl bromide is recommended for gravity distribution in smaller bins or farm storages. For forced distribution systems, 30 : 70 EDB to methyl bromide is used. Majumder et al, (1963) found that the presence of methyl bromide improved the penetration and distribution of EDB in columns of grain and milled materials, the most effective proportions varying according to the material under treatment.

Dawson (1967) has taken out a United States patent for a yelled fumigant composition made by agitating different amounts of methyl bromide and ethylene dibromide in mixture with 3 to 5 percent by weight of colloidal silica. The gel thus produced retards the evaporation of the constituent fumigants and thus affords a convenient method for applying them to grain.

Insect toxicity. From the limited amount of laboratory evidence available to date, it appears that mixtures of these two fumigants are intrinsically more toxic to stored product insects than methyl bromide alone and at least equally as toxic as ethylene dibromide alone (Kazmaier and Fuller, 1959). These authors also concluded that the mixtures killed the postembryonic stages of Tribolium confusum more rapidly than either fumigant alone.

Penetration of flour. Heuser (1964) studied the pattern of diffusion through flour at 25C of the two components of a 1 : 1 by weight mixture mixture of EDB and methyl bromide. He found that the methyl bromide components in insecticidal concentrations could penetrate to 18 in (45 cm) after 48 hours, but EDB only penetrated effectively to 2 in (5 cm) after the same time. He concluded that if EDB is used "in hightemperature zones to produce a longer lasting insecticidal effect in the surface layers than is obtained with M. B. (methyl bromide) for example where the gasproofing of fumigation sheeting is suspect, then its addition should be regarded as a supplement only, and not as in any Way replacing a proportion of the methyl bromide dose".

Ethylene dichloride

Compared with most modern fumigants, ethylene dichloride (EDC) is intrinsically not very toxic to insects but its mixture with CT, usually in the proportion by volume of 3 : 1 EDC to CT, has been used throughout the world as a successful fumigant for stored grain in a variety of structures. It has proved effective under tropical conditions in Africa (Hall, 1963). Its use for this purpose is contingent on a long exposure period, usually not less than 14 days, before the grain is turned or the fumigant aerated (Thompson, 1964). Such long exposure periods do not appear to have adverse effects on the grain. It may be applied either by gravity distribution or forced distribution systems.

This mixture has also been widely used as a seed fumigant. Properly used it does not have an adverse effect on the germination of seeds (Kamel and Shahba, 1958; Cotton, 1963; Parkin 1963).

Methyl bromide-phosphine

A method for the combined use of methyl bromide and phosphine has been tested for treatments in oil-seed expeller, a dense material in which methyl bromide does not easily penetrate (Wohigemuth et al, 1976). The phosphine is found to diffuse into the expeller to give control while methyl bromide is effective on the outer surfaces. Furthermore, mixtures of methyl bromide and phosphine have been shown to have a joint action that improves the insecticidal effectiveness of both compounds (Bond, 1978; Bond and Morse, 1982; El Lakwah, 1978).

In this treatment, the phosphine-producing formulation is distributed evenly over the surface of the goods and, after sealing, the methyl bromide is applied through a tube by a double jet to vaporize in the Free space above the goods. Dosage of 56 g/m methyl bromide and 4.4 g/m phosphine at 29.5C for 72 hours gave satisfactory control of Khapra beetle larvae.

Methyl chloroform

Methyl chloroform (1,1,1 - trichloroethane) has been tested under field conditions in mixtures with ethylene dichloride and the trials have shown that the two compounds interact physically to improve distribution of each component through grain bulks (UK, 1978). When applied with methyl chloroform, EDC is carried to a greater depth than when used alone, and the two compounds seem to act jointly to give a much enhanced toxic effect on insects. The physical properties of methyl chloroform relating to distribution and persistence compare favourably with carbon tetrachloride as does its toxicity to insects and mites. The available toxicological evidence suggests that it is a much safer compound than carbon tetrachloride for a pest control operator to use and tests on residues suggest that it should not be a toxic hazard in grain. The potential of methyl chloroform as a substitute for carbon tetrachloride in fumigant mixtures may be indicated by further testing in the future.


8. Space fumigation at atmospheric pressure

The term "space fumigation" is convenient for the designation of a wide range of treatments in enclosed spaces, which either contain infested materials or residual insect populations. The calculation of the dosage of fumigant to be applied is based primarily on the volume of the space. Other factors, such as the amount of a given commodity in the space, are sometimes introduced into the calculation for dosage. If facilities are available for the determination of fumigant concentrations at frequent intervals during exposure, it is advantageous to calculate the dosage on the basis of minimum concentration to be maintained in every part of the free air space during the required exposure time.

Fumigation may be successfully carried out in any structure that can be made sufficiently gas-tight for the length of time required. Infestations may often be dealt with on the spot without the necessity of moving the affected material.

Sealing Methods and Materials

Since sealing techniques are common to all types of fumigation, these are outlined first. In practice, the choice of materials is largely influencer) by their availability.

NARROW CRACKS AND SMALL HOLES

Masking tape, heavy kraft paper applied with flour paste, caulking compound, paint-on adhesives, froth packs of Styrofoam, etc. may be used to cover or seal small cracks and holes. A vinyl plastic sealing compound is available that can be sprayed over all holes, cracks and crevices. This method, sometimes called "cocooning", is effective but requires a compressor for air, a spray gun and a hose. Instructions for carrying out the process are provided by manufacturers of the plastic; a full description is also given by Roop (1944). Webley and Harris (1979) describe the sealing and fumigation, using phosphine, of mud-brick stores in Mali.

LARGER CRACKS, CREVICES AND OPENINGS

Heavy kraft paper applied with flour paste and impregnateti with a heavy grease or sheet of polyethylene sealed with masking tape may be used for larger openings. The vinyl sealing compound described above in the cocooning method can be applied to bridge larger openings and it will provide a relatively gas-impermeable, durable seal.

VENTILATORS OR OUTSIDE OPENINGS

Heavy kraft paper can sometimes be used but polyethylene sheeting is relatively inexpensive and can be re-used several times. It can be tied with ropes to make a tight seal around a ventilator.


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